[gostls13.git] / src / cmd / go / internal / work / exec.go

// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Action graph execution.

package work

import (
        "bytes"
        "context"
        "crypto/sha256"
        "encoding/json"
        "errors"
        "fmt"
        "internal/coverage"
        "internal/lazyregexp"
        "io"
        "io/fs"
        "log"
        "math/rand"
        "os"
        "os/exec"
        "path/filepath"
        "regexp"
        "runtime"
        "sort"
        "strconv"
        "strings"
        "sync"
        "time"

        "cmd/go/internal/base"
        "cmd/go/internal/cache"
        "cmd/go/internal/cfg"
        "cmd/go/internal/fsys"
        "cmd/go/internal/load"
        "cmd/go/internal/modload"
        "cmd/go/internal/str"
        "cmd/go/internal/trace"
        "cmd/internal/quoted"
        "cmd/internal/sys"
)

const defaultCFlags = "-O2 -g"

// actionList returns the list of actions in the dag rooted at root
// as visited in a depth-first post-order traversal.
func actionList(root *Action) []*Action {
        seen := map[*Action]bool{}
        all := []*Action{}
        var walk func(*Action)
        walk = func(a *Action) {
                if seen[a] {
                        return
                }
                seen[a] = true
                for _, a1 := range a.Deps {
                        walk(a1)
                }
                all = append(all, a)
        }
        walk(root)
        return all
}

// do runs the action graph rooted at root.
func (b *Builder) Do(ctx context.Context, root *Action) {
        ctx, span := trace.StartSpan(ctx, "exec.Builder.Do ("+root.Mode+" "+root.Target+")")
        defer span.Done()

        if !b.IsCmdList {
                // If we're doing real work, take time at the end to trim the cache.
                c := cache.Default()
                defer c.Trim()
        }

        // Build list of all actions, assigning depth-first post-order priority.
        // The original implementation here was a true queue
        // (using a channel) but it had the effect of getting
        // distracted by low-level leaf actions to the detriment
        // of completing higher-level actions. The order of
        // work does not matter much to overall execution time,
        // but when running "go test std" it is nice to see each test
        // results as soon as possible. The priorities assigned
        // ensure that, all else being equal, the execution prefers
        // to do what it would have done first in a simple depth-first
        // dependency order traversal.
        all := actionList(root)
        for i, a := range all {
                a.priority = i
        }

        // Write action graph, without timing information, in case we fail and exit early.
        writeActionGraph := func() {
                if file := cfg.DebugActiongraph; file != "" {
                        if strings.HasSuffix(file, ".go") {
                                // Do not overwrite Go source code in:
                                //      go build -debug-actiongraph x.go
                                base.Fatalf("go: refusing to write action graph to %v\n", file)
                        }
                        js := actionGraphJSON(root)
                        if err := os.WriteFile(file, []byte(js), 0666); err != nil {
                                fmt.Fprintf(os.Stderr, "go: writing action graph: %v\n", err)
                                base.SetExitStatus(1)
                        }
                }
        }
        writeActionGraph()

        b.readySema = make(chan bool, len(all))

        // Initialize per-action execution state.
        for _, a := range all {
                for _, a1 := range a.Deps {
                        a1.triggers = append(a1.triggers, a)
                }
                a.pending = len(a.Deps)
                if a.pending == 0 {
                        b.ready.push(a)
                        b.readySema <- true
                }
        }

        // Handle runs a single action and takes care of triggering
        // any actions that are runnable as a result.
        handle := func(ctx context.Context, a *Action) {
                if a.json != nil {
                        a.json.TimeStart = time.Now()
                }
                var err error
                if a.Func != nil && (!a.Failed || a.IgnoreFail) {
                        // TODO(matloob): Better action descriptions
                        desc := "Executing action "
                        if a.Package != nil {
                                desc += "(" + a.Mode + " " + a.Package.Desc() + ")"
                        }
                        ctx, span := trace.StartSpan(ctx, desc)
                        a.traceSpan = span
                        for _, d := range a.Deps {
                                trace.Flow(ctx, d.traceSpan, a.traceSpan)
                        }
                        err = a.Func(b, ctx, a)
                        span.Done()
                }
                if a.json != nil {
                        a.json.TimeDone = time.Now()
                }

                // The actions run in parallel but all the updates to the
                // shared work state are serialized through b.exec.
                b.exec.Lock()
                defer b.exec.Unlock()

                if err != nil {
                        if b.AllowErrors {
                                if a.Package.Error == nil {
                                        a.Package.Error = &load.PackageError{Err: err}
                                }
                        } else {
                                base.Errorf("%s", err)
                        }
                        a.Failed = true
                }

                for _, a0 := range a.triggers {
                        if a.Failed {
                                a0.Failed = true
                        }
                        if a0.pending--; a0.pending == 0 {
                                b.ready.push(a0)
                                b.readySema <- true
                        }
                }

                if a == root {
                        close(b.readySema)
                }
        }

        var wg sync.WaitGroup

        // Kick off goroutines according to parallelism.
        // If we are using the -n flag (just printing commands)
        // drop the parallelism to 1, both to make the output
        // deterministic and because there is no real work anyway.
        par := cfg.BuildP
        if cfg.BuildN {
                par = 1
        }
        for i := 0; i < par; i++ {
                wg.Add(1)
                go func() {
                        ctx := trace.StartGoroutine(ctx)
                        defer wg.Done()
                        for {
                                select {
                                case _, ok := <-b.readySema:
                                        if !ok {
                                                return
                                        }
                                        // Receiving a value from b.readySema entitles
                                        // us to take from the ready queue.
                                        b.exec.Lock()
                                        a := b.ready.pop()
                                        b.exec.Unlock()
                                        handle(ctx, a)
                                case <-base.Interrupted:
                                        base.SetExitStatus(1)
                                        return
                                }
                        }
                }()
        }

        wg.Wait()

        // Write action graph again, this time with timing information.
        writeActionGraph()
}

// buildActionID computes the action ID for a build action.
func (b *Builder) buildActionID(a *Action) cache.ActionID {
        p := a.Package
        h := cache.NewHash("build " + p.ImportPath)

        // Configuration independent of compiler toolchain.
        // Note: buildmode has already been accounted for in buildGcflags
        // and should not be inserted explicitly. Most buildmodes use the
        // same compiler settings and can reuse each other's results.
        // If not, the reason is already recorded in buildGcflags.
        fmt.Fprintf(h, "compile\n")

        // Include information about the origin of the package that
        // may be embedded in the debug info for the object file.
        if cfg.BuildTrimpath {
                // When -trimpath is used with a package built from the module cache,
                // its debug information refers to the module path and version
                // instead of the directory.
                if p.Module != nil {
                        fmt.Fprintf(h, "module %s@%s\n", p.Module.Path, p.Module.Version)
                }
        } else if p.Goroot {
                // The Go compiler always hides the exact value of $GOROOT
                // when building things in GOROOT.
                //
                // The C compiler does not, but for packages in GOROOT we rewrite the path
                // as though -trimpath were set, so that we don't invalidate the build cache
                // (and especially any precompiled C archive files) when changing
                // GOROOT_FINAL. (See https://go.dev/issue/50183.)
                //
                // b.WorkDir is always either trimmed or rewritten to
                // the literal string "/tmp/go-build".
        } else if !strings.HasPrefix(p.Dir, b.WorkDir) {
                // -trimpath is not set and no other rewrite rules apply,
                // so the object file may refer to the absolute directory
                // containing the package.
                fmt.Fprintf(h, "dir %s\n", p.Dir)
        }

        if p.Module != nil {
                fmt.Fprintf(h, "go %s\n", p.Module.GoVersion)
        }
        fmt.Fprintf(h, "goos %s goarch %s\n", cfg.Goos, cfg.Goarch)
        fmt.Fprintf(h, "import %q\n", p.ImportPath)
        fmt.Fprintf(h, "omitdebug %v standard %v local %v prefix %q\n", p.Internal.OmitDebug, p.Standard, p.Internal.Local, p.Internal.LocalPrefix)
        if cfg.BuildTrimpath {
                fmt.Fprintln(h, "trimpath")
        }
        if p.Internal.ForceLibrary {
                fmt.Fprintf(h, "forcelibrary\n")
        }
        if len(p.CgoFiles)+len(p.SwigFiles)+len(p.SwigCXXFiles) > 0 {
                fmt.Fprintf(h, "cgo %q\n", b.toolID("cgo"))
                cppflags, cflags, cxxflags, fflags, ldflags, _ := b.CFlags(p)

                ccExe := b.ccExe()
                fmt.Fprintf(h, "CC=%q %q %q %q\n", ccExe, cppflags, cflags, ldflags)
                // Include the C compiler tool ID so that if the C
                // compiler changes we rebuild the package.
                // But don't do that for standard library packages like net,
                // so that the prebuilt .a files from a Go binary install
                // don't need to be rebuilt with the local compiler.
                if !p.Standard {
                        if ccID, err := b.gccToolID(ccExe[0], "c"); err == nil {
                                fmt.Fprintf(h, "CC ID=%q\n", ccID)
                        }
                }
                if len(p.CXXFiles)+len(p.SwigCXXFiles) > 0 {
                        cxxExe := b.cxxExe()
                        fmt.Fprintf(h, "CXX=%q %q\n", cxxExe, cxxflags)
                        if cxxID, err := b.gccToolID(cxxExe[0], "c++"); err == nil {
                                fmt.Fprintf(h, "CXX ID=%q\n", cxxID)
                        }
                }
                if len(p.FFiles) > 0 {
                        fcExe := b.fcExe()
                        fmt.Fprintf(h, "FC=%q %q\n", fcExe, fflags)
                        if fcID, err := b.gccToolID(fcExe[0], "f95"); err == nil {
                                fmt.Fprintf(h, "FC ID=%q\n", fcID)
                        }
                }
                // TODO(rsc): Should we include the SWIG version?
        }
        if p.Internal.CoverMode != "" {
                fmt.Fprintf(h, "cover %q %q\n", p.Internal.CoverMode, b.toolID("cover"))
        }
        if p.Internal.FuzzInstrument {
                if fuzzFlags := fuzzInstrumentFlags(); fuzzFlags != nil {
                        fmt.Fprintf(h, "fuzz %q\n", fuzzFlags)
                }
        }
        if p.Internal.BuildInfo != "" {
                fmt.Fprintf(h, "modinfo %q\n", p.Internal.BuildInfo)
        }

        // Configuration specific to compiler toolchain.
        switch cfg.BuildToolchainName {
        default:
                base.Fatalf("buildActionID: unknown build toolchain %q", cfg.BuildToolchainName)
        case "gc":
                fmt.Fprintf(h, "compile %s %q %q\n", b.toolID("compile"), forcedGcflags, p.Internal.Gcflags)
                if len(p.SFiles) > 0 {
                        fmt.Fprintf(h, "asm %q %q %q\n", b.toolID("asm"), forcedAsmflags, p.Internal.Asmflags)
                }

                // GOARM, GOMIPS, etc.
                key, val := cfg.GetArchEnv()
                fmt.Fprintf(h, "%s=%s\n", key, val)

                if cfg.CleanGOEXPERIMENT != "" {
                        fmt.Fprintf(h, "GOEXPERIMENT=%q\n", cfg.CleanGOEXPERIMENT)
                }

                // TODO(rsc): Convince compiler team not to add more magic environment variables,
                // or perhaps restrict the environment variables passed to subprocesses.
                // Because these are clumsy, undocumented special-case hacks
                // for debugging the compiler, they are not settable using 'go env -w',
                // and so here we use os.Getenv, not cfg.Getenv.
                magic := []string{
                        "GOCLOBBERDEADHASH",
                        "GOSSAFUNC",
                        "GOSSADIR",
                        "GOSSAHASH",
                }
                for _, env := range magic {
                        if x := os.Getenv(env); x != "" {
                                fmt.Fprintf(h, "magic %s=%s\n", env, x)
                        }
                }
                if os.Getenv("GOSSAHASH") != "" {
                        for i := 0; ; i++ {
                                env := fmt.Sprintf("GOSSAHASH%d", i)
                                x := os.Getenv(env)
                                if x == "" {
                                        break
                                }
                                fmt.Fprintf(h, "magic %s=%s\n", env, x)
                        }
                }
                if os.Getenv("GSHS_LOGFILE") != "" {
                        // Clumsy hack. Compiler writes to this log file,
                        // so do not allow use of cache at all.
                        // We will still write to the cache but it will be
                        // essentially unfindable.
                        fmt.Fprintf(h, "nocache %d\n", time.Now().UnixNano())
                }

        case "gccgo":
                id, err := b.gccToolID(BuildToolchain.compiler(), "go")
                if err != nil {
                        base.Fatalf("%v", err)
                }
                fmt.Fprintf(h, "compile %s %q %q\n", id, forcedGccgoflags, p.Internal.Gccgoflags)
                fmt.Fprintf(h, "pkgpath %s\n", gccgoPkgpath(p))
                fmt.Fprintf(h, "ar %q\n", BuildToolchain.(gccgoToolchain).ar())
                if len(p.SFiles) > 0 {
                        id, _ = b.gccToolID(BuildToolchain.compiler(), "assembler-with-cpp")
                        // Ignore error; different assembler versions
                        // are unlikely to make any difference anyhow.
                        fmt.Fprintf(h, "asm %q\n", id)
                }
        }

        // Input files.
        inputFiles := str.StringList(
                p.GoFiles,
                p.CgoFiles,
                p.CFiles,
                p.CXXFiles,
                p.FFiles,
                p.MFiles,
                p.HFiles,
                p.SFiles,
                p.SysoFiles,
                p.SwigFiles,
                p.SwigCXXFiles,
                p.EmbedFiles,
        )
        for _, file := range inputFiles {
                fmt.Fprintf(h, "file %s %s\n", file, b.fileHash(filepath.Join(p.Dir, file)))
        }
        for _, a1 := range a.Deps {
                p1 := a1.Package
                if p1 != nil {
                        fmt.Fprintf(h, "import %s %s\n", p1.ImportPath, contentID(a1.buildID))
                }
        }

        return h.Sum()
}

// needCgoHdr reports whether the actions triggered by this one
// expect to be able to access the cgo-generated header file.
func (b *Builder) needCgoHdr(a *Action) bool {
        // If this build triggers a header install, run cgo to get the header.
        if !b.IsCmdList && (a.Package.UsesCgo() || a.Package.UsesSwig()) && (cfg.BuildBuildmode == "c-archive" || cfg.BuildBuildmode == "c-shared") {
                for _, t1 := range a.triggers {
                        if t1.Mode == "install header" {
                                return true
                        }
                }
                for _, t1 := range a.triggers {
                        for _, t2 := range t1.triggers {
                                if t2.Mode == "install header" {
                                        return true
                                }
                        }
                }
        }
        return false
}

// allowedVersion reports whether the version v is an allowed version of go
// (one that we can compile).
// v is known to be of the form "1.23".
func allowedVersion(v string) bool {
        // Special case: no requirement.
        if v == "" {
                return true
        }
        // Special case "1.0" means "go1", which is OK.
        if v == "1.0" {
                return true
        }
        // Otherwise look through release tags of form "go1.23" for one that matches.
        for _, tag := range cfg.BuildContext.ReleaseTags {
                if strings.HasPrefix(tag, "go") && tag[2:] == v {
                        return true
                }
        }
        return false
}

const (
        needBuild uint32 = 1 << iota
        needCgoHdr
        needVet
        needCompiledGoFiles
        needStale
)

// build is the action for building a single package.
// Note that any new influence on this logic must be reported in b.buildActionID above as well.
func (b *Builder) build(ctx context.Context, a *Action) (err error) {
        p := a.Package

        bit := func(x uint32, b bool) uint32 {
                if b {
                        return x
                }
                return 0
        }

        cachedBuild := false
        need := bit(needBuild, !b.IsCmdList && a.needBuild || b.NeedExport) |
                bit(needCgoHdr, b.needCgoHdr(a)) |
                bit(needVet, a.needVet) |
                bit(needCompiledGoFiles, b.NeedCompiledGoFiles)

        if !p.BinaryOnly {
                if b.useCache(a, b.buildActionID(a), p.Target, need&needBuild != 0) {
                        // We found the main output in the cache.
                        // If we don't need any other outputs, we can stop.
                        // Otherwise, we need to write files to a.Objdir (needVet, needCgoHdr).
                        // Remember that we might have them in cache
                        // and check again after we create a.Objdir.
                        cachedBuild = true
                        a.output = []byte{} // start saving output in case we miss any cache results
                        need &^= needBuild
                        if b.NeedExport {
                                p.Export = a.built
                                p.BuildID = a.buildID
                        }
                        if need&needCompiledGoFiles != 0 {
                                if err := b.loadCachedSrcFiles(a); err == nil {
                                        need &^= needCompiledGoFiles
                                }
                        }
                }

                // Source files might be cached, even if the full action is not
                // (e.g., go list -compiled -find).
                if !cachedBuild && need&needCompiledGoFiles != 0 {
                        if err := b.loadCachedSrcFiles(a); err == nil {
                                need &^= needCompiledGoFiles
                        }
                }

                if need == 0 {
                        return nil
                }
                defer b.flushOutput(a)
        }

        defer func() {
                if err != nil {
                        err = fmt.Errorf("go build %s: %v", p.ImportPath, err)
                }
                if err != nil && b.IsCmdList && b.NeedError && p.Error == nil {
                        p.Error = &load.PackageError{Err: err}
                }
        }()
        if cfg.BuildN {
                // In -n mode, print a banner between packages.
                // The banner is five lines so that when changes to
                // different sections of the bootstrap script have to
                // be merged, the banners give patch something
                // to use to find its context.
                b.Print("\n#\n# " + p.ImportPath + "\n#\n\n")
        }

        if cfg.BuildV {
                b.Print(p.ImportPath + "\n")
        }

        if p.BinaryOnly {
                p.Stale = true
                p.StaleReason = "binary-only packages are no longer supported"
                if b.IsCmdList {
                        return nil
                }
                return errors.New("binary-only packages are no longer supported")
        }

        if err := b.Mkdir(a.Objdir); err != nil {
                return err
        }
        objdir := a.Objdir

        // Load cached cgo header, but only if we're skipping the main build (cachedBuild==true).
        if cachedBuild && need&needCgoHdr != 0 {
                if err := b.loadCachedCgoHdr(a); err == nil {
                        need &^= needCgoHdr
                }
        }

        // Load cached vet config, but only if that's all we have left
        // (need == needVet, not testing just the one bit).
        // If we are going to do a full build anyway,
        // we're going to regenerate the files below anyway.
        if need == needVet {
                if err := b.loadCachedVet(a); err == nil {
                        need &^= needVet
                }
        }
        if need == 0 {
                return nil
        }

        if err := AllowInstall(a); err != nil {
                return err
        }

        // make target directory
        dir, _ := filepath.Split(a.Target)
        if dir != "" {
                if err := b.Mkdir(dir); err != nil {
                        return err
                }
        }

        gofiles := str.StringList(p.GoFiles)
        cgofiles := str.StringList(p.CgoFiles)
        cfiles := str.StringList(p.CFiles)
        sfiles := str.StringList(p.SFiles)
        cxxfiles := str.StringList(p.CXXFiles)
        var objects, cgoObjects, pcCFLAGS, pcLDFLAGS []string

        if p.UsesCgo() || p.UsesSwig() {
                if pcCFLAGS, pcLDFLAGS, err = b.getPkgConfigFlags(p); err != nil {
                        return
                }
        }

        // Compute overlays for .c/.cc/.h/etc. and if there are any overlays
        // put correct contents of all those files in the objdir, to ensure
        // the correct headers are included. nonGoOverlay is the overlay that
        // points from nongo files to the copied files in objdir.
        nonGoFileLists := [][]string{p.CFiles, p.SFiles, p.CXXFiles, p.HFiles, p.FFiles}
OverlayLoop:
        for _, fs := range nonGoFileLists {
                for _, f := range fs {
                        if _, ok := fsys.OverlayPath(mkAbs(p.Dir, f)); ok {
                                a.nonGoOverlay = make(map[string]string)
                                break OverlayLoop
                        }
                }
        }
        if a.nonGoOverlay != nil {
                for _, fs := range nonGoFileLists {
                        for i := range fs {
                                from := mkAbs(p.Dir, fs[i])
                                opath, _ := fsys.OverlayPath(from)
                                dst := objdir + filepath.Base(fs[i])
                                if err := b.copyFile(dst, opath, 0666, false); err != nil {
                                        return err
                                }
                                a.nonGoOverlay[from] = dst
                        }
                }
        }

        // Run SWIG on each .swig and .swigcxx file.
        // Each run will generate two files, a .go file and a .c or .cxx file.
        // The .go file will use import "C" and is to be processed by cgo.
        if p.UsesSwig() {
                outGo, outC, outCXX, err := b.swig(a, p, objdir, pcCFLAGS)
                if err != nil {
                        return err
                }
                cgofiles = append(cgofiles, outGo...)
                cfiles = append(cfiles, outC...)
                cxxfiles = append(cxxfiles, outCXX...)
        }

        // If we're doing coverage, preprocess the .go files and put them in the work directory
        if p.Internal.CoverMode != "" {
                outfiles := []string{}
                infiles := []string{}
                for i, file := range str.StringList(gofiles, cgofiles) {
                        if base.IsTestFile(file) {
                                continue // Not covering this file.
                        }

                        var sourceFile string
                        var coverFile string
                        var key string
                        if base, found := strings.CutSuffix(file, ".cgo1.go"); found {
                                // cgo files have absolute paths
                                base = filepath.Base(base)
                                sourceFile = file
                                coverFile = objdir + base + ".cgo1.go"
                                key = base + ".go"
                        } else {
                                sourceFile = filepath.Join(p.Dir, file)
                                coverFile = objdir + file
                                key = file
                        }
                        coverFile = strings.TrimSuffix(coverFile, ".go") + ".cover.go"
                        if cfg.Experiment.CoverageRedesign {
                                infiles = append(infiles, sourceFile)
                                outfiles = append(outfiles, coverFile)
                        } else {
                                cover := p.Internal.CoverVars[key]
                                if cover == nil {
                                        continue // Not covering this file.
                                }
                                if err := b.cover(a, coverFile, sourceFile, cover.Var); err != nil {
                                        return err
                                }
                        }
                        if i < len(gofiles) {
                                gofiles[i] = coverFile
                        } else {
                                cgofiles[i-len(gofiles)] = coverFile
                        }
                }

                if cfg.Experiment.CoverageRedesign {
                        if len(infiles) != 0 {
                                // Coverage instrumentation creates new top level
                                // variables in the target package for things like
                                // meta-data containers, counter vars, etc. To avoid
                                // collisions with user variables, suffix the var name
                                // with 12 hex digits from the SHA-256 hash of the
                                // import path. Choice of 12 digits is historical/arbitrary,
                                // we just need enough of the hash to avoid accidents,
                                // as opposed to precluding determined attempts by
                                // users to break things.
                                sum := sha256.Sum256([]byte(a.Package.ImportPath))
                                coverVar := fmt.Sprintf("goCover_%x_", sum[:6])
                                mode := a.Package.Internal.CoverMode
                                if mode == "" {
                                        panic("covermode should be set at this point")
                                }
                                pkgcfg := a.Objdir + "pkgcfg.txt"
                                covoutfiles := a.Objdir + "coveroutfiles.txt"
                                if err := b.cover2(a, pkgcfg, covoutfiles, infiles, outfiles, coverVar, mode); err != nil {
                                        return err
                                }
                        } else {
                                // If there are no input files passed to cmd/cover,
                                // then we don't want to pass -covercfg when building
                                // the package with the compiler, so set covermode to
                                // the empty string so as to signal that we need to do
                                // that.
                                p.Internal.CoverMode = ""
                        }
                }
        }

        // Run cgo.
        if p.UsesCgo() || p.UsesSwig() {
                // In a package using cgo, cgo compiles the C, C++ and assembly files with gcc.
                // There is one exception: runtime/cgo's job is to bridge the
                // cgo and non-cgo worlds, so it necessarily has files in both.
                // In that case gcc only gets the gcc_* files.
                var gccfiles []string
                gccfiles = append(gccfiles, cfiles...)
                cfiles = nil
                if p.Standard && p.ImportPath == "runtime/cgo" {
                        filter := func(files, nongcc, gcc []string) ([]string, []string) {
                                for _, f := range files {
                                        if strings.HasPrefix(f, "gcc_") {
                                                gcc = append(gcc, f)
                                        } else {
                                                nongcc = append(nongcc, f)
                                        }
                                }
                                return nongcc, gcc
                        }
                        sfiles, gccfiles = filter(sfiles, sfiles[:0], gccfiles)
                } else {
                        for _, sfile := range sfiles {
                                data, err := os.ReadFile(filepath.Join(p.Dir, sfile))
                                if err == nil {
                                        if bytes.HasPrefix(data, []byte("TEXT")) || bytes.Contains(data, []byte("\nTEXT")) ||
                                                bytes.HasPrefix(data, []byte("DATA")) || bytes.Contains(data, []byte("\nDATA")) ||
                                                bytes.HasPrefix(data, []byte("GLOBL")) || bytes.Contains(data, []byte("\nGLOBL")) {
                                                return fmt.Errorf("package using cgo has Go assembly file %s", sfile)
                                        }
                                }
                        }
                        gccfiles = append(gccfiles, sfiles...)
                        sfiles = nil
                }

                outGo, outObj, err := b.cgo(a, base.Tool("cgo"), objdir, pcCFLAGS, pcLDFLAGS, mkAbsFiles(p.Dir, cgofiles), gccfiles, cxxfiles, p.MFiles, p.FFiles)

                // The files in cxxfiles have now been handled by b.cgo.
                cxxfiles = nil

                if err != nil {
                        return err
                }
                if cfg.BuildToolchainName == "gccgo" {
                        cgoObjects = append(cgoObjects, a.Objdir+"_cgo_flags")
                }
                cgoObjects = append(cgoObjects, outObj...)
                gofiles = append(gofiles, outGo...)

                switch cfg.BuildBuildmode {
                case "c-archive", "c-shared":
                        b.cacheCgoHdr(a)
                }
        }

        var srcfiles []string // .go and non-.go
        srcfiles = append(srcfiles, gofiles...)
        srcfiles = append(srcfiles, sfiles...)
        srcfiles = append(srcfiles, cfiles...)
        srcfiles = append(srcfiles, cxxfiles...)
        b.cacheSrcFiles(a, srcfiles)

        // Running cgo generated the cgo header.
        need &^= needCgoHdr

        // Sanity check only, since Package.load already checked as well.
        if len(gofiles) == 0 {
                return &load.NoGoError{Package: p}
        }

        // Prepare Go vet config if needed.
        if need&needVet != 0 {
                buildVetConfig(a, srcfiles)
                need &^= needVet
        }
        if need&needCompiledGoFiles != 0 {
                if err := b.loadCachedSrcFiles(a); err != nil {
                        return fmt.Errorf("loading compiled Go files from cache: %w", err)
                }
                need &^= needCompiledGoFiles
        }
        if need == 0 {
                // Nothing left to do.
                return nil
        }

        // Collect symbol ABI requirements from assembly.
        symabis, err := BuildToolchain.symabis(b, a, sfiles)
        if err != nil {
                return err
        }

        // Prepare Go import config.
        // We start it off with a comment so it can't be empty, so icfg.Bytes() below is never nil.
        // It should never be empty anyway, but there have been bugs in the past that resulted
        // in empty configs, which then unfortunately turn into "no config passed to compiler",
        // and the compiler falls back to looking in pkg itself, which mostly works,
        // except when it doesn't.
        var icfg bytes.Buffer
        fmt.Fprintf(&icfg, "# import config\n")
        for i, raw := range p.Internal.RawImports {
                final := p.Imports[i]
                if final != raw {
                        fmt.Fprintf(&icfg, "importmap %s=%s\n", raw, final)
                }
        }
        for _, a1 := range a.Deps {
                p1 := a1.Package
                if p1 == nil || p1.ImportPath == "" || a1.built == "" {
                        continue
                }
                fmt.Fprintf(&icfg, "packagefile %s=%s\n", p1.ImportPath, a1.built)
        }

        // Prepare Go embed config if needed.
        // Unlike the import config, it's okay for the embed config to be empty.
        var embedcfg []byte
        if len(p.Internal.Embed) > 0 {
                var embed struct {
                        Patterns map[string][]string
                        Files    map[string]string
                }
                embed.Patterns = p.Internal.Embed
                embed.Files = make(map[string]string)
                for _, file := range p.EmbedFiles {
                        embed.Files[file] = filepath.Join(p.Dir, file)
                }
                js, err := json.MarshalIndent(&embed, "", "\t")
                if err != nil {
                        return fmt.Errorf("marshal embedcfg: %v", err)
                }
                embedcfg = js
        }

        if p.Internal.BuildInfo != "" && cfg.ModulesEnabled {
                prog := modload.ModInfoProg(p.Internal.BuildInfo, cfg.BuildToolchainName == "gccgo")
                if len(prog) > 0 {
                        if err := b.writeFile(objdir+"_gomod_.go", prog); err != nil {
                                return err
                        }
                        gofiles = append(gofiles, objdir+"_gomod_.go")
                }
        }

        // Compile Go.
        objpkg := objdir + "_pkg_.a"
        ofile, out, err := BuildToolchain.gc(b, a, objpkg, icfg.Bytes(), embedcfg, symabis, len(sfiles) > 0, gofiles)
        if len(out) > 0 {
                output := b.processOutput(out)
                if p.Module != nil && !allowedVersion(p.Module.GoVersion) {
                        output += "note: module requires Go " + p.Module.GoVersion + "\n"
                }

                if err != nil {
                        return errors.New(fmt.Sprint(formatOutput(b.WorkDir, p.Dir, p.Desc(), output)))
                } else {
                        b.showOutput(a, p.Dir, p.Desc(), output)
                }
        }
        if err != nil {
                if p.Module != nil && !allowedVersion(p.Module.GoVersion) {
                        b.showOutput(a, p.Dir, p.Desc(), "note: module requires Go "+p.Module.GoVersion+"\n")
                }
                return err
        }
        if ofile != objpkg {
                objects = append(objects, ofile)
        }

        // Copy .h files named for goos or goarch or goos_goarch
        // to names using GOOS and GOARCH.
        // For example, defs_linux_amd64.h becomes defs_GOOS_GOARCH.h.
        _goos_goarch := "_" + cfg.Goos + "_" + cfg.Goarch
        _goos := "_" + cfg.Goos
        _goarch := "_" + cfg.Goarch
        for _, file := range p.HFiles {
                name, ext := fileExtSplit(file)
                switch {
                case strings.HasSuffix(name, _goos_goarch):
                        targ := file[:len(name)-len(_goos_goarch)] + "_GOOS_GOARCH." + ext
                        if err := b.copyFile(objdir+targ, filepath.Join(p.Dir, file), 0666, true); err != nil {
                                return err
                        }
                case strings.HasSuffix(name, _goarch):
                        targ := file[:len(name)-len(_goarch)] + "_GOARCH." + ext
                        if err := b.copyFile(objdir+targ, filepath.Join(p.Dir, file), 0666, true); err != nil {
                                return err
                        }
                case strings.HasSuffix(name, _goos):
                        targ := file[:len(name)-len(_goos)] + "_GOOS." + ext
                        if err := b.copyFile(objdir+targ, filepath.Join(p.Dir, file), 0666, true); err != nil {
                                return err
                        }
                }
        }

        for _, file := range cfiles {
                out := file[:len(file)-len(".c")] + ".o"
                if err := BuildToolchain.cc(b, a, objdir+out, file); err != nil {
                        return err
                }
                objects = append(objects, out)
        }

        // Assemble .s files.
        if len(sfiles) > 0 {
                ofiles, err := BuildToolchain.asm(b, a, sfiles)
                if err != nil {
                        return err
                }
                objects = append(objects, ofiles...)
        }

        // For gccgo on ELF systems, we write the build ID as an assembler file.
        // This lets us set the SHF_EXCLUDE flag.
        // This is read by readGccgoArchive in cmd/internal/buildid/buildid.go.
        if a.buildID != "" && cfg.BuildToolchainName == "gccgo" {
                switch cfg.Goos {
                case "aix", "android", "dragonfly", "freebsd", "illumos", "linux", "netbsd", "openbsd", "solaris":
                        asmfile, err := b.gccgoBuildIDFile(a)
                        if err != nil {
                                return err
                        }
                        ofiles, err := BuildToolchain.asm(b, a, []string{asmfile})
                        if err != nil {
                                return err
                        }
                        objects = append(objects, ofiles...)
                }
        }

        // NOTE(rsc): On Windows, it is critically important that the
        // gcc-compiled objects (cgoObjects) be listed after the ordinary
        // objects in the archive. I do not know why this is.
        // https://golang.org/issue/2601
        objects = append(objects, cgoObjects...)

        // Add system object files.
        for _, syso := range p.SysoFiles {
                objects = append(objects, filepath.Join(p.Dir, syso))
        }

        // Pack into archive in objdir directory.
        // If the Go compiler wrote an archive, we only need to add the
        // object files for non-Go sources to the archive.
        // If the Go compiler wrote an archive and the package is entirely
        // Go sources, there is no pack to execute at all.
        if len(objects) > 0 {
                if err := BuildToolchain.pack(b, a, objpkg, objects); err != nil {
                        return err
                }
        }

        if err := b.updateBuildID(a, objpkg, true); err != nil {
                return err
        }

        a.built = objpkg
        return nil
}

func (b *Builder) cacheObjdirFile(a *Action, c *cache.Cache, name string) error {
        f, err := os.Open(a.Objdir + name)
        if err != nil {
                return err
        }
        defer f.Close()
        _, _, err = c.Put(cache.Subkey(a.actionID, name), f)
        return err
}

func (b *Builder) findCachedObjdirFile(a *Action, c *cache.Cache, name string) (string, error) {
        file, _, err := c.GetFile(cache.Subkey(a.actionID, name))
        if err != nil {
                return "", fmt.Errorf("loading cached file %s: %w", name, err)
        }
        return file, nil
}

func (b *Builder) loadCachedObjdirFile(a *Action, c *cache.Cache, name string) error {
        cached, err := b.findCachedObjdirFile(a, c, name)
        if err != nil {
                return err
        }
        return b.copyFile(a.Objdir+name, cached, 0666, true)
}

func (b *Builder) cacheCgoHdr(a *Action) {
        c := cache.Default()
        b.cacheObjdirFile(a, c, "_cgo_install.h")
}

func (b *Builder) loadCachedCgoHdr(a *Action) error {
        c := cache.Default()
        return b.loadCachedObjdirFile(a, c, "_cgo_install.h")
}

func (b *Builder) cacheSrcFiles(a *Action, srcfiles []string) {
        c := cache.Default()
        var buf bytes.Buffer
        for _, file := range srcfiles {
                if !strings.HasPrefix(file, a.Objdir) {
                        // not generated
                        buf.WriteString("./")
                        buf.WriteString(file)
                        buf.WriteString("\n")
                        continue
                }
                name := file[len(a.Objdir):]
                buf.WriteString(name)
                buf.WriteString("\n")
                if err := b.cacheObjdirFile(a, c, name); err != nil {
                        return
                }
        }
        c.PutBytes(cache.Subkey(a.actionID, "srcfiles"), buf.Bytes())
}

func (b *Builder) loadCachedVet(a *Action) error {
        c := cache.Default()
        list, _, err := c.GetBytes(cache.Subkey(a.actionID, "srcfiles"))
        if err != nil {
                return fmt.Errorf("reading srcfiles list: %w", err)
        }
        var srcfiles []string
        for _, name := range strings.Split(string(list), "\n") {
                if name == "" { // end of list
                        continue
                }
                if strings.HasPrefix(name, "./") {
                        srcfiles = append(srcfiles, name[2:])
                        continue
                }
                if err := b.loadCachedObjdirFile(a, c, name); err != nil {
                        return err
                }
                srcfiles = append(srcfiles, a.Objdir+name)
        }
        buildVetConfig(a, srcfiles)
        return nil
}

func (b *Builder) loadCachedSrcFiles(a *Action) error {
        c := cache.Default()
        list, _, err := c.GetBytes(cache.Subkey(a.actionID, "srcfiles"))
        if err != nil {
                return fmt.Errorf("reading srcfiles list: %w", err)
        }
        var files []string
        for _, name := range strings.Split(string(list), "\n") {
                if name == "" { // end of list
                        continue
                }
                if strings.HasPrefix(name, "./") {
                        files = append(files, name[len("./"):])
                        continue
                }
                file, err := b.findCachedObjdirFile(a, c, name)
                if err != nil {
                        return fmt.Errorf("finding %s: %w", name, err)
                }
                files = append(files, file)
        }
        a.Package.CompiledGoFiles = files
        return nil
}

// vetConfig is the configuration passed to vet describing a single package.
type vetConfig struct {
        ID           string   // package ID (example: "fmt [fmt.test]")
        Compiler     string   // compiler name (gc, gccgo)
        Dir          string   // directory containing package
        ImportPath   string   // canonical import path ("package path")
        GoFiles      []string // absolute paths to package source files
        NonGoFiles   []string // absolute paths to package non-Go files
        IgnoredFiles []string // absolute paths to ignored source files

        ImportMap   map[string]string // map import path in source code to package path
        PackageFile map[string]string // map package path to .a file with export data
        Standard    map[string]bool   // map package path to whether it's in the standard library
        PackageVetx map[string]string // map package path to vetx data from earlier vet run
        VetxOnly    bool              // only compute vetx data; don't report detected problems
        VetxOutput  string            // write vetx data to this output file

        SucceedOnTypecheckFailure bool // awful hack; see #18395 and below
}

func buildVetConfig(a *Action, srcfiles []string) {
        // Classify files based on .go extension.
        // srcfiles does not include raw cgo files.
        var gofiles, nongofiles []string
        for _, name := range srcfiles {
                if strings.HasSuffix(name, ".go") {
                        gofiles = append(gofiles, name)
                } else {
                        nongofiles = append(nongofiles, name)
                }
        }

        ignored := str.StringList(a.Package.IgnoredGoFiles, a.Package.IgnoredOtherFiles)

        // Pass list of absolute paths to vet,
        // so that vet's error messages will use absolute paths,
        // so that we can reformat them relative to the directory
        // in which the go command is invoked.
        vcfg := &vetConfig{
                ID:           a.Package.ImportPath,
                Compiler:     cfg.BuildToolchainName,
                Dir:          a.Package.Dir,
                GoFiles:      mkAbsFiles(a.Package.Dir, gofiles),
                NonGoFiles:   mkAbsFiles(a.Package.Dir, nongofiles),
                IgnoredFiles: mkAbsFiles(a.Package.Dir, ignored),
                ImportPath:   a.Package.ImportPath,
                ImportMap:    make(map[string]string),
                PackageFile:  make(map[string]string),
                Standard:     make(map[string]bool),
        }
        a.vetCfg = vcfg
        for i, raw := range a.Package.Internal.RawImports {
                final := a.Package.Imports[i]
                vcfg.ImportMap[raw] = final
        }

        // Compute the list of mapped imports in the vet config
        // so that we can add any missing mappings below.
        vcfgMapped := make(map[string]bool)
        for _, p := range vcfg.ImportMap {
                vcfgMapped[p] = true
        }

        for _, a1 := range a.Deps {
                p1 := a1.Package
                if p1 == nil || p1.ImportPath == "" {
                        continue
                }
                // Add import mapping if needed
                // (for imports like "runtime/cgo" that appear only in generated code).
                if !vcfgMapped[p1.ImportPath] {
                        vcfg.ImportMap[p1.ImportPath] = p1.ImportPath
                }
                if a1.built != "" {
                        vcfg.PackageFile[p1.ImportPath] = a1.built
                }
                if p1.Standard {
                        vcfg.Standard[p1.ImportPath] = true
                }
        }
}

// VetTool is the path to an alternate vet tool binary.
// The caller is expected to set it (if needed) before executing any vet actions.
var VetTool string

// VetFlags are the default flags to pass to vet.
// The caller is expected to set them before executing any vet actions.
var VetFlags []string

// VetExplicit records whether the vet flags were set explicitly on the command line.
var VetExplicit bool

func (b *Builder) vet(ctx context.Context, a *Action) error {
        // a.Deps[0] is the build of the package being vetted.
        // a.Deps[1] is the build of the "fmt" package.

        a.Failed = false // vet of dependency may have failed but we can still succeed

        if a.Deps[0].Failed {
                // The build of the package has failed. Skip vet check.
                // Vet could return export data for non-typecheck errors,
                // but we ignore it because the package cannot be compiled.
                return nil
        }

        vcfg := a.Deps[0].vetCfg
        if vcfg == nil {
                // Vet config should only be missing if the build failed.
                return fmt.Errorf("vet config not found")
        }

        vcfg.VetxOnly = a.VetxOnly
        vcfg.VetxOutput = a.Objdir + "vet.out"
        vcfg.PackageVetx = make(map[string]string)

        h := cache.NewHash("vet " + a.Package.ImportPath)
        fmt.Fprintf(h, "vet %q\n", b.toolID("vet"))

        vetFlags := VetFlags

        // In GOROOT, we enable all the vet tests during 'go test',
        // not just the high-confidence subset. This gets us extra
        // checking for the standard library (at some compliance cost)
        // and helps us gain experience about how well the checks
        // work, to help decide which should be turned on by default.
        // The command-line still wins.
        //
        // Note that this flag change applies even when running vet as
        // a dependency of vetting a package outside std.
        // (Otherwise we'd have to introduce a whole separate
        // space of "vet fmt as a dependency of a std top-level vet"
        // versus "vet fmt as a dependency of a non-std top-level vet".)
        // This is OK as long as the packages that are farther down the
        // dependency tree turn on *more* analysis, as here.
        // (The unsafeptr check does not write any facts for use by
        // later vet runs, nor does unreachable.)
        if a.Package.Goroot && !VetExplicit && VetTool == "" {
                // Turn off -unsafeptr checks.
                // There's too much unsafe.Pointer code
                // that vet doesn't like in low-level packages
                // like runtime, sync, and reflect.
                // Note that $GOROOT/src/buildall.bash
                // does the same for the misc-compile trybots
                // and should be updated if these flags are
                // changed here.
                vetFlags = []string{"-unsafeptr=false"}

                // Also turn off -unreachable checks during go test.
                // During testing it is very common to make changes
                // like hard-coded forced returns or panics that make
                // code unreachable. It's unreasonable to insist on files
                // not having any unreachable code during "go test".
                // (buildall.bash still runs with -unreachable enabled
                // for the overall whole-tree scan.)
                if cfg.CmdName == "test" {
                        vetFlags = append(vetFlags, "-unreachable=false")
                }
        }

        // Note: We could decide that vet should compute export data for
        // all analyses, in which case we don't need to include the flags here.
        // But that would mean that if an analysis causes problems like
        // unexpected crashes there would be no way to turn it off.
        // It seems better to let the flags disable export analysis too.
        fmt.Fprintf(h, "vetflags %q\n", vetFlags)

        fmt.Fprintf(h, "pkg %q\n", a.Deps[0].actionID)
        for _, a1 := range a.Deps {
                if a1.Mode == "vet" && a1.built != "" {
                        fmt.Fprintf(h, "vetout %q %s\n", a1.Package.ImportPath, b.fileHash(a1.built))
                        vcfg.PackageVetx[a1.Package.ImportPath] = a1.built
                }
        }
        key := cache.ActionID(h.Sum())

        if vcfg.VetxOnly && !cfg.BuildA {
                c := cache.Default()
                if file, _, err := c.GetFile(key); err == nil {
                        a.built = file
                        return nil
                }
        }

        js, err := json.MarshalIndent(vcfg, "", "\t")
        if err != nil {
                return fmt.Errorf("internal error marshaling vet config: %v", err)
        }
        js = append(js, '\n')
        if err := b.writeFile(a.Objdir+"vet.cfg", js); err != nil {
                return err
        }

        // TODO(rsc): Why do we pass $GCCGO to go vet?
        env := b.cCompilerEnv()
        if cfg.BuildToolchainName == "gccgo" {
                env = append(env, "GCCGO="+BuildToolchain.compiler())
        }

        p := a.Package
        tool := VetTool
        if tool == "" {
                tool = base.Tool("vet")
        }
        runErr := b.run(a, p.Dir, p.ImportPath, env, cfg.BuildToolexec, tool, vetFlags, a.Objdir+"vet.cfg")

        // If vet wrote export data, save it for input to future vets.
        if f, err := os.Open(vcfg.VetxOutput); err == nil {
                a.built = vcfg.VetxOutput
                cache.Default().Put(key, f)
                f.Close()
        }

        return runErr
}

// linkActionID computes the action ID for a link action.
func (b *Builder) linkActionID(a *Action) cache.ActionID {
        p := a.Package
        h := cache.NewHash("link " + p.ImportPath)

        // Toolchain-independent configuration.
        fmt.Fprintf(h, "link\n")
        fmt.Fprintf(h, "buildmode %s goos %s goarch %s\n", cfg.BuildBuildmode, cfg.Goos, cfg.Goarch)
        fmt.Fprintf(h, "import %q\n", p.ImportPath)
        fmt.Fprintf(h, "omitdebug %v standard %v local %v prefix %q\n", p.Internal.OmitDebug, p.Standard, p.Internal.Local, p.Internal.LocalPrefix)
        if cfg.BuildTrimpath {
                fmt.Fprintln(h, "trimpath")
        }

        // Toolchain-dependent configuration, shared with b.linkSharedActionID.
        b.printLinkerConfig(h, p)

        // Input files.
        for _, a1 := range a.Deps {
                p1 := a1.Package
                if p1 != nil {
                        if a1.built != "" || a1.buildID != "" {
                                buildID := a1.buildID
                                if buildID == "" {
                                        buildID = b.buildID(a1.built)
                                }
                                fmt.Fprintf(h, "packagefile %s=%s\n", p1.ImportPath, contentID(buildID))
                        }
                        // Because we put package main's full action ID into the binary's build ID,
                        // we must also put the full action ID into the binary's action ID hash.
                        if p1.Name == "main" {
                                fmt.Fprintf(h, "packagemain %s\n", a1.buildID)
                        }
                        if p1.Shlib != "" {
                                fmt.Fprintf(h, "packageshlib %s=%s\n", p1.ImportPath, contentID(b.buildID(p1.Shlib)))
                        }
                }
        }

        return h.Sum()
}

// printLinkerConfig prints the linker config into the hash h,
// as part of the computation of a linker-related action ID.
func (b *Builder) printLinkerConfig(h io.Writer, p *load.Package) {
        switch cfg.BuildToolchainName {
        default:
                base.Fatalf("linkActionID: unknown toolchain %q", cfg.BuildToolchainName)

        case "gc":
                fmt.Fprintf(h, "link %s %q %s\n", b.toolID("link"), forcedLdflags, ldBuildmode)
                if p != nil {
                        fmt.Fprintf(h, "linkflags %q\n", p.Internal.Ldflags)
                }

                // GOARM, GOMIPS, etc.
                key, val := cfg.GetArchEnv()
                fmt.Fprintf(h, "%s=%s\n", key, val)

                if cfg.CleanGOEXPERIMENT != "" {
                        fmt.Fprintf(h, "GOEXPERIMENT=%q\n", cfg.CleanGOEXPERIMENT)
                }

                // The linker writes source file paths that say GOROOT_FINAL, but
                // only if -trimpath is not specified (see ld() in gc.go).
                gorootFinal := cfg.GOROOT_FINAL
                if cfg.BuildTrimpath {
                        gorootFinal = trimPathGoRootFinal
                }
                fmt.Fprintf(h, "GOROOT=%s\n", gorootFinal)

                // GO_EXTLINK_ENABLED controls whether the external linker is used.
                fmt.Fprintf(h, "GO_EXTLINK_ENABLED=%s\n", cfg.Getenv("GO_EXTLINK_ENABLED"))

                // TODO(rsc): Do cgo settings and flags need to be included?
                // Or external linker settings and flags?

        case "gccgo":
                id, err := b.gccToolID(BuildToolchain.linker(), "go")
                if err != nil {
                        base.Fatalf("%v", err)
                }
                fmt.Fprintf(h, "link %s %s\n", id, ldBuildmode)
                // TODO(iant): Should probably include cgo flags here.
        }
}

// link is the action for linking a single command.
// Note that any new influence on this logic must be reported in b.linkActionID above as well.
func (b *Builder) link(ctx context.Context, a *Action) (err error) {
        if b.useCache(a, b.linkActionID(a), a.Package.Target, !b.IsCmdList) || b.IsCmdList {
                return nil
        }
        defer b.flushOutput(a)

        if err := b.Mkdir(a.Objdir); err != nil {
                return err
        }

        importcfg := a.Objdir + "importcfg.link"
        if err := b.writeLinkImportcfg(a, importcfg); err != nil {
                return err
        }

        if err := AllowInstall(a); err != nil {
                return err
        }

        // make target directory
        dir, _ := filepath.Split(a.Target)
        if dir != "" {
                if err := b.Mkdir(dir); err != nil {
                        return err
                }
        }

        if err := BuildToolchain.ld(b, a, a.Target, importcfg, a.Deps[0].built); err != nil {
                return err
        }

        // Update the binary with the final build ID.
        // But if OmitDebug is set, don't rewrite the binary, because we set OmitDebug
        // on binaries that we are going to run and then delete.
        // There's no point in doing work on such a binary.
        // Worse, opening the binary for write here makes it
        // essentially impossible to safely fork+exec due to a fundamental
        // incompatibility between ETXTBSY and threads on modern Unix systems.
        // See golang.org/issue/22220.
        // We still call updateBuildID to update a.buildID, which is important
        // for test result caching, but passing rewrite=false (final arg)
        // means we don't actually rewrite the binary, nor store the
        // result into the cache. That's probably a net win:
        // less cache space wasted on large binaries we are not likely to
        // need again. (On the other hand it does make repeated go test slower.)
        // It also makes repeated go run slower, which is a win in itself:
        // we don't want people to treat go run like a scripting environment.
        if err := b.updateBuildID(a, a.Target, !a.Package.Internal.OmitDebug); err != nil {
                return err
        }

        a.built = a.Target
        return nil
}

func (b *Builder) writeLinkImportcfg(a *Action, file string) error {
        // Prepare Go import cfg.
        var icfg bytes.Buffer
        for _, a1 := range a.Deps {
                p1 := a1.Package
                if p1 == nil {
                        continue
                }
                fmt.Fprintf(&icfg, "packagefile %s=%s\n", p1.ImportPath, a1.built)
                if p1.Shlib != "" {
                        fmt.Fprintf(&icfg, "packageshlib %s=%s\n", p1.ImportPath, p1.Shlib)
                }
        }
        fmt.Fprintf(&icfg, "modinfo %q\n", modload.ModInfoData(a.Package.Internal.BuildInfo))
        return b.writeFile(file, icfg.Bytes())
}

// PkgconfigCmd returns a pkg-config binary name
// defaultPkgConfig is defined in zdefaultcc.go, written by cmd/dist.
func (b *Builder) PkgconfigCmd() string {
        return envList("PKG_CONFIG", cfg.DefaultPkgConfig)[0]
}

// splitPkgConfigOutput parses the pkg-config output into a slice of
// flags. This implements the algorithm from pkgconf/libpkgconf/argvsplit.c.
func splitPkgConfigOutput(out []byte) ([]string, error) {
        if len(out) == 0 {
                return nil, nil
        }
        var flags []string
        flag := make([]byte, 0, len(out))
        escaped := false
        quote := byte(0)

        for _, c := range out {
                if escaped {
                        if quote != 0 {
                                switch c {
                                case '$', '`', '"', '\\':
                                default:
                                        flag = append(flag, '\\')
                                }
                                flag = append(flag, c)
                        } else {
                                flag = append(flag, c)
                        }
                        escaped = false
                } else if quote != 0 {
                        if c == quote {
                                quote = 0
                        } else {
                                switch c {
                                case '\\':
                                        escaped = true
                                default:
                                        flag = append(flag, c)
                                }
                        }
                } else if strings.IndexByte(" \t\n\v\f\r", c) < 0 {
                        switch c {
                        case '\\':
                                escaped = true
                        case '\'', '"':
                                quote = c
                        default:
                                flag = append(flag, c)
                        }
                } else if len(flag) != 0 {
                        flags = append(flags, string(flag))
                        flag = flag[:0]
                }
        }
        if escaped {
                return nil, errors.New("broken character escaping in pkgconf output ")
        }
        if quote != 0 {
                return nil, errors.New("unterminated quoted string in pkgconf output ")
        } else if len(flag) != 0 {
                flags = append(flags, string(flag))
        }

        return flags, nil
}

// Calls pkg-config if needed and returns the cflags/ldflags needed to build the package.
func (b *Builder) getPkgConfigFlags(p *load.Package) (cflags, ldflags []string, err error) {
        if pcargs := p.CgoPkgConfig; len(pcargs) > 0 {
                // pkg-config permits arguments to appear anywhere in
                // the command line. Move them all to the front, before --.
                var pcflags []string
                var pkgs []string
                for _, pcarg := range pcargs {
                        if pcarg == "--" {
                                // We're going to add our own "--" argument.
                        } else if strings.HasPrefix(pcarg, "--") {
                                pcflags = append(pcflags, pcarg)
                        } else {
                                pkgs = append(pkgs, pcarg)
                        }
                }
                for _, pkg := range pkgs {
                        if !load.SafeArg(pkg) {
                                return nil, nil, fmt.Errorf("invalid pkg-config package name: %s", pkg)
                        }
                }
                var out []byte
                out, err = b.runOut(nil, p.Dir, nil, b.PkgconfigCmd(), "--cflags", pcflags, "--", pkgs)
                if err != nil {
                        prefix, suffix := formatOutput(b.WorkDir, p.Dir, b.PkgconfigCmd()+" --cflags "+strings.Join(pcflags, " ")+" -- "+strings.Join(pkgs, " "), string(out))
                        return nil, nil, errors.New(fmt.Sprint(prefix, suffix+err.Error()))
                }
                if len(out) > 0 {
                        cflags, err = splitPkgConfigOutput(out)
                        if err != nil {
                                return nil, nil, err
                        }
                        if err := checkCompilerFlags("CFLAGS", "pkg-config --cflags", cflags); err != nil {
                                return nil, nil, err
                        }
                }
                out, err = b.runOut(nil, p.Dir, nil, b.PkgconfigCmd(), "--libs", pcflags, "--", pkgs)
                if err != nil {
                        prefix, suffix := formatOutput(b.WorkDir, p.Dir, b.PkgconfigCmd()+" --libs "+strings.Join(pcflags, " ")+" -- "+strings.Join(pkgs, " "), string(out))
                        return nil, nil, errors.New(fmt.Sprint(prefix, suffix+err.Error()))
                }
                if len(out) > 0 {
                        // NOTE: we don't attempt to parse quotes and unescapes here. pkg-config
                        // is typically used within shell backticks, which treats quotes literally.
                        ldflags = strings.Fields(string(out))
                        if err := checkLinkerFlags("LDFLAGS", "pkg-config --libs", ldflags); err != nil {
                                return nil, nil, err
                        }
                }
        }

        return
}

func (b *Builder) installShlibname(ctx context.Context, a *Action) error {
        if err := AllowInstall(a); err != nil {
                return err
        }

        // TODO: BuildN
        a1 := a.Deps[0]
        err := os.WriteFile(a.Target, []byte(filepath.Base(a1.Target)+"\n"), 0666)
        if err != nil {
                return err
        }
        if cfg.BuildX {
                b.Showcmd("", "echo '%s' > %s # internal", filepath.Base(a1.Target), a.Target)
        }
        return nil
}

func (b *Builder) linkSharedActionID(a *Action) cache.ActionID {
        h := cache.NewHash("linkShared")

        // Toolchain-independent configuration.
        fmt.Fprintf(h, "linkShared\n")
        fmt.Fprintf(h, "goos %s goarch %s\n", cfg.Goos, cfg.Goarch)

        // Toolchain-dependent configuration, shared with b.linkActionID.
        b.printLinkerConfig(h, nil)

        // Input files.
        for _, a1 := range a.Deps {
                p1 := a1.Package
                if a1.built == "" {
                        continue
                }
                if p1 != nil {
                        fmt.Fprintf(h, "packagefile %s=%s\n", p1.ImportPath, contentID(b.buildID(a1.built)))
                        if p1.Shlib != "" {
                                fmt.Fprintf(h, "packageshlib %s=%s\n", p1.ImportPath, contentID(b.buildID(p1.Shlib)))
                        }
                }
        }
        // Files named on command line are special.
        for _, a1 := range a.Deps[0].Deps {
                p1 := a1.Package
                fmt.Fprintf(h, "top %s=%s\n", p1.ImportPath, contentID(b.buildID(a1.built)))
        }

        return h.Sum()
}

func (b *Builder) linkShared(ctx context.Context, a *Action) (err error) {
        if b.useCache(a, b.linkSharedActionID(a), a.Target, !b.IsCmdList) || b.IsCmdList {
                return nil
        }
        defer b.flushOutput(a)

        if err := AllowInstall(a); err != nil {
                return err
        }

        if err := b.Mkdir(a.Objdir); err != nil {
                return err
        }

        importcfg := a.Objdir + "importcfg.link"
        if err := b.writeLinkImportcfg(a, importcfg); err != nil {
                return err
        }

        // TODO(rsc): There is a missing updateBuildID here,
        // but we have to decide where to store the build ID in these files.
        a.built = a.Target
        return BuildToolchain.ldShared(b, a, a.Deps[0].Deps, a.Target, importcfg, a.Deps)
}

// BuildInstallFunc is the action for installing a single package or executable.
func BuildInstallFunc(b *Builder, ctx context.Context, a *Action) (err error) {
        defer func() {
                if err != nil {
                        // a.Package == nil is possible for the go install -buildmode=shared
                        // action that installs libmangledname.so, which corresponds to
                        // a list of packages, not just one.
                        sep, path := "", ""
                        if a.Package != nil {
                                sep, path = " ", a.Package.ImportPath
                        }
                        err = fmt.Errorf("go %s%s%s: %v", cfg.CmdName, sep, path, err)
                }
        }()

        a1 := a.Deps[0]
        a.buildID = a1.buildID
        if a.json != nil {
                a.json.BuildID = a.buildID
        }

        // If we are using the eventual install target as an up-to-date
        // cached copy of the thing we built, then there's no need to
        // copy it into itself (and that would probably fail anyway).
        // In this case a1.built == a.Target because a1.built == p.Target,
        // so the built target is not in the a1.Objdir tree that b.cleanup(a1) removes.
        if a1.built == a.Target {
                a.built = a.Target
                if !a.buggyInstall {
                        b.cleanup(a1)
                }
                // Whether we're smart enough to avoid a complete rebuild
                // depends on exactly what the staleness and rebuild algorithms
                // are, as well as potentially the state of the Go build cache.
                // We don't really want users to be able to infer (or worse start depending on)
                // those details from whether the modification time changes during
                // "go install", so do a best-effort update of the file times to make it
                // look like we rewrote a.Target even if we did not. Updating the mtime
                // may also help other mtime-based systems that depend on our
                // previous mtime updates that happened more often.
                // This is still not perfect - we ignore the error result, and if the file was
                // unwritable for some reason then pretending to have written it is also
                // confusing - but it's probably better than not doing the mtime update.
                //
                // But don't do that for the special case where building an executable
                // with -linkshared implicitly installs all its dependent libraries.
                // We want to hide that awful detail as much as possible, so don't
                // advertise it by touching the mtimes (usually the libraries are up
                // to date).
                if !a.buggyInstall && !b.IsCmdList {
                        if cfg.BuildN {
                                b.Showcmd("", "touch %s", a.Target)
                        } else if err := AllowInstall(a); err == nil {
                                now := time.Now()
                                os.Chtimes(a.Target, now, now)
                        }
                }
                return nil
        }

        // If we're building for go list -export,
        // never install anything; just keep the cache reference.
        if b.IsCmdList {
                a.built = a1.built
                return nil
        }
        if err := AllowInstall(a); err != nil {
                return err
        }

        if err := b.Mkdir(a.Objdir); err != nil {
                return err
        }

        perm := fs.FileMode(0666)
        if a1.Mode == "link" {
                switch cfg.BuildBuildmode {
                case "c-archive", "c-shared", "plugin":
                default:
                        perm = 0777
                }
        }

        // make target directory
        dir, _ := filepath.Split(a.Target)
        if dir != "" {
                if err := b.Mkdir(dir); err != nil {
                        return err
                }
        }

        if !a.buggyInstall {
                defer b.cleanup(a1)
        }

        return b.moveOrCopyFile(a.Target, a1.built, perm, false)
}

// AllowInstall returns a non-nil error if this invocation of the go command is
// allowed to install a.Target.
//
// The build of cmd/go running under its own test is forbidden from installing
// to its original GOROOT. The var is exported so it can be set by TestMain.
var AllowInstall = func(*Action) error { return nil }

// cleanup removes a's object dir to keep the amount of
// on-disk garbage down in a large build. On an operating system
// with aggressive buffering, cleaning incrementally like
// this keeps the intermediate objects from hitting the disk.
func (b *Builder) cleanup(a *Action) {
        if !cfg.BuildWork {
                if cfg.BuildX {
                        // Don't say we are removing the directory if
                        // we never created it.
                        if _, err := os.Stat(a.Objdir); err == nil || cfg.BuildN {
                                b.Showcmd("", "rm -r %s", a.Objdir)
                        }
                }
                os.RemoveAll(a.Objdir)
        }
}

// moveOrCopyFile is like 'mv src dst' or 'cp src dst'.
func (b *Builder) moveOrCopyFile(dst, src string, perm fs.FileMode, force bool) error {
        if cfg.BuildN {
                b.Showcmd("", "mv %s %s", src, dst)
                return nil
        }

        // If we can update the mode and rename to the dst, do it.
        // Otherwise fall back to standard copy.

        // If the source is in the build cache, we need to copy it.
        if strings.HasPrefix(src, cache.DefaultDir()) {
                return b.copyFile(dst, src, perm, force)
        }

        // On Windows, always copy the file, so that we respect the NTFS
        // permissions of the parent folder. https://golang.org/issue/22343.
        // What matters here is not cfg.Goos (the system we are building
        // for) but runtime.GOOS (the system we are building on).
        if runtime.GOOS == "windows" {
                return b.copyFile(dst, src, perm, force)
        }

        // If the destination directory has the group sticky bit set,
        // we have to copy the file to retain the correct permissions.
        // https://golang.org/issue/18878
        if fi, err := os.Stat(filepath.Dir(dst)); err == nil {
                if fi.IsDir() && (fi.Mode()&fs.ModeSetgid) != 0 {
                        return b.copyFile(dst, src, perm, force)
                }
        }

        // The perm argument is meant to be adjusted according to umask,
        // but we don't know what the umask is.
        // Create a dummy file to find out.
        // This avoids build tags and works even on systems like Plan 9
        // where the file mask computation incorporates other information.
        mode := perm
        f, err := os.OpenFile(filepath.Clean(dst)+"-go-tmp-umask", os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
        if err == nil {
                fi, err := f.Stat()
                if err == nil {
                        mode = fi.Mode() & 0777
                }
                name := f.Name()
                f.Close()
                os.Remove(name)
        }

        if err := os.Chmod(src, mode); err == nil {
                if err := os.Rename(src, dst); err == nil {
                        if cfg.BuildX {
                                b.Showcmd("", "mv %s %s", src, dst)
                        }
                        return nil
                }
        }

        return b.copyFile(dst, src, perm, force)
}

// copyFile is like 'cp src dst'.
func (b *Builder) copyFile(dst, src string, perm fs.FileMode, force bool) error {
        if cfg.BuildN || cfg.BuildX {
                b.Showcmd("", "cp %s %s", src, dst)
                if cfg.BuildN {
                        return nil
                }
        }

        sf, err := os.Open(src)
        if err != nil {
                return err
        }
        defer sf.Close()

        // Be careful about removing/overwriting dst.
        // Do not remove/overwrite if dst exists and is a directory
        // or a non-empty non-object file.
        if fi, err := os.Stat(dst); err == nil {
                if fi.IsDir() {
                        return fmt.Errorf("build output %q already exists and is a directory", dst)
                }
                if !force && fi.Mode().IsRegular() && fi.Size() != 0 && !isObject(dst) {
                        return fmt.Errorf("build output %q already exists and is not an object file", dst)
                }
        }

        // On Windows, remove lingering ~ file from last attempt.
        if runtime.GOOS == "windows" {
                if _, err := os.Stat(dst + "~"); err == nil {
                        os.Remove(dst + "~")
                }
        }

        mayberemovefile(dst)
        df, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
        if err != nil && runtime.GOOS == "windows" {
                // Windows does not allow deletion of a binary file
                // while it is executing. Try to move it out of the way.
                // If the move fails, which is likely, we'll try again the
                // next time we do an install of this binary.
                if err := os.Rename(dst, dst+"~"); err == nil {
                        os.Remove(dst + "~")
                }
                df, err = os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
        }
        if err != nil {
                return fmt.Errorf("copying %s: %w", src, err) // err should already refer to dst
        }

        _, err = io.Copy(df, sf)
        df.Close()
        if err != nil {
                mayberemovefile(dst)
                return fmt.Errorf("copying %s to %s: %v", src, dst, err)
        }
        return nil
}

// writeFile writes the text to file.
func (b *Builder) writeFile(file string, text []byte) error {
        if cfg.BuildN || cfg.BuildX {
                b.Showcmd("", "cat >%s << 'EOF' # internal\n%sEOF", file, text)
        }
        if cfg.BuildN {
                return nil
        }
        return os.WriteFile(file, text, 0666)
}

// Install the cgo export header file, if there is one.
func (b *Builder) installHeader(ctx context.Context, a *Action) error {
        src := a.Objdir + "_cgo_install.h"
        if _, err := os.Stat(src); os.IsNotExist(err) {
                // If the file does not exist, there are no exported
                // functions, and we do not install anything.
                // TODO(rsc): Once we know that caching is rebuilding
                // at the right times (not missing rebuilds), here we should
                // probably delete the installed header, if any.
                if cfg.BuildX {
                        b.Showcmd("", "# %s not created", src)
                }
                return nil
        }

        if err := AllowInstall(a); err != nil {
                return err
        }

        dir, _ := filepath.Split(a.Target)
        if dir != "" {
                if err := b.Mkdir(dir); err != nil {
                        return err
                }
        }

        return b.moveOrCopyFile(a.Target, src, 0666, true)
}

// cover runs, in effect,
//
//      go tool cover -mode=b.coverMode -var="varName" -o dst.go src.go
func (b *Builder) cover(a *Action, dst, src string, varName string) error {
        return b.run(a, a.Objdir, "cover "+a.Package.ImportPath, nil,
                cfg.BuildToolexec,
                base.Tool("cover"),
                "-mode", a.Package.Internal.CoverMode,
                "-var", varName,
                "-o", dst,
                src)
}

// cover2 runs, in effect,
//
//      go tool cover -pkgcfg=<config file> -mode=b.coverMode -var="varName" -o <outfiles> <infiles>
func (b *Builder) cover2(a *Action, pkgcfg, covoutputs string, infiles, outfiles []string, varName string, mode string) error {
        if err := b.writeCoverPkgInputs(a, pkgcfg, covoutputs, outfiles); err != nil {
                return err
        }
        args := []string{base.Tool("cover"),
                "-pkgcfg", pkgcfg,
                "-mode", mode,
                "-var", varName,
                "-outfilelist", covoutputs,
        }
        args = append(args, infiles...)
        return b.run(a, a.Objdir, "cover "+a.Package.ImportPath, nil,
                cfg.BuildToolexec, args)
}

func (b *Builder) writeCoverPkgInputs(a *Action, pconfigfile string, covoutputsfile string, outfiles []string) error {
        p := a.Package
        p.Internal.CoverageCfg = a.Objdir + "coveragecfg"
        pcfg := coverage.CoverPkgConfig{
                PkgPath: p.ImportPath,
                PkgName: p.Name,
                // Note: coverage granularity is currently hard-wired to
                // 'perblock'; there isn't a way using "go build -cover" or "go
                // test -cover" to select it. This may change in the future
                // depending on user demand.
                Granularity: "perblock",
                OutConfig:   p.Internal.CoverageCfg,
        }
        if a.Package.Module != nil {
                pcfg.ModulePath = a.Package.Module.Path
        }
        data, err := json.Marshal(pcfg)
        if err != nil {
                return err
        }
        if err := b.writeFile(pconfigfile, data); err != nil {
                return err
        }
        var sb strings.Builder
        for i := range outfiles {
                fmt.Fprintf(&sb, "%s\n", outfiles[i])
        }
        return b.writeFile(covoutputsfile, []byte(sb.String()))
}

var objectMagic = [][]byte{
        {'!', '<', 'a', 'r', 'c', 'h', '>', '\n'}, // Package archive
        {'<', 'b', 'i', 'g', 'a', 'f', '>', '\n'}, // Package AIX big archive
        {'\x7F', 'E', 'L', 'F'},                   // ELF
        {0xFE, 0xED, 0xFA, 0xCE},                  // Mach-O big-endian 32-bit
        {0xFE, 0xED, 0xFA, 0xCF},                  // Mach-O big-endian 64-bit
        {0xCE, 0xFA, 0xED, 0xFE},                  // Mach-O little-endian 32-bit
        {0xCF, 0xFA, 0xED, 0xFE},                  // Mach-O little-endian 64-bit
        {0x4d, 0x5a, 0x90, 0x00, 0x03, 0x00},      // PE (Windows) as generated by 6l/8l and gcc
        {0x4d, 0x5a, 0x78, 0x00, 0x01, 0x00},      // PE (Windows) as generated by llvm for dll
        {0x00, 0x00, 0x01, 0xEB},                  // Plan 9 i386
        {0x00, 0x00, 0x8a, 0x97},                  // Plan 9 amd64
        {0x00, 0x00, 0x06, 0x47},                  // Plan 9 arm
        {0x00, 0x61, 0x73, 0x6D},                  // WASM
        {0x01, 0xDF},                              // XCOFF 32bit
        {0x01, 0xF7},                              // XCOFF 64bit
}

func isObject(s string) bool {
        f, err := os.Open(s)
        if err != nil {
                return false
        }
        defer f.Close()
        buf := make([]byte, 64)
        io.ReadFull(f, buf)
        for _, magic := range objectMagic {
                if bytes.HasPrefix(buf, magic) {
                        return true
                }
        }
        return false
}

// mayberemovefile removes a file only if it is a regular file
// When running as a user with sufficient privileges, we may delete
// even device files, for example, which is not intended.
func mayberemovefile(s string) {
        if fi, err := os.Lstat(s); err == nil && !fi.Mode().IsRegular() {
                return
        }
        os.Remove(s)
}

// fmtcmd formats a command in the manner of fmt.Sprintf but also:
//
//      If dir is non-empty and the script is not in dir right now,
//      fmtcmd inserts "cd dir\n" before the command.
//
//      fmtcmd replaces the value of b.WorkDir with $WORK.
//      fmtcmd replaces the value of goroot with $GOROOT.
//      fmtcmd replaces the value of b.gobin with $GOBIN.
//
//      fmtcmd replaces the name of the current directory with dot (.)
//      but only when it is at the beginning of a space-separated token.
func (b *Builder) fmtcmd(dir string, format string, args ...any) string {
        cmd := fmt.Sprintf(format, args...)
        if dir != "" && dir != "/" {
                dot := " ."
                if dir[len(dir)-1] == filepath.Separator {
                        dot += string(filepath.Separator)
                }
                cmd = strings.ReplaceAll(" "+cmd, " "+dir, dot)[1:]
                if b.scriptDir != dir {
                        b.scriptDir = dir
                        cmd = "cd " + dir + "\n" + cmd
                }
        }
        if b.WorkDir != "" && !strings.HasPrefix(cmd, "cat ") {
                cmd = strings.ReplaceAll(cmd, b.WorkDir, "$WORK")
                escaped := strconv.Quote(b.WorkDir)
                escaped = escaped[1 : len(escaped)-1] // strip quote characters
                if escaped != b.WorkDir {
                        cmd = strings.ReplaceAll(cmd, escaped, "$WORK")
                }
        }
        return cmd
}

// showcmd prints the given command to standard output
// for the implementation of -n or -x.
func (b *Builder) Showcmd(dir string, format string, args ...any) {
        b.output.Lock()
        defer b.output.Unlock()
        b.Print(b.fmtcmd(dir, format, args...) + "\n")
}

// showOutput prints "# desc" followed by the given output.
// The output is expected to contain references to 'dir', usually
// the source directory for the package that has failed to build.
// showOutput rewrites mentions of dir with a relative path to dir
// when the relative path is shorter. This is usually more pleasant.
// For example, if fmt doesn't compile and we are in src/html,
// the output is
//
//      $ go build
//      # fmt
//      ../fmt/print.go:1090: undefined: asdf
//      $
//
// instead of
//
//      $ go build
//      # fmt
//      /usr/gopher/go/src/fmt/print.go:1090: undefined: asdf
//      $
//
// showOutput also replaces references to the work directory with $WORK.
//
// If a is not nil and a.output is not nil, showOutput appends to that slice instead of
// printing to b.Print.
func (b *Builder) showOutput(a *Action, dir, desc, out string) {
        prefix, suffix := formatOutput(b.WorkDir, dir, desc, out)
        if a != nil && a.output != nil {
                a.output = append(a.output, prefix...)
                a.output = append(a.output, suffix...)
                return
        }

        b.output.Lock()
        defer b.output.Unlock()
        b.Print(prefix, suffix)
}

// formatOutput prints "# desc" followed by the given output.
// The output is expected to contain references to 'dir', usually
// the source directory for the package that has failed to build.
// formatOutput rewrites mentions of dir with a relative path to dir
// when the relative path is shorter. This is usually more pleasant.
// For example, if fmt doesn't compile and we are in src/html,
// the output is
//
//      $ go build
//      # fmt
//      ../fmt/print.go:1090: undefined: asdf
//      $
//
// instead of
//
//      $ go build
//      # fmt
//      /usr/gopher/go/src/fmt/print.go:1090: undefined: asdf
//      $
//
// formatOutput also replaces references to the work directory with $WORK.
// formatOutput returns the output in a prefix with the description and a
// suffix with the actual output.
func formatOutput(workDir, dir, desc, out string) (prefix, suffix string) {
        prefix = "# " + desc
        suffix = "\n" + out
        if reldir := base.ShortPath(dir); reldir != dir {
                suffix = strings.ReplaceAll(suffix, " "+dir, " "+reldir)
                suffix = strings.ReplaceAll(suffix, "\n"+dir, "\n"+reldir)
                suffix = strings.ReplaceAll(suffix, "\n\t"+dir, "\n\t"+reldir)
        }
        suffix = strings.ReplaceAll(suffix, " "+workDir, " $WORK")

        return prefix, suffix
}

var cgoLine = lazyregexp.New(`\[[^\[\]]+\.(cgo1|cover)\.go:[0-9]+(:[0-9]+)?\]`)
var cgoTypeSigRe = lazyregexp.New(`\b_C2?(type|func|var|macro)_\B`)

// run runs the command given by cmdline in the directory dir.
// If the command fails, run prints information about the failure
// and returns a non-nil error.
func (b *Builder) run(a *Action, dir string, desc string, env []string, cmdargs ...any) error {
        out, err := b.runOut(a, dir, env, cmdargs...)
        if len(out) > 0 {
                if desc == "" {
                        desc = b.fmtcmd(dir, "%s", strings.Join(str.StringList(cmdargs...), " "))
                }
                if err != nil {
                        err = errors.New(fmt.Sprint(formatOutput(b.WorkDir, dir, desc, b.processOutput(out))))
                } else {
                        b.showOutput(a, dir, desc, b.processOutput(out))
                }
        }
        return err
}

// processOutput prepares the output of runOut to be output to the console.
func (b *Builder) processOutput(out []byte) string {
        if out[len(out)-1] != '\n' {
                out = append(out, '\n')
        }
        messages := string(out)
        // Fix up output referring to cgo-generated code to be more readable.
        // Replace x.go:19[/tmp/.../x.cgo1.go:18] with x.go:19.
        // Replace *[100]_Ctype_foo with *[100]C.foo.
        // If we're using -x, assume we're debugging and want the full dump, so disable the rewrite.
        if !cfg.BuildX && cgoLine.MatchString(messages) {
                messages = cgoLine.ReplaceAllString(messages, "")
                messages = cgoTypeSigRe.ReplaceAllString(messages, "C.")
        }
        return messages
}

// runOut runs the command given by cmdline in the directory dir.
// It returns the command output and any errors that occurred.
// It accumulates execution time in a.
func (b *Builder) runOut(a *Action, dir string, env []string, cmdargs ...any) ([]byte, error) {
        cmdline := str.StringList(cmdargs...)

        for _, arg := range cmdline {
                // GNU binutils commands, including gcc and gccgo, interpret an argument
                // @foo anywhere in the command line (even following --) as meaning
                // "read and insert arguments from the file named foo."
                // Don't say anything that might be misinterpreted that way.
                if strings.HasPrefix(arg, "@") {
                        return nil, fmt.Errorf("invalid command-line argument %s in command: %s", arg, joinUnambiguously(cmdline))
                }
        }

        if cfg.BuildN || cfg.BuildX {
                var envcmdline string
                for _, e := range env {
                        if j := strings.IndexByte(e, '='); j != -1 {
                                if strings.ContainsRune(e[j+1:], '\'') {
                                        envcmdline += fmt.Sprintf("%s=%q", e[:j], e[j+1:])
                                } else {
                                        envcmdline += fmt.Sprintf("%s='%s'", e[:j], e[j+1:])
                                }
                                envcmdline += " "
                        }
                }
                envcmdline += joinUnambiguously(cmdline)
                b.Showcmd(dir, "%s", envcmdline)
                if cfg.BuildN {
                        return nil, nil
                }
        }

        var buf bytes.Buffer
        cmd := exec.Command(cmdline[0], cmdline[1:]...)
        if cmd.Path != "" {
                cmd.Args[0] = cmd.Path
        }
        cmd.Stdout = &buf
        cmd.Stderr = &buf
        cleanup := passLongArgsInResponseFiles(cmd)
        defer cleanup()
        if dir != "." {
                cmd.Dir = dir
        }
        cmd.Env = cmd.Environ() // Pre-allocate with correct PWD.

        // Add the TOOLEXEC_IMPORTPATH environment variable for -toolexec tools.
        // It doesn't really matter if -toolexec isn't being used.
        // Note that a.Package.Desc is not really an import path,
        // but this is consistent with 'go list -f {{.ImportPath}}'.
        // Plus, it is useful to uniquely identify packages in 'go list -json'.
        if a != nil && a.Package != nil {
                cmd.Env = append(cmd.Env, "TOOLEXEC_IMPORTPATH="+a.Package.Desc())
        }

        cmd.Env = append(cmd.Env, env...)
        start := time.Now()
        err := cmd.Run()
        if a != nil && a.json != nil {
                aj := a.json
                aj.Cmd = append(aj.Cmd, joinUnambiguously(cmdline))
                aj.CmdReal += time.Since(start)
                if ps := cmd.ProcessState; ps != nil {
                        aj.CmdUser += ps.UserTime()
                        aj.CmdSys += ps.SystemTime()
                }
        }

        // err can be something like 'exit status 1'.
        // Add information about what program was running.
        // Note that if buf.Bytes() is non-empty, the caller usually
        // shows buf.Bytes() and does not print err at all, so the
        // prefix here does not make most output any more verbose.
        if err != nil {
                err = errors.New(cmdline[0] + ": " + err.Error())
        }
        return buf.Bytes(), err
}

// joinUnambiguously prints the slice, quoting where necessary to make the
// output unambiguous.
// TODO: See issue 5279. The printing of commands needs a complete redo.
func joinUnambiguously(a []string) string {
        var buf strings.Builder
        for i, s := range a {
                if i > 0 {
                        buf.WriteByte(' ')
                }
                q := strconv.Quote(s)
                // A gccgo command line can contain -( and -).
                // Make sure we quote them since they are special to the shell.
                // The trimpath argument can also contain > (part of =>) and ;. Quote those too.
                if s == "" || strings.ContainsAny(s, " ()>;") || len(q) > len(s)+2 {
                        buf.WriteString(q)
                } else {
                        buf.WriteString(s)
                }
        }
        return buf.String()
}

// cCompilerEnv returns environment variables to set when running the
// C compiler. This is needed to disable escape codes in clang error
// messages that confuse tools like cgo.
func (b *Builder) cCompilerEnv() []string {
        return []string{"TERM=dumb"}
}

// mkdir makes the named directory.
func (b *Builder) Mkdir(dir string) error {
        // Make Mkdir(a.Objdir) a no-op instead of an error when a.Objdir == "".
        if dir == "" {
                return nil
        }

        b.exec.Lock()
        defer b.exec.Unlock()
        // We can be a little aggressive about being
        // sure directories exist. Skip repeated calls.
        if b.mkdirCache[dir] {
                return nil
        }
        b.mkdirCache[dir] = true

        if cfg.BuildN || cfg.BuildX {
                b.Showcmd("", "mkdir -p %s", dir)
                if cfg.BuildN {
                        return nil
                }
        }

        if err := os.MkdirAll(dir, 0777); err != nil {
                return err
        }
        return nil
}

// symlink creates a symlink newname -> oldname.
func (b *Builder) Symlink(oldname, newname string) error {
        // It's not an error to try to recreate an existing symlink.
        if link, err := os.Readlink(newname); err == nil && link == oldname {
                return nil
        }

        if cfg.BuildN || cfg.BuildX {
                b.Showcmd("", "ln -s %s %s", oldname, newname)
                if cfg.BuildN {
                        return nil
                }
        }
        return os.Symlink(oldname, newname)
}

// mkAbs returns an absolute path corresponding to
// evaluating f in the directory dir.
// We always pass absolute paths of source files so that
// the error messages will include the full path to a file
// in need of attention.
func mkAbs(dir, f string) string {
        // Leave absolute paths alone.
        // Also, during -n mode we use the pseudo-directory $WORK
        // instead of creating an actual work directory that won't be used.
        // Leave paths beginning with $WORK alone too.
        if filepath.IsAbs(f) || strings.HasPrefix(f, "$WORK") {
                return f
        }
        return filepath.Join(dir, f)
}

type toolchain interface {
        // gc runs the compiler in a specific directory on a set of files
        // and returns the name of the generated output file.
        gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, out []byte, err error)
        // cc runs the toolchain's C compiler in a directory on a C file
        // to produce an output file.
        cc(b *Builder, a *Action, ofile, cfile string) error
        // asm runs the assembler in a specific directory on specific files
        // and returns a list of named output files.
        asm(b *Builder, a *Action, sfiles []string) ([]string, error)
        // symabis scans the symbol ABIs from sfiles and returns the
        // path to the output symbol ABIs file, or "" if none.
        symabis(b *Builder, a *Action, sfiles []string) (string, error)
        // pack runs the archive packer in a specific directory to create
        // an archive from a set of object files.
        // typically it is run in the object directory.
        pack(b *Builder, a *Action, afile string, ofiles []string) error
        // ld runs the linker to create an executable starting at mainpkg.
        ld(b *Builder, root *Action, out, importcfg, mainpkg string) error
        // ldShared runs the linker to create a shared library containing the pkgs built by toplevelactions
        ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error

        compiler() string
        linker() string
}

type noToolchain struct{}

func noCompiler() error {
        log.Fatalf("unknown compiler %q", cfg.BuildContext.Compiler)
        return nil
}

func (noToolchain) compiler() string {
        noCompiler()
        return ""
}

func (noToolchain) linker() string {
        noCompiler()
        return ""
}

func (noToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, out []byte, err error) {
        return "", nil, noCompiler()
}

func (noToolchain) asm(b *Builder, a *Action, sfiles []string) ([]string, error) {
        return nil, noCompiler()
}

func (noToolchain) symabis(b *Builder, a *Action, sfiles []string) (string, error) {
        return "", noCompiler()
}

func (noToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
        return noCompiler()
}

func (noToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string) error {
        return noCompiler()
}

func (noToolchain) ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
        return noCompiler()
}

func (noToolchain) cc(b *Builder, a *Action, ofile, cfile string) error {
        return noCompiler()
}

// gcc runs the gcc C compiler to create an object from a single C file.
func (b *Builder) gcc(a *Action, p *load.Package, workdir, out string, flags []string, cfile string) error {
        return b.ccompile(a, p, out, flags, cfile, b.GccCmd(p.Dir, workdir))
}

// gxx runs the g++ C++ compiler to create an object from a single C++ file.
func (b *Builder) gxx(a *Action, p *load.Package, workdir, out string, flags []string, cxxfile string) error {
        return b.ccompile(a, p, out, flags, cxxfile, b.GxxCmd(p.Dir, workdir))
}

// gfortran runs the gfortran Fortran compiler to create an object from a single Fortran file.
func (b *Builder) gfortran(a *Action, p *load.Package, workdir, out string, flags []string, ffile string) error {
        return b.ccompile(a, p, out, flags, ffile, b.gfortranCmd(p.Dir, workdir))
}

// ccompile runs the given C or C++ compiler and creates an object from a single source file.
func (b *Builder) ccompile(a *Action, p *load.Package, outfile string, flags []string, file string, compiler []string) error {
        file = mkAbs(p.Dir, file)
        desc := p.ImportPath
        outfile = mkAbs(p.Dir, outfile)

        // Elide source directory paths if -trimpath or GOROOT_FINAL is set.
        // This is needed for source files (e.g., a .c file in a package directory).
        // TODO(golang.org/issue/36072): cgo also generates files with #line
        // directives pointing to the source directory. It should not generate those
        // when -trimpath is enabled.
        if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
                if cfg.BuildTrimpath || p.Goroot {
                        // Keep in sync with Action.trimpath.
                        // The trimmed paths are a little different, but we need to trim in the
                        // same situations.
                        var from, toPath string
                        if m := p.Module; m != nil {
                                from = m.Dir
                                toPath = m.Path + "@" + m.Version
                        } else {
                                from = p.Dir
                                toPath = p.ImportPath
                        }
                        // -fdebug-prefix-map requires an absolute "to" path (or it joins the path
                        // with the working directory). Pick something that makes sense for the
                        // target platform.
                        var to string
                        if cfg.BuildContext.GOOS == "windows" {
                                to = filepath.Join(`\\_\_`, toPath)
                        } else {
                                to = filepath.Join("/_", toPath)
                        }
                        flags = append(flags[:len(flags):len(flags)], "-fdebug-prefix-map="+from+"="+to)
                }
        }

        overlayPath := file
        if p, ok := a.nonGoOverlay[overlayPath]; ok {
                overlayPath = p
        }
        output, err := b.runOut(a, filepath.Dir(overlayPath), b.cCompilerEnv(), compiler, flags, "-o", outfile, "-c", filepath.Base(overlayPath))
        if len(output) > 0 {
                // On FreeBSD 11, when we pass -g to clang 3.8 it
                // invokes its internal assembler with -dwarf-version=2.
                // When it sees .section .note.GNU-stack, it warns
                // "DWARF2 only supports one section per compilation unit".
                // This warning makes no sense, since the section is empty,
                // but it confuses people.
                // We work around the problem by detecting the warning
                // and dropping -g and trying again.
                if bytes.Contains(output, []byte("DWARF2 only supports one section per compilation unit")) {
                        newFlags := make([]string, 0, len(flags))
                        for _, f := range flags {
                                if !strings.HasPrefix(f, "-g") {
                                        newFlags = append(newFlags, f)
                                }
                        }
                        if len(newFlags) < len(flags) {
                                return b.ccompile(a, p, outfile, newFlags, file, compiler)
                        }
                }

                if err != nil || os.Getenv("GO_BUILDER_NAME") != "" {
                        err = errors.New(fmt.Sprintf(formatOutput(b.WorkDir, p.Dir, desc, b.processOutput(output))))
                } else {
                        b.showOutput(a, p.Dir, desc, b.processOutput(output))
                }
        }
        return err
}

// gccld runs the gcc linker to create an executable from a set of object files.
// Any error output is only displayed for BuildN or BuildX.
func (b *Builder) gccld(a *Action, p *load.Package, objdir, outfile string, flags []string, objs []string) error {
        var cmd []string
        if len(p.CXXFiles) > 0 || len(p.SwigCXXFiles) > 0 {
                cmd = b.GxxCmd(p.Dir, objdir)
        } else {
                cmd = b.GccCmd(p.Dir, objdir)
        }

        cmdargs := []any{cmd, "-o", outfile, objs, flags}
        dir := p.Dir
        out, err := b.runOut(a, base.Cwd(), b.cCompilerEnv(), cmdargs...)

        if len(out) > 0 {
                // Filter out useless linker warnings caused by bugs outside Go.
                // See also cmd/link/internal/ld's hostlink method.
                var save [][]byte
                var skipLines int
                for _, line := range bytes.SplitAfter(out, []byte("\n")) {
                        // golang.org/issue/26073 - Apple Xcode bug
                        if bytes.Contains(line, []byte("ld: warning: text-based stub file")) {
                                continue
                        }

                        if skipLines > 0 {
                                skipLines--
                                continue
                        }

                        // Remove duplicate main symbol with runtime/cgo on AIX.
                        // With runtime/cgo, two main are available:
                        // One is generated by cgo tool with {return 0;}.
                        // The other one is the main calling runtime.rt0_go
                        // in runtime/cgo.
                        // The second can't be used by cgo programs because
                        // runtime.rt0_go is unknown to them.
                        // Therefore, we let ld remove this main version
                        // and used the cgo generated one.
                        if p.ImportPath == "runtime/cgo" && bytes.Contains(line, []byte("ld: 0711-224 WARNING: Duplicate symbol: .main")) {
                                skipLines = 1
                                continue
                        }

                        save = append(save, line)
                }
                out = bytes.Join(save, nil)
                if len(out) > 0 && (cfg.BuildN || cfg.BuildX) {
                        b.showOutput(nil, dir, p.ImportPath, b.processOutput(out))
                }
        }
        return err
}

// gccCmd returns a gcc command line prefix
// defaultCC is defined in zdefaultcc.go, written by cmd/dist.
func (b *Builder) GccCmd(incdir, workdir string) []string {
        return b.compilerCmd(b.ccExe(), incdir, workdir)
}

// gxxCmd returns a g++ command line prefix
// defaultCXX is defined in zdefaultcc.go, written by cmd/dist.
func (b *Builder) GxxCmd(incdir, workdir string) []string {
        return b.compilerCmd(b.cxxExe(), incdir, workdir)
}

// gfortranCmd returns a gfortran command line prefix.
func (b *Builder) gfortranCmd(incdir, workdir string) []string {
        return b.compilerCmd(b.fcExe(), incdir, workdir)
}

// ccExe returns the CC compiler setting without all the extra flags we add implicitly.
func (b *Builder) ccExe() []string {
        return envList("CC", cfg.DefaultCC(cfg.Goos, cfg.Goarch))
}

// cxxExe returns the CXX compiler setting without all the extra flags we add implicitly.
func (b *Builder) cxxExe() []string {
        return envList("CXX", cfg.DefaultCXX(cfg.Goos, cfg.Goarch))
}

// fcExe returns the FC compiler setting without all the extra flags we add implicitly.
func (b *Builder) fcExe() []string {
        return envList("FC", "gfortran")
}

// compilerCmd returns a command line prefix for the given environment
// variable and using the default command when the variable is empty.
func (b *Builder) compilerCmd(compiler []string, incdir, workdir string) []string {
        a := append(compiler, "-I", incdir)

        // Definitely want -fPIC but on Windows gcc complains
        // "-fPIC ignored for target (all code is position independent)"
        if cfg.Goos != "windows" {
                a = append(a, "-fPIC")
        }
        a = append(a, b.gccArchArgs()...)
        // gcc-4.5 and beyond require explicit "-pthread" flag
        // for multithreading with pthread library.
        if cfg.BuildContext.CgoEnabled {
                switch cfg.Goos {
                case "windows":
                        a = append(a, "-mthreads")
                default:
                        a = append(a, "-pthread")
                }
        }

        if cfg.Goos == "aix" {
                // mcmodel=large must always be enabled to allow large TOC.
                a = append(a, "-mcmodel=large")
        }

        // disable ASCII art in clang errors, if possible
        if b.gccSupportsFlag(compiler, "-fno-caret-diagnostics") {
                a = append(a, "-fno-caret-diagnostics")
        }
        // clang is too smart about command-line arguments
        if b.gccSupportsFlag(compiler, "-Qunused-arguments") {
                a = append(a, "-Qunused-arguments")
        }

        // zig cc passes --gc-sections to the underlying linker, which then causes
        // undefined symbol errors when compiling with cgo but without C code.
        // https://github.com/golang/go/issues/52690
        if b.gccSupportsFlag(compiler, "-Wl,--no-gc-sections") {
                a = append(a, "-Wl,--no-gc-sections")
        }

        // disable word wrapping in error messages
        a = append(a, "-fmessage-length=0")

        // Tell gcc not to include the work directory in object files.
        if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
                if workdir == "" {
                        workdir = b.WorkDir
                }
                workdir = strings.TrimSuffix(workdir, string(filepath.Separator))
                a = append(a, "-fdebug-prefix-map="+workdir+"=/tmp/go-build")
        }

        // Tell gcc not to include flags in object files, which defeats the
        // point of -fdebug-prefix-map above.
        if b.gccSupportsFlag(compiler, "-gno-record-gcc-switches") {
                a = append(a, "-gno-record-gcc-switches")
        }

        // On OS X, some of the compilers behave as if -fno-common
        // is always set, and the Mach-O linker in 6l/8l assumes this.
        // See https://golang.org/issue/3253.
        if cfg.Goos == "darwin" || cfg.Goos == "ios" {
                a = append(a, "-fno-common")
        }

        return a
}

// gccNoPie returns the flag to use to request non-PIE. On systems
// with PIE (position independent executables) enabled by default,
// -no-pie must be passed when doing a partial link with -Wl,-r.
// But -no-pie is not supported by all compilers, and clang spells it -nopie.
func (b *Builder) gccNoPie(linker []string) string {
        if b.gccSupportsFlag(linker, "-no-pie") {
                return "-no-pie"
        }
        if b.gccSupportsFlag(linker, "-nopie") {
                return "-nopie"
        }
        return ""
}

// gccSupportsFlag checks to see if the compiler supports a flag.
func (b *Builder) gccSupportsFlag(compiler []string, flag string) bool {
        key := [2]string{compiler[0], flag}

        b.exec.Lock()
        defer b.exec.Unlock()
        if b, ok := b.flagCache[key]; ok {
                return b
        }
        if b.flagCache == nil {
                b.flagCache = make(map[[2]string]bool)
        }

        tmp := os.DevNull

        // On the iOS builder the command
        //   $CC -Wl,--no-gc-sections -x c - -o /dev/null < /dev/null
        // is failing with:
        //   Unable to remove existing file: Invalid argument
        if runtime.GOOS == "windows" || runtime.GOOS == "ios" {
                f, err := os.CreateTemp(b.WorkDir, "")
                if err != nil {
                        return false
                }
                f.Close()
                tmp = f.Name()
                defer os.Remove(tmp)
        }

        // We used to write an empty C file, but that gets complicated with go
        // build -n. We tried using a file that does not exist, but that fails on
        // systems with GCC version 4.2.1; that is the last GPLv2 version of GCC,
        // so some systems have frozen on it. Now we pass an empty file on stdin,
        // which should work at least for GCC and clang.
        //
        // If the argument is "-Wl,", then it is testing the linker. In that case,
        // skip "-c". If it's not "-Wl,", then we are testing the compiler and can
        // omit the linking step with "-c".
        //
        // Using the same CFLAGS/LDFLAGS here and for building the program.
        cmdArgs := str.StringList(compiler, flag)
        if strings.HasPrefix(flag, "-Wl,") /* linker flag */ {
                ldflags, err := buildFlags("LDFLAGS", defaultCFlags, nil, checkLinkerFlags)
                if err != nil {
                        return false
                }
                cmdArgs = append(cmdArgs, ldflags...)
        } else { /* compiler flag, add "-c" */
                cflags, err := buildFlags("CFLAGS", defaultCFlags, nil, checkCompilerFlags)
                if err != nil {
                        return false
                }
                cmdArgs = append(cmdArgs, cflags...)
                cmdArgs = append(cmdArgs, "-c")
        }

        cmdArgs = append(cmdArgs, "-x", "c", "-", "-o", tmp)

        if cfg.BuildN || cfg.BuildX {
                b.Showcmd(b.WorkDir, "%s || true", joinUnambiguously(cmdArgs))
                if cfg.BuildN {
                        return false
                }
        }
        cmd := exec.Command(cmdArgs[0], cmdArgs[1:]...)
        cmd.Dir = b.WorkDir
        cmd.Env = append(cmd.Environ(), "LC_ALL=C")
        out, _ := cmd.CombinedOutput()
        // GCC says "unrecognized command line option".
        // clang says "unknown argument".
        // tcc says "unsupported"
        // AIX says "not recognized"
        // Older versions of GCC say "unrecognised debug output level".
        // For -fsplit-stack GCC says "'-fsplit-stack' is not supported".
        supported := !bytes.Contains(out, []byte("unrecognized")) &&
                !bytes.Contains(out, []byte("unknown")) &&
                !bytes.Contains(out, []byte("unrecognised")) &&
                !bytes.Contains(out, []byte("is not supported")) &&
                !bytes.Contains(out, []byte("not recognized")) &&
                !bytes.Contains(out, []byte("unsupported"))
        b.flagCache[key] = supported
        return supported
}

// gccArchArgs returns arguments to pass to gcc based on the architecture.
func (b *Builder) gccArchArgs() []string {
        switch cfg.Goarch {
        case "386":
                return []string{"-m32"}
        case "amd64":
                if cfg.Goos == "darwin" {
                        return []string{"-arch", "x86_64", "-m64"}
                }
                return []string{"-m64"}
        case "arm64":
                if cfg.Goos == "darwin" {
                        return []string{"-arch", "arm64"}
                }
        case "arm":
                return []string{"-marm"} // not thumb
        case "s390x":
                return []string{"-m64", "-march=z196"}
        case "mips64", "mips64le":
                args := []string{"-mabi=64"}
                if cfg.GOMIPS64 == "hardfloat" {
                        return append(args, "-mhard-float")
                } else if cfg.GOMIPS64 == "softfloat" {
                        return append(args, "-msoft-float")
                }
        case "mips", "mipsle":
                args := []string{"-mabi=32", "-march=mips32"}
                if cfg.GOMIPS == "hardfloat" {
                        return append(args, "-mhard-float", "-mfp32", "-mno-odd-spreg")
                } else if cfg.GOMIPS == "softfloat" {
                        return append(args, "-msoft-float")
                }
        case "loong64":
                return []string{"-mabi=lp64d"}
        case "ppc64":
                if cfg.Goos == "aix" {
                        return []string{"-maix64"}
                }
        }
        return nil
}

// envList returns the value of the given environment variable broken
// into fields, using the default value when the variable is empty.
//
// The environment variable must be quoted correctly for
// quoted.Split. This should be done before building
// anything, for example, in BuildInit.
func envList(key, def string) []string {
        v := cfg.Getenv(key)
        if v == "" {
                v = def
        }
        args, err := quoted.Split(v)
        if err != nil {
                panic(fmt.Sprintf("could not parse environment variable %s with value %q: %v", key, v, err))
        }
        return args
}

// CFlags returns the flags to use when invoking the C, C++ or Fortran compilers, or cgo.
func (b *Builder) CFlags(p *load.Package) (cppflags, cflags, cxxflags, fflags, ldflags []string, err error) {
        if cppflags, err = buildFlags("CPPFLAGS", "", p.CgoCPPFLAGS, checkCompilerFlags); err != nil {
                return
        }
        if cflags, err = buildFlags("CFLAGS", defaultCFlags, p.CgoCFLAGS, checkCompilerFlags); err != nil {
                return
        }
        if cxxflags, err = buildFlags("CXXFLAGS", defaultCFlags, p.CgoCXXFLAGS, checkCompilerFlags); err != nil {
                return
        }
        if fflags, err = buildFlags("FFLAGS", defaultCFlags, p.CgoFFLAGS, checkCompilerFlags); err != nil {
                return
        }
        if ldflags, err = buildFlags("LDFLAGS", defaultCFlags, p.CgoLDFLAGS, checkLinkerFlags); err != nil {
                return
        }

        return
}

func buildFlags(name, defaults string, fromPackage []string, check func(string, string, []string) error) ([]string, error) {
        if err := check(name, "#cgo "+name, fromPackage); err != nil {
                return nil, err
        }
        return str.StringList(envList("CGO_"+name, defaults), fromPackage), nil
}

var cgoRe = lazyregexp.New(`[/\\:]`)

func (b *Builder) cgo(a *Action, cgoExe, objdir string, pcCFLAGS, pcLDFLAGS, cgofiles, gccfiles, gxxfiles, mfiles, ffiles []string) (outGo, outObj []string, err error) {
        p := a.Package
        cgoCPPFLAGS, cgoCFLAGS, cgoCXXFLAGS, cgoFFLAGS, cgoLDFLAGS, err := b.CFlags(p)
        if err != nil {
                return nil, nil, err
        }

        cgoCPPFLAGS = append(cgoCPPFLAGS, pcCFLAGS...)
        cgoLDFLAGS = append(cgoLDFLAGS, pcLDFLAGS...)
        // If we are compiling Objective-C code, then we need to link against libobjc
        if len(mfiles) > 0 {
                cgoLDFLAGS = append(cgoLDFLAGS, "-lobjc")
        }

        // Likewise for Fortran, except there are many Fortran compilers.
        // Support gfortran out of the box and let others pass the correct link options
        // via CGO_LDFLAGS
        if len(ffiles) > 0 {
                fc := cfg.Getenv("FC")
                if fc == "" {
                        fc = "gfortran"
                }
                if strings.Contains(fc, "gfortran") {
                        cgoLDFLAGS = append(cgoLDFLAGS, "-lgfortran")
                }
        }

        if cfg.BuildMSan {
                cgoCFLAGS = append([]string{"-fsanitize=memory"}, cgoCFLAGS...)
                cgoLDFLAGS = append([]string{"-fsanitize=memory"}, cgoLDFLAGS...)
        }
        if cfg.BuildASan {
                cgoCFLAGS = append([]string{"-fsanitize=address"}, cgoCFLAGS...)
                cgoLDFLAGS = append([]string{"-fsanitize=address"}, cgoLDFLAGS...)
        }

        // Allows including _cgo_export.h, as well as the user's .h files,
        // from .[ch] files in the package.
        cgoCPPFLAGS = append(cgoCPPFLAGS, "-I", objdir)

        // cgo
        // TODO: CGO_FLAGS?
        gofiles := []string{objdir + "_cgo_gotypes.go"}
        cfiles := []string{"_cgo_export.c"}
        for _, fn := range cgofiles {
                f := strings.TrimSuffix(filepath.Base(fn), ".go")
                gofiles = append(gofiles, objdir+f+".cgo1.go")
                cfiles = append(cfiles, f+".cgo2.c")
        }

        // TODO: make cgo not depend on $GOARCH?

        cgoflags := []string{}
        if p.Standard && p.ImportPath == "runtime/cgo" {
                cgoflags = append(cgoflags, "-import_runtime_cgo=false")
        }
        if p.Standard && (p.ImportPath == "runtime/race" || p.ImportPath == "runtime/msan" || p.ImportPath == "runtime/cgo" || p.ImportPath == "runtime/asan") {
                cgoflags = append(cgoflags, "-import_syscall=false")
        }

        // Update $CGO_LDFLAGS with p.CgoLDFLAGS.
        // These flags are recorded in the generated _cgo_gotypes.go file
        // using //go:cgo_ldflag directives, the compiler records them in the
        // object file for the package, and then the Go linker passes them
        // along to the host linker. At this point in the code, cgoLDFLAGS
        // consists of the original $CGO_LDFLAGS (unchecked) and all the
        // flags put together from source code (checked).
        cgoenv := b.cCompilerEnv()
        if len(cgoLDFLAGS) > 0 {
                flags := make([]string, len(cgoLDFLAGS))
                for i, f := range cgoLDFLAGS {
                        flags[i] = strconv.Quote(f)
                }
                cgoenv = append(cgoenv, "CGO_LDFLAGS="+strings.Join(flags, " "))
        }

        if cfg.BuildToolchainName == "gccgo" {
                if b.gccSupportsFlag([]string{BuildToolchain.compiler()}, "-fsplit-stack") {
                        cgoCFLAGS = append(cgoCFLAGS, "-fsplit-stack")
                }
                cgoflags = append(cgoflags, "-gccgo")
                if pkgpath := gccgoPkgpath(p); pkgpath != "" {
                        cgoflags = append(cgoflags, "-gccgopkgpath="+pkgpath)
                }
        }

        switch cfg.BuildBuildmode {
        case "c-archive", "c-shared":
                // Tell cgo that if there are any exported functions
                // it should generate a header file that C code can
                // #include.
                cgoflags = append(cgoflags, "-exportheader="+objdir+"_cgo_install.h")
        }

        execdir := p.Dir

        // Rewrite overlaid paths in cgo files.
        // cgo adds //line and #line pragmas in generated files with these paths.
        var trimpath []string
        for i := range cgofiles {
                path := mkAbs(p.Dir, cgofiles[i])
                if opath, ok := fsys.OverlayPath(path); ok {
                        cgofiles[i] = opath
                        trimpath = append(trimpath, opath+"=>"+path)
                }
        }
        if len(trimpath) > 0 {
                cgoflags = append(cgoflags, "-trimpath", strings.Join(trimpath, ";"))
        }

        if err := b.run(a, execdir, p.ImportPath, cgoenv, cfg.BuildToolexec, cgoExe, "-objdir", objdir, "-importpath", p.ImportPath, cgoflags, "--", cgoCPPFLAGS, cgoCFLAGS, cgofiles); err != nil {
                return nil, nil, err
        }
        outGo = append(outGo, gofiles...)

        // Use sequential object file names to keep them distinct
        // and short enough to fit in the .a header file name slots.
        // We no longer collect them all into _all.o, and we'd like
        // tools to see both the .o suffix and unique names, so
        // we need to make them short enough not to be truncated
        // in the final archive.
        oseq := 0
        nextOfile := func() string {
                oseq++
                return objdir + fmt.Sprintf("_x%03d.o", oseq)
        }

        // gcc
        cflags := str.StringList(cgoCPPFLAGS, cgoCFLAGS)
        for _, cfile := range cfiles {
                ofile := nextOfile()
                if err := b.gcc(a, p, a.Objdir, ofile, cflags, objdir+cfile); err != nil {
                        return nil, nil, err
                }
                outObj = append(outObj, ofile)
        }

        for _, file := range gccfiles {
                ofile := nextOfile()
                if err := b.gcc(a, p, a.Objdir, ofile, cflags, file); err != nil {
                        return nil, nil, err
                }
                outObj = append(outObj, ofile)
        }

        cxxflags := str.StringList(cgoCPPFLAGS, cgoCXXFLAGS)
        for _, file := range gxxfiles {
                ofile := nextOfile()
                if err := b.gxx(a, p, a.Objdir, ofile, cxxflags, file); err != nil {
                        return nil, nil, err
                }
                outObj = append(outObj, ofile)
        }

        for _, file := range mfiles {
                ofile := nextOfile()
                if err := b.gcc(a, p, a.Objdir, ofile, cflags, file); err != nil {
                        return nil, nil, err
                }
                outObj = append(outObj, ofile)
        }

        fflags := str.StringList(cgoCPPFLAGS, cgoFFLAGS)
        for _, file := range ffiles {
                ofile := nextOfile()
                if err := b.gfortran(a, p, a.Objdir, ofile, fflags, file); err != nil {
                        return nil, nil, err
                }
                outObj = append(outObj, ofile)
        }

        switch cfg.BuildToolchainName {
        case "gc":
                importGo := objdir + "_cgo_import.go"
                dynOutGo, dynOutObj, err := b.dynimport(a, p, objdir, importGo, cgoExe, cflags, cgoLDFLAGS, outObj)
                if err != nil {
                        return nil, nil, err
                }
                if dynOutGo != "" {
                        outGo = append(outGo, dynOutGo)
                }
                if dynOutObj != "" {
                        outObj = append(outObj, dynOutObj)
                }

        case "gccgo":
                defunC := objdir + "_cgo_defun.c"
                defunObj := objdir + "_cgo_defun.o"
                if err := BuildToolchain.cc(b, a, defunObj, defunC); err != nil {
                        return nil, nil, err
                }
                outObj = append(outObj, defunObj)

        default:
                noCompiler()
        }

        // Double check the //go:cgo_ldflag comments in the generated files.
        // The compiler only permits such comments in files whose base name
        // starts with "_cgo_". Make sure that the comments in those files
        // are safe. This is a backstop against people somehow smuggling
        // such a comment into a file generated by cgo.
        if cfg.BuildToolchainName == "gc" && !cfg.BuildN {
                var flags []string
                for _, f := range outGo {
                        if !strings.HasPrefix(filepath.Base(f), "_cgo_") {
                                continue
                        }

                        src, err := os.ReadFile(f)
                        if err != nil {
                                return nil, nil, err
                        }

                        const cgoLdflag = "//go:cgo_ldflag"
                        idx := bytes.Index(src, []byte(cgoLdflag))
                        for idx >= 0 {
                                // We are looking at //go:cgo_ldflag.
                                // Find start of line.
                                start := bytes.LastIndex(src[:idx], []byte("\n"))
                                if start == -1 {
                                        start = 0
                                }

                                // Find end of line.
                                end := bytes.Index(src[idx:], []byte("\n"))
                                if end == -1 {
                                        end = len(src)
                                } else {
                                        end += idx
                                }

                                // Check for first line comment in line.
                                // We don't worry about /* */ comments,
                                // which normally won't appear in files
                                // generated by cgo.
                                commentStart := bytes.Index(src[start:], []byte("//"))
                                commentStart += start
                                // If that line comment is //go:cgo_ldflag,
                                // it's a match.
                                if bytes.HasPrefix(src[commentStart:], []byte(cgoLdflag)) {
                                        // Pull out the flag, and unquote it.
                                        // This is what the compiler does.
                                        flag := string(src[idx+len(cgoLdflag) : end])
                                        flag = strings.TrimSpace(flag)
                                        flag = strings.Trim(flag, `"`)
                                        flags = append(flags, flag)
                                }
                                src = src[end:]
                                idx = bytes.Index(src, []byte(cgoLdflag))
                        }
                }

                // We expect to find the contents of cgoLDFLAGS in flags.
                if len(cgoLDFLAGS) > 0 {
                outer:
                        for i := range flags {
                                for j, f := range cgoLDFLAGS {
                                        if f != flags[i+j] {
                                                continue outer
                                        }
                                }
                                flags = append(flags[:i], flags[i+len(cgoLDFLAGS):]...)
                                break
                        }
                }

                if err := checkLinkerFlags("LDFLAGS", "go:cgo_ldflag", flags); err != nil {
                        return nil, nil, err
                }
        }

        return outGo, outObj, nil
}

// dynimport creates a Go source file named importGo containing
// //go:cgo_import_dynamic directives for each symbol or library
// dynamically imported by the object files outObj.
// dynOutGo, if not empty, is a new Go file to build as part of the package.
// dynOutObj, if not empty, is a new file to add to the generated archive.
func (b *Builder) dynimport(a *Action, p *load.Package, objdir, importGo, cgoExe string, cflags, cgoLDFLAGS, outObj []string) (dynOutGo, dynOutObj string, err error) {
        cfile := objdir + "_cgo_main.c"
        ofile := objdir + "_cgo_main.o"
        if err := b.gcc(a, p, objdir, ofile, cflags, cfile); err != nil {
                return "", "", err
        }

        // Gather .syso files from this package and all (transitive) dependencies.
        var syso []string
        seen := make(map[*Action]bool)
        var gatherSyso func(*Action)
        gatherSyso = func(a1 *Action) {
                if seen[a1] {
                        return
                }
                seen[a1] = true
                if p1 := a1.Package; p1 != nil {
                        syso = append(syso, mkAbsFiles(p1.Dir, p1.SysoFiles)...)
                }
                for _, a2 := range a1.Deps {
                        gatherSyso(a2)
                }
        }
        gatherSyso(a)
        sort.Strings(syso)
        str.Uniq(&syso)
        linkobj := str.StringList(ofile, outObj, syso)
        dynobj := objdir + "_cgo_.o"

        ldflags := cgoLDFLAGS
        if (cfg.Goarch == "arm" && cfg.Goos == "linux") || cfg.Goos == "android" {
                if !str.Contains(ldflags, "-no-pie") {
                        // we need to use -pie for Linux/ARM to get accurate imported sym (added in https://golang.org/cl/5989058)
                        // this seems to be outdated, but we don't want to break existing builds depending on this (Issue 45940)
                        ldflags = append(ldflags, "-pie")
                }
                if str.Contains(ldflags, "-pie") && str.Contains(ldflags, "-static") {
                        // -static -pie doesn't make sense, and causes link errors.
                        // Issue 26197.
                        n := make([]string, 0, len(ldflags)-1)
                        for _, flag := range ldflags {
                                if flag != "-static" {
                                        n = append(n, flag)
                                }
                        }
                        ldflags = n
                }
        }
        if err := b.gccld(a, p, objdir, dynobj, ldflags, linkobj); err != nil {
                // We only need this information for internal linking.
                // If this link fails, mark the object as requiring
                // external linking. This link can fail for things like
                // syso files that have unexpected dependencies.
                // cmd/link explicitly looks for the name "dynimportfail".
                // See issue #52863.
                fail := objdir + "dynimportfail"
                if cfg.BuildN || cfg.BuildX {
                        b.Showcmd("", "echo > %s", fail)
                }
                if !cfg.BuildN {
                        if err := os.WriteFile(fail, nil, 0666); err != nil {
                                return "", "", err
                        }
                }
                return "", fail, nil
        }

        // cgo -dynimport
        var cgoflags []string
        if p.Standard && p.ImportPath == "runtime/cgo" {
                cgoflags = []string{"-dynlinker"} // record path to dynamic linker
        }
        err = b.run(a, base.Cwd(), p.ImportPath, b.cCompilerEnv(), cfg.BuildToolexec, cgoExe, "-dynpackage", p.Name, "-dynimport", dynobj, "-dynout", importGo, cgoflags)
        if err != nil {
                return "", "", err
        }
        return importGo, "", nil
}

// Run SWIG on all SWIG input files.
// TODO: Don't build a shared library, once SWIG emits the necessary
// pragmas for external linking.
func (b *Builder) swig(a *Action, p *load.Package, objdir string, pcCFLAGS []string) (outGo, outC, outCXX []string, err error) {
        if err := b.swigVersionCheck(); err != nil {
                return nil, nil, nil, err
        }

        intgosize, err := b.swigIntSize(objdir)
        if err != nil {
                return nil, nil, nil, err
        }

        for _, f := range p.SwigFiles {
                goFile, cFile, err := b.swigOne(a, p, f, objdir, pcCFLAGS, false, intgosize)
                if err != nil {
                        return nil, nil, nil, err
                }
                if goFile != "" {
                        outGo = append(outGo, goFile)
                }
                if cFile != "" {
                        outC = append(outC, cFile)
                }
        }
        for _, f := range p.SwigCXXFiles {
                goFile, cxxFile, err := b.swigOne(a, p, f, objdir, pcCFLAGS, true, intgosize)
                if err != nil {
                        return nil, nil, nil, err
                }
                if goFile != "" {
                        outGo = append(outGo, goFile)
                }
                if cxxFile != "" {
                        outCXX = append(outCXX, cxxFile)
                }
        }
        return outGo, outC, outCXX, nil
}

// Make sure SWIG is new enough.
var (
        swigCheckOnce sync.Once
        swigCheck     error
)

func (b *Builder) swigDoVersionCheck() error {
        out, err := b.runOut(nil, ".", nil, "swig", "-version")
        if err != nil {
                return err
        }
        re := regexp.MustCompile(`[vV]ersion +(\d+)([.]\d+)?([.]\d+)?`)
        matches := re.FindSubmatch(out)
        if matches == nil {
                // Can't find version number; hope for the best.
                return nil
        }

        major, err := strconv.Atoi(string(matches[1]))
        if err != nil {
                // Can't find version number; hope for the best.
                return nil
        }
        const errmsg = "must have SWIG version >= 3.0.6"
        if major < 3 {
                return errors.New(errmsg)
        }
        if major > 3 {
                // 4.0 or later
                return nil
        }

        // We have SWIG version 3.x.
        if len(matches[2]) > 0 {
                minor, err := strconv.Atoi(string(matches[2][1:]))
                if err != nil {
                        return nil
                }
                if minor > 0 {
                        // 3.1 or later
                        return nil
                }
        }

        // We have SWIG version 3.0.x.
        if len(matches[3]) > 0 {
                patch, err := strconv.Atoi(string(matches[3][1:]))
                if err != nil {
                        return nil
                }
                if patch < 6 {
                        // Before 3.0.6.
                        return errors.New(errmsg)
                }
        }

        return nil
}

func (b *Builder) swigVersionCheck() error {
        swigCheckOnce.Do(func() {
                swigCheck = b.swigDoVersionCheck()
        })
        return swigCheck
}

// Find the value to pass for the -intgosize option to swig.
var (
        swigIntSizeOnce  sync.Once
        swigIntSize      string
        swigIntSizeError error
)

// This code fails to build if sizeof(int) <= 32
const swigIntSizeCode = `
package main
const i int = 1 << 32
`

// Determine the size of int on the target system for the -intgosize option
// of swig >= 2.0.9. Run only once.
func (b *Builder) swigDoIntSize(objdir string) (intsize string, err error) {
        if cfg.BuildN {
                return "$INTBITS", nil
        }
        src := filepath.Join(b.WorkDir, "swig_intsize.go")
        if err = os.WriteFile(src, []byte(swigIntSizeCode), 0666); err != nil {
                return
        }
        srcs := []string{src}

        p := load.GoFilesPackage(context.TODO(), load.PackageOpts{}, srcs)

        if _, _, e := BuildToolchain.gc(b, &Action{Mode: "swigDoIntSize", Package: p, Objdir: objdir}, "", nil, nil, "", false, srcs); e != nil {
                return "32", nil
        }
        return "64", nil
}

// Determine the size of int on the target system for the -intgosize option
// of swig >= 2.0.9.
func (b *Builder) swigIntSize(objdir string) (intsize string, err error) {
        swigIntSizeOnce.Do(func() {
                swigIntSize, swigIntSizeError = b.swigDoIntSize(objdir)
        })
        return swigIntSize, swigIntSizeError
}

// Run SWIG on one SWIG input file.
func (b *Builder) swigOne(a *Action, p *load.Package, file, objdir string, pcCFLAGS []string, cxx bool, intgosize string) (outGo, outC string, err error) {
        cgoCPPFLAGS, cgoCFLAGS, cgoCXXFLAGS, _, _, err := b.CFlags(p)
        if err != nil {
                return "", "", err
        }

        var cflags []string
        if cxx {
                cflags = str.StringList(cgoCPPFLAGS, pcCFLAGS, cgoCXXFLAGS)
        } else {
                cflags = str.StringList(cgoCPPFLAGS, pcCFLAGS, cgoCFLAGS)
        }

        n := 5 // length of ".swig"
        if cxx {
                n = 8 // length of ".swigcxx"
        }
        base := file[:len(file)-n]
        goFile := base + ".go"
        gccBase := base + "_wrap."
        gccExt := "c"
        if cxx {
                gccExt = "cxx"
        }

        gccgo := cfg.BuildToolchainName == "gccgo"

        // swig
        args := []string{
                "-go",
                "-cgo",
                "-intgosize", intgosize,
                "-module", base,
                "-o", objdir + gccBase + gccExt,
                "-outdir", objdir,
        }

        for _, f := range cflags {
                if len(f) > 3 && f[:2] == "-I" {
                        args = append(args, f)
                }
        }

        if gccgo {
                args = append(args, "-gccgo")
                if pkgpath := gccgoPkgpath(p); pkgpath != "" {
                        args = append(args, "-go-pkgpath", pkgpath)
                }
        }
        if cxx {
                args = append(args, "-c++")
        }

        out, err := b.runOut(a, p.Dir, nil, "swig", args, file)
        if err != nil {
                if len(out) > 0 {
                        if bytes.Contains(out, []byte("-intgosize")) || bytes.Contains(out, []byte("-cgo")) {
                                return "", "", errors.New("must have SWIG version >= 3.0.6")
                        }
                        // swig error
                        return "", "", errors.New(fmt.Sprint(formatOutput(b.WorkDir, p.Dir, p.Desc(), b.processOutput(out))))
                }
                return "", "", err
        }
        if len(out) > 0 {
                b.showOutput(a, p.Dir, p.Desc(), b.processOutput(out)) // swig warning
        }

        // If the input was x.swig, the output is x.go in the objdir.
        // But there might be an x.go in the original dir too, and if it
        // uses cgo as well, cgo will be processing both and will
        // translate both into x.cgo1.go in the objdir, overwriting one.
        // Rename x.go to _x_swig.go to avoid this problem.
        // We ignore files in the original dir that begin with underscore
        // so _x_swig.go cannot conflict with an original file we were
        // going to compile.
        goFile = objdir + goFile
        newGoFile := objdir + "_" + base + "_swig.go"
        if err := os.Rename(goFile, newGoFile); err != nil {
                return "", "", err
        }
        return newGoFile, objdir + gccBase + gccExt, nil
}

// disableBuildID adjusts a linker command line to avoid creating a
// build ID when creating an object file rather than an executable or
// shared library. Some systems, such as Ubuntu, always add
// --build-id to every link, but we don't want a build ID when we are
// producing an object file. On some of those system a plain -r (not
// -Wl,-r) will turn off --build-id, but clang 3.0 doesn't support a
// plain -r. I don't know how to turn off --build-id when using clang
// other than passing a trailing --build-id=none. So that is what we
// do, but only on systems likely to support it, which is to say,
// systems that normally use gold or the GNU linker.
func (b *Builder) disableBuildID(ldflags []string) []string {
        switch cfg.Goos {
        case "android", "dragonfly", "linux", "netbsd":
                ldflags = append(ldflags, "-Wl,--build-id=none")
        }
        return ldflags
}

// mkAbsFiles converts files into a list of absolute files,
// assuming they were originally relative to dir,
// and returns that new list.
func mkAbsFiles(dir string, files []string) []string {
        abs := make([]string, len(files))
        for i, f := range files {
                if !filepath.IsAbs(f) {
                        f = filepath.Join(dir, f)
                }
                abs[i] = f
        }
        return abs
}

// passLongArgsInResponseFiles modifies cmd such that, for
// certain programs, long arguments are passed in "response files", a
// file on disk with the arguments, with one arg per line. An actual
// argument starting with '@' means that the rest of the argument is
// a filename of arguments to expand.
//
// See issues 18468 (Windows) and 37768 (Darwin).
func passLongArgsInResponseFiles(cmd *exec.Cmd) (cleanup func()) {
        cleanup = func() {} // no cleanup by default

        var argLen int
        for _, arg := range cmd.Args {
                argLen += len(arg)
        }

        // If we're not approaching 32KB of args, just pass args normally.
        // (use 30KB instead to be conservative; not sure how accounting is done)
        if !useResponseFile(cmd.Path, argLen) {
                return
        }

        tf, err := os.CreateTemp("", "args")
        if err != nil {
                log.Fatalf("error writing long arguments to response file: %v", err)
        }
        cleanup = func() { os.Remove(tf.Name()) }
        var buf bytes.Buffer
        for _, arg := range cmd.Args[1:] {
                fmt.Fprintf(&buf, "%s\n", encodeArg(arg))
        }
        if _, err := tf.Write(buf.Bytes()); err != nil {
                tf.Close()
                cleanup()
                log.Fatalf("error writing long arguments to response file: %v", err)
        }
        if err := tf.Close(); err != nil {
                cleanup()
                log.Fatalf("error writing long arguments to response file: %v", err)
        }
        cmd.Args = []string{cmd.Args[0], "@" + tf.Name()}
        return cleanup
}

func useResponseFile(path string, argLen int) bool {
        // Unless the program uses objabi.Flagparse, which understands
        // response files, don't use response files.
        // TODO: Note that other toolchains like CC are missing here for now.
        prog := strings.TrimSuffix(filepath.Base(path), ".exe")
        switch prog {
        case "compile", "link", "cgo", "asm":
        default:
                return false
        }

        if argLen > sys.ExecArgLengthLimit {
                return true
        }

        // On the Go build system, use response files about 10% of the
        // time, just to exercise this codepath.
        isBuilder := os.Getenv("GO_BUILDER_NAME") != ""
        if isBuilder && rand.Intn(10) == 0 {
                return true
        }

        return false
}

// encodeArg encodes an argument for response file writing.
func encodeArg(arg string) string {
        // If there aren't any characters we need to reencode, fastpath out.
        if !strings.ContainsAny(arg, "\\\n") {
                return arg
        }
        var b strings.Builder
        for _, r := range arg {
                switch r {
                case '\\':
                        b.WriteByte('\\')
                        b.WriteByte('\\')
                case '\n':
                        b.WriteByte('\\')
                        b.WriteByte('n')
                default:
                        b.WriteRune(r)
                }
        }
        return b.String()
}