[gostls13.git] / src / cmd / go / internal / work / action.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 creation (planning).

package work

import (
        "bufio"
        "bytes"
        "container/heap"
        "context"
        "debug/elf"
        "encoding/json"
        "fmt"
        "os"
        "path/filepath"
        "strings"
        "sync"
        "time"

        "cmd/go/internal/base"
        "cmd/go/internal/cache"
        "cmd/go/internal/cfg"
        "cmd/go/internal/load"
        "cmd/go/internal/robustio"
        "cmd/go/internal/trace"
        "cmd/internal/buildid"
)

// A Builder holds global state about a build.
// It does not hold per-package state, because we
// build packages in parallel, and the builder is shared.
type Builder struct {
        WorkDir     string               // the temporary work directory (ends in filepath.Separator)
        actionCache map[cacheKey]*Action // a cache of already-constructed actions
        mkdirCache  map[string]bool      // a cache of created directories
        flagCache   map[[2]string]bool   // a cache of supported compiler flags
        Print       func(args ...any) (int, error)

        IsCmdList           bool // running as part of go list; set p.Stale and additional fields below
        NeedError           bool // list needs p.Error
        NeedExport          bool // list needs p.Export
        NeedCompiledGoFiles bool // list needs p.CompiledGoFiles
        AllowErrors         bool // errors don't immediately exit the program

        objdirSeq int // counter for NewObjdir
        pkgSeq    int

        output    sync.Mutex
        scriptDir string // current directory in printed script

        exec      sync.Mutex
        readySema chan bool
        ready     actionQueue

        id           sync.Mutex
        toolIDCache  map[string]string // tool name -> tool ID
        buildIDCache map[string]string // file name -> build ID
}

// NOTE: Much of Action would not need to be exported if not for test.
// Maybe test functionality should move into this package too?

// An Action represents a single action in the action graph.
type Action struct {
        Mode       string                                         // description of action operation
        Package    *load.Package                                  // the package this action works on
        Deps       []*Action                                      // actions that must happen before this one
        Func       func(*Builder, context.Context, *Action) error // the action itself (nil = no-op)
        IgnoreFail bool                                           // whether to run f even if dependencies fail
        TestOutput *bytes.Buffer                                  // test output buffer
        Args       []string                                       // additional args for runProgram

        triggers []*Action // inverse of deps

        buggyInstall bool // is this a buggy install (see -linkshared)?

        TryCache func(*Builder, *Action) bool // callback for cache bypass

        // Generated files, directories.
        Objdir   string         // directory for intermediate objects
        Target   string         // goal of the action: the created package or executable
        built    string         // the actual created package or executable
        actionID cache.ActionID // cache ID of action input
        buildID  string         // build ID of action output

        VetxOnly  bool       // Mode=="vet": only being called to supply info about dependencies
        needVet   bool       // Mode=="build": need to fill in vet config
        needBuild bool       // Mode=="build": need to do actual build (can be false if needVet is true)
        vetCfg    *vetConfig // vet config
        output    []byte     // output redirect buffer (nil means use b.Print)

        // Execution state.
        pending      int               // number of deps yet to complete
        priority     int               // relative execution priority
        Failed       bool              // whether the action failed
        json         *actionJSON       // action graph information
        nonGoOverlay map[string]string // map from non-.go source files to copied files in objdir. Nil if no overlay is used.
        traceSpan    *trace.Span
}

// BuildActionID returns the action ID section of a's build ID.
func (a *Action) BuildActionID() string { return actionID(a.buildID) }

// BuildContentID returns the content ID section of a's build ID.
func (a *Action) BuildContentID() string { return contentID(a.buildID) }

// BuildID returns a's build ID.
func (a *Action) BuildID() string { return a.buildID }

// BuiltTarget returns the actual file that was built. This differs
// from Target when the result was cached.
func (a *Action) BuiltTarget() string { return a.built }

// An actionQueue is a priority queue of actions.
type actionQueue []*Action

// Implement heap.Interface
func (q *actionQueue) Len() int           { return len(*q) }
func (q *actionQueue) Swap(i, j int)      { (*q)[i], (*q)[j] = (*q)[j], (*q)[i] }
func (q *actionQueue) Less(i, j int) bool { return (*q)[i].priority < (*q)[j].priority }
func (q *actionQueue) Push(x any)         { *q = append(*q, x.(*Action)) }
func (q *actionQueue) Pop() any {
        n := len(*q) - 1
        x := (*q)[n]
        *q = (*q)[:n]
        return x
}

func (q *actionQueue) push(a *Action) {
        if a.json != nil {
                a.json.TimeReady = time.Now()
        }
        heap.Push(q, a)
}

func (q *actionQueue) pop() *Action {
        return heap.Pop(q).(*Action)
}

type actionJSON struct {
        ID         int
        Mode       string
        Package    string
        Deps       []int     `json:",omitempty"`
        IgnoreFail bool      `json:",omitempty"`
        Args       []string  `json:",omitempty"`
        Link       bool      `json:",omitempty"`
        Objdir     string    `json:",omitempty"`
        Target     string    `json:",omitempty"`
        Priority   int       `json:",omitempty"`
        Failed     bool      `json:",omitempty"`
        Built      string    `json:",omitempty"`
        VetxOnly   bool      `json:",omitempty"`
        NeedVet    bool      `json:",omitempty"`
        NeedBuild  bool      `json:",omitempty"`
        ActionID   string    `json:",omitempty"`
        BuildID    string    `json:",omitempty"`
        TimeReady  time.Time `json:",omitempty"`
        TimeStart  time.Time `json:",omitempty"`
        TimeDone   time.Time `json:",omitempty"`

        Cmd     []string      // `json:",omitempty"`
        CmdReal time.Duration `json:",omitempty"`
        CmdUser time.Duration `json:",omitempty"`
        CmdSys  time.Duration `json:",omitempty"`
}

// cacheKey is the key for the action cache.
type cacheKey struct {
        mode string
        p    *load.Package
}

func actionGraphJSON(a *Action) string {
        var workq []*Action
        var inWorkq = make(map[*Action]int)

        add := func(a *Action) {
                if _, ok := inWorkq[a]; ok {
                        return
                }
                inWorkq[a] = len(workq)
                workq = append(workq, a)
        }
        add(a)

        for i := 0; i < len(workq); i++ {
                for _, dep := range workq[i].Deps {
                        add(dep)
                }
        }

        var list []*actionJSON
        for id, a := range workq {
                if a.json == nil {
                        a.json = &actionJSON{
                                Mode:       a.Mode,
                                ID:         id,
                                IgnoreFail: a.IgnoreFail,
                                Args:       a.Args,
                                Objdir:     a.Objdir,
                                Target:     a.Target,
                                Failed:     a.Failed,
                                Priority:   a.priority,
                                Built:      a.built,
                                VetxOnly:   a.VetxOnly,
                                NeedBuild:  a.needBuild,
                                NeedVet:    a.needVet,
                        }
                        if a.Package != nil {
                                // TODO(rsc): Make this a unique key for a.Package somehow.
                                a.json.Package = a.Package.ImportPath
                        }
                        for _, a1 := range a.Deps {
                                a.json.Deps = append(a.json.Deps, inWorkq[a1])
                        }
                }
                list = append(list, a.json)
        }

        js, err := json.MarshalIndent(list, "", "\t")
        if err != nil {
                fmt.Fprintf(os.Stderr, "go: writing debug action graph: %v\n", err)
                return ""
        }
        return string(js)
}

// BuildMode specifies the build mode:
// are we just building things or also installing the results?
type BuildMode int

const (
        ModeBuild BuildMode = iota
        ModeInstall
        ModeBuggyInstall

        ModeVetOnly = 1 << 8
)

// NewBuilder returns a new Builder ready for use.
//
// If workDir is the empty string, NewBuilder creates a WorkDir if needed
// and arranges for it to be removed in case of an unclean exit.
// The caller must Close the builder explicitly to clean up the WorkDir
// before a clean exit.
func NewBuilder(workDir string) *Builder {
        b := new(Builder)

        b.Print = func(a ...any) (int, error) {
                return fmt.Fprint(os.Stderr, a...)
        }
        b.actionCache = make(map[cacheKey]*Action)
        b.mkdirCache = make(map[string]bool)
        b.toolIDCache = make(map[string]string)
        b.buildIDCache = make(map[string]string)

        if workDir != "" {
                b.WorkDir = workDir
        } else if cfg.BuildN {
                b.WorkDir = "$WORK"
        } else {
                if !buildInitStarted {
                        panic("internal error: NewBuilder called before BuildInit")
                }
                tmp, err := os.MkdirTemp(cfg.Getenv("GOTMPDIR"), "go-build")
                if err != nil {
                        base.Fatalf("go: creating work dir: %v", err)
                }
                if !filepath.IsAbs(tmp) {
                        abs, err := filepath.Abs(tmp)
                        if err != nil {
                                os.RemoveAll(tmp)
                                base.Fatalf("go: creating work dir: %v", err)
                        }
                        tmp = abs
                }
                b.WorkDir = tmp
                builderWorkDirs.Store(b, b.WorkDir)
                if cfg.BuildX || cfg.BuildWork {
                        fmt.Fprintf(os.Stderr, "WORK=%s\n", b.WorkDir)
                }
        }

        if err := CheckGOOSARCHPair(cfg.Goos, cfg.Goarch); err != nil {
                fmt.Fprintf(os.Stderr, "go: %v\n", err)
                base.SetExitStatus(2)
                base.Exit()
        }

        for _, tag := range cfg.BuildContext.BuildTags {
                if strings.Contains(tag, ",") {
                        fmt.Fprintf(os.Stderr, "go: -tags space-separated list contains comma\n")
                        base.SetExitStatus(2)
                        base.Exit()
                }
        }

        return b
}

var builderWorkDirs sync.Map // *Builder → WorkDir

func (b *Builder) Close() error {
        wd, ok := builderWorkDirs.Load(b)
        if !ok {
                return nil
        }
        defer builderWorkDirs.Delete(b)

        if b.WorkDir != wd.(string) {
                base.Errorf("go: internal error: Builder WorkDir unexpectedly changed from %s to %s", wd, b.WorkDir)
        }

        if !cfg.BuildWork {
                if err := robustio.RemoveAll(b.WorkDir); err != nil {
                        return err
                }
        }
        b.WorkDir = ""
        return nil
}

func closeBuilders() {
        leakedBuilders := 0
        builderWorkDirs.Range(func(bi, _ any) bool {
                leakedBuilders++
                if err := bi.(*Builder).Close(); err != nil {
                        base.Errorf("go: %v", err)
                }
                return true
        })

        if leakedBuilders > 0 && base.GetExitStatus() == 0 {
                fmt.Fprintf(os.Stderr, "go: internal error: Builder leaked on successful exit\n")
                base.SetExitStatus(1)
        }
}

func CheckGOOSARCHPair(goos, goarch string) error {
        if _, ok := cfg.OSArchSupportsCgo[goos+"/"+goarch]; !ok && cfg.BuildContext.Compiler == "gc" {
                return fmt.Errorf("unsupported GOOS/GOARCH pair %s/%s", goos, goarch)
        }
        return nil
}

// NewObjdir returns the name of a fresh object directory under b.WorkDir.
// It is up to the caller to call b.Mkdir on the result at an appropriate time.
// The result ends in a slash, so that file names in that directory
// can be constructed with direct string addition.
//
// NewObjdir must be called only from a single goroutine at a time,
// so it is safe to call during action graph construction, but it must not
// be called during action graph execution.
func (b *Builder) NewObjdir() string {
        b.objdirSeq++
        return filepath.Join(b.WorkDir, fmt.Sprintf("b%03d", b.objdirSeq)) + string(filepath.Separator)
}

// readpkglist returns the list of packages that were built into the shared library
// at shlibpath. For the native toolchain this list is stored, newline separated, in
// an ELF note with name "Go\x00\x00" and type 1. For GCCGO it is extracted from the
// .go_export section.
func readpkglist(shlibpath string) (pkgs []*load.Package) {
        var stk load.ImportStack
        if cfg.BuildToolchainName == "gccgo" {
                f, _ := elf.Open(shlibpath)
                sect := f.Section(".go_export")
                data, _ := sect.Data()
                scanner := bufio.NewScanner(bytes.NewBuffer(data))
                for scanner.Scan() {
                        t := scanner.Text()
                        var found bool
                        if t, found = strings.CutPrefix(t, "pkgpath "); found {
                                t = strings.TrimSuffix(t, ";")
                                pkgs = append(pkgs, load.LoadImportWithFlags(t, base.Cwd(), nil, &stk, nil, 0))
                        }
                }
        } else {
                pkglistbytes, err := buildid.ReadELFNote(shlibpath, "Go\x00\x00", 1)
                if err != nil {
                        base.Fatalf("readELFNote failed: %v", err)
                }
                scanner := bufio.NewScanner(bytes.NewBuffer(pkglistbytes))
                for scanner.Scan() {
                        t := scanner.Text()
                        pkgs = append(pkgs, load.LoadImportWithFlags(t, base.Cwd(), nil, &stk, nil, 0))
                }
        }
        return
}

// cacheAction looks up {mode, p} in the cache and returns the resulting action.
// If the cache has no such action, f() is recorded and returned.
// TODO(rsc): Change the second key from *load.Package to interface{},
// to make the caching in linkShared less awkward?
func (b *Builder) cacheAction(mode string, p *load.Package, f func() *Action) *Action {
        a := b.actionCache[cacheKey{mode, p}]
        if a == nil {
                a = f()
                b.actionCache[cacheKey{mode, p}] = a
        }
        return a
}

// AutoAction returns the "right" action for go build or go install of p.
func (b *Builder) AutoAction(mode, depMode BuildMode, p *load.Package) *Action {
        if p.Name == "main" {
                return b.LinkAction(mode, depMode, p)
        }
        return b.CompileAction(mode, depMode, p)
}

// CompileAction returns the action for compiling and possibly installing
// (according to mode) the given package. The resulting action is only
// for building packages (archives), never for linking executables.
// depMode is the action (build or install) to use when building dependencies.
// To turn package main into an executable, call b.Link instead.
func (b *Builder) CompileAction(mode, depMode BuildMode, p *load.Package) *Action {
        vetOnly := mode&ModeVetOnly != 0
        mode &^= ModeVetOnly

        if mode != ModeBuild && (p.Internal.Local || p.Module != nil) && p.Target == "" {
                // Imported via local path or using modules. No permanent target.
                mode = ModeBuild
        }
        if mode != ModeBuild && p.Name == "main" {
                // We never install the .a file for a main package.
                mode = ModeBuild
        }

        // Construct package build action.
        a := b.cacheAction("build", p, func() *Action {
                a := &Action{
                        Mode:    "build",
                        Package: p,
                        Func:    (*Builder).build,
                        Objdir:  b.NewObjdir(),
                }

                if p.Error == nil || !p.Error.IsImportCycle {
                        for _, p1 := range p.Internal.Imports {
                                a.Deps = append(a.Deps, b.CompileAction(depMode, depMode, p1))
                        }
                }

                if p.Standard {
                        switch p.ImportPath {
                        case "builtin", "unsafe":
                                // Fake packages - nothing to build.
                                a.Mode = "built-in package"
                                a.Func = nil
                                return a
                        }

                        // gccgo standard library is "fake" too.
                        if cfg.BuildToolchainName == "gccgo" {
                                // the target name is needed for cgo.
                                a.Mode = "gccgo stdlib"
                                a.Target = p.Target
                                a.Func = nil
                                return a
                        }
                }

                return a
        })

        // Find the build action; the cache entry may have been replaced
        // by the install action during (*Builder).installAction.
        buildAction := a
        switch buildAction.Mode {
        case "build", "built-in package", "gccgo stdlib":
                // ok
        case "build-install":
                buildAction = a.Deps[0]
        default:
                panic("lost build action: " + buildAction.Mode)
        }
        buildAction.needBuild = buildAction.needBuild || !vetOnly

        // Construct install action.
        if mode == ModeInstall || mode == ModeBuggyInstall {
                a = b.installAction(a, mode)
        }

        return a
}

// VetAction returns the action for running go vet on package p.
// It depends on the action for compiling p.
// If the caller may be causing p to be installed, it is up to the caller
// to make sure that the install depends on (runs after) vet.
func (b *Builder) VetAction(mode, depMode BuildMode, p *load.Package) *Action {
        a := b.vetAction(mode, depMode, p)
        a.VetxOnly = false
        return a
}

func (b *Builder) vetAction(mode, depMode BuildMode, p *load.Package) *Action {
        // Construct vet action.
        a := b.cacheAction("vet", p, func() *Action {
                a1 := b.CompileAction(mode|ModeVetOnly, depMode, p)

                // vet expects to be able to import "fmt".
                var stk load.ImportStack
                stk.Push("vet")
                p1 := load.LoadImportWithFlags("fmt", p.Dir, p, &stk, nil, 0)
                stk.Pop()
                aFmt := b.CompileAction(ModeBuild, depMode, p1)

                var deps []*Action
                if a1.buggyInstall {
                        // (*Builder).vet expects deps[0] to be the package
                        // and deps[1] to be "fmt". If we see buggyInstall
                        // here then a1 is an install of a shared library,
                        // and the real package is a1.Deps[0].
                        deps = []*Action{a1.Deps[0], aFmt, a1}
                } else {
                        deps = []*Action{a1, aFmt}
                }
                for _, p1 := range p.Internal.Imports {
                        deps = append(deps, b.vetAction(mode, depMode, p1))
                }

                a := &Action{
                        Mode:       "vet",
                        Package:    p,
                        Deps:       deps,
                        Objdir:     a1.Objdir,
                        VetxOnly:   true,
                        IgnoreFail: true, // it's OK if vet of dependencies "fails" (reports problems)
                }
                if a1.Func == nil {
                        // Built-in packages like unsafe.
                        return a
                }
                deps[0].needVet = true
                a.Func = (*Builder).vet
                return a
        })
        return a
}

// LinkAction returns the action for linking p into an executable
// and possibly installing the result (according to mode).
// depMode is the action (build or install) to use when compiling dependencies.
func (b *Builder) LinkAction(mode, depMode BuildMode, p *load.Package) *Action {
        // Construct link action.
        a := b.cacheAction("link", p, func() *Action {
                a := &Action{
                        Mode:    "link",
                        Package: p,
                }

                a1 := b.CompileAction(ModeBuild, depMode, p)
                a.Func = (*Builder).link
                a.Deps = []*Action{a1}
                a.Objdir = a1.Objdir

                // An executable file. (This is the name of a temporary file.)
                // Because we run the temporary file in 'go run' and 'go test',
                // the name will show up in ps listings. If the caller has specified
                // a name, use that instead of a.out. The binary is generated
                // in an otherwise empty subdirectory named exe to avoid
                // naming conflicts. The only possible conflict is if we were
                // to create a top-level package named exe.
                name := "a.out"
                if p.Internal.ExeName != "" {
                        name = p.Internal.ExeName
                } else if (cfg.Goos == "darwin" || cfg.Goos == "windows") && cfg.BuildBuildmode == "c-shared" && p.Target != "" {
                        // On OS X, the linker output name gets recorded in the
                        // shared library's LC_ID_DYLIB load command.
                        // The code invoking the linker knows to pass only the final
                        // path element. Arrange that the path element matches what
                        // we'll install it as; otherwise the library is only loadable as "a.out".
                        // On Windows, DLL file name is recorded in PE file
                        // export section, so do like on OS X.
                        _, name = filepath.Split(p.Target)
                }
                a.Target = a.Objdir + filepath.Join("exe", name) + cfg.ExeSuffix
                a.built = a.Target
                b.addTransitiveLinkDeps(a, a1, "")

                // Sequence the build of the main package (a1) strictly after the build
                // of all other dependencies that go into the link. It is likely to be after
                // them anyway, but just make sure. This is required by the build ID-based
                // shortcut in (*Builder).useCache(a1), which will call b.linkActionID(a).
                // In order for that linkActionID call to compute the right action ID, all the
                // dependencies of a (except a1) must have completed building and have
                // recorded their build IDs.
                a1.Deps = append(a1.Deps, &Action{Mode: "nop", Deps: a.Deps[1:]})
                return a
        })

        if mode == ModeInstall || mode == ModeBuggyInstall {
                a = b.installAction(a, mode)
        }

        return a
}

// installAction returns the action for installing the result of a1.
func (b *Builder) installAction(a1 *Action, mode BuildMode) *Action {
        // Because we overwrite the build action with the install action below,
        // a1 may already be an install action fetched from the "build" cache key,
        // and the caller just doesn't realize.
        if strings.HasSuffix(a1.Mode, "-install") {
                if a1.buggyInstall && mode == ModeInstall {
                        //  Congratulations! The buggy install is now a proper install.
                        a1.buggyInstall = false
                }
                return a1
        }

        // If there's no actual action to build a1,
        // there's nothing to install either.
        // This happens if a1 corresponds to reusing an already-built object.
        if a1.Func == nil {
                return a1
        }

        p := a1.Package
        return b.cacheAction(a1.Mode+"-install", p, func() *Action {
                // The install deletes the temporary build result,
                // so we need all other actions, both past and future,
                // that attempt to depend on the build to depend instead
                // on the install.

                // Make a private copy of a1 (the build action),
                // no longer accessible to any other rules.
                buildAction := new(Action)
                *buildAction = *a1

                // Overwrite a1 with the install action.
                // This takes care of updating past actions that
                // point at a1 for the build action; now they will
                // point at a1 and get the install action.
                // We also leave a1 in the action cache as the result
                // for "build", so that actions not yet created that
                // try to depend on the build will instead depend
                // on the install.
                *a1 = Action{
                        Mode:    buildAction.Mode + "-install",
                        Func:    BuildInstallFunc,
                        Package: p,
                        Objdir:  buildAction.Objdir,
                        Deps:    []*Action{buildAction},
                        Target:  p.Target,
                        built:   p.Target,

                        buggyInstall: mode == ModeBuggyInstall,
                }

                b.addInstallHeaderAction(a1)
                return a1
        })
}

// addTransitiveLinkDeps adds to the link action a all packages
// that are transitive dependencies of a1.Deps.
// That is, if a is a link of package main, a1 is the compile of package main
// and a1.Deps is the actions for building packages directly imported by
// package main (what the compiler needs). The linker needs all packages
// transitively imported by the whole program; addTransitiveLinkDeps
// makes sure those are present in a.Deps.
// If shlib is non-empty, then a corresponds to the build and installation of shlib,
// so any rebuild of shlib should not be added as a dependency.
func (b *Builder) addTransitiveLinkDeps(a, a1 *Action, shlib string) {
        // Expand Deps to include all built packages, for the linker.
        // Use breadth-first search to find rebuilt-for-test packages
        // before the standard ones.
        // TODO(rsc): Eliminate the standard ones from the action graph,
        // which will require doing a little bit more rebuilding.
        workq := []*Action{a1}
        haveDep := map[string]bool{}
        if a1.Package != nil {
                haveDep[a1.Package.ImportPath] = true
        }
        for i := 0; i < len(workq); i++ {
                a1 := workq[i]
                for _, a2 := range a1.Deps {
                        // TODO(rsc): Find a better discriminator than the Mode strings, once the dust settles.
                        if a2.Package == nil || (a2.Mode != "build-install" && a2.Mode != "build") || haveDep[a2.Package.ImportPath] {
                                continue
                        }
                        haveDep[a2.Package.ImportPath] = true
                        a.Deps = append(a.Deps, a2)
                        if a2.Mode == "build-install" {
                                a2 = a2.Deps[0] // walk children of "build" action
                        }
                        workq = append(workq, a2)
                }
        }

        // If this is go build -linkshared, then the link depends on the shared libraries
        // in addition to the packages themselves. (The compile steps do not.)
        if cfg.BuildLinkshared {
                haveShlib := map[string]bool{shlib: true}
                for _, a1 := range a.Deps {
                        p1 := a1.Package
                        if p1 == nil || p1.Shlib == "" || haveShlib[filepath.Base(p1.Shlib)] {
                                continue
                        }
                        haveShlib[filepath.Base(p1.Shlib)] = true
                        // TODO(rsc): The use of ModeInstall here is suspect, but if we only do ModeBuild,
                        // we'll end up building an overall library or executable that depends at runtime
                        // on other libraries that are out-of-date, which is clearly not good either.
                        // We call it ModeBuggyInstall to make clear that this is not right.
                        a.Deps = append(a.Deps, b.linkSharedAction(ModeBuggyInstall, ModeBuggyInstall, p1.Shlib, nil))
                }
        }
}

// addInstallHeaderAction adds an install header action to a, if needed.
// The action a should be an install action as generated by either
// b.CompileAction or b.LinkAction with mode=ModeInstall,
// and so a.Deps[0] is the corresponding build action.
func (b *Builder) addInstallHeaderAction(a *Action) {
        // Install header for cgo in c-archive and c-shared modes.
        p := a.Package
        if p.UsesCgo() && (cfg.BuildBuildmode == "c-archive" || cfg.BuildBuildmode == "c-shared") {
                hdrTarget := a.Target[:len(a.Target)-len(filepath.Ext(a.Target))] + ".h"
                if cfg.BuildContext.Compiler == "gccgo" && cfg.BuildO == "" {
                        // For the header file, remove the "lib"
                        // added by go/build, so we generate pkg.h
                        // rather than libpkg.h.
                        dir, file := filepath.Split(hdrTarget)
                        file = strings.TrimPrefix(file, "lib")
                        hdrTarget = filepath.Join(dir, file)
                }
                ah := &Action{
                        Mode:    "install header",
                        Package: a.Package,
                        Deps:    []*Action{a.Deps[0]},
                        Func:    (*Builder).installHeader,
                        Objdir:  a.Deps[0].Objdir,
                        Target:  hdrTarget,
                }
                a.Deps = append(a.Deps, ah)
        }
}

// buildmodeShared takes the "go build" action a1 into the building of a shared library of a1.Deps.
// That is, the input a1 represents "go build pkgs" and the result represents "go build -buildmode=shared pkgs".
func (b *Builder) buildmodeShared(mode, depMode BuildMode, args []string, pkgs []*load.Package, a1 *Action) *Action {
        name, err := libname(args, pkgs)
        if err != nil {
                base.Fatalf("%v", err)
        }
        return b.linkSharedAction(mode, depMode, name, a1)
}

// linkSharedAction takes a grouping action a1 corresponding to a list of built packages
// and returns an action that links them together into a shared library with the name shlib.
// If a1 is nil, shlib should be an absolute path to an existing shared library,
// and then linkSharedAction reads that library to find out the package list.
func (b *Builder) linkSharedAction(mode, depMode BuildMode, shlib string, a1 *Action) *Action {
        fullShlib := shlib
        shlib = filepath.Base(shlib)
        a := b.cacheAction("build-shlib "+shlib, nil, func() *Action {
                if a1 == nil {
                        // TODO(rsc): Need to find some other place to store config,
                        // not in pkg directory. See golang.org/issue/22196.
                        pkgs := readpkglist(fullShlib)
                        a1 = &Action{
                                Mode: "shlib packages",
                        }
                        for _, p := range pkgs {
                                a1.Deps = append(a1.Deps, b.CompileAction(mode, depMode, p))
                        }
                }

                // Fake package to hold ldflags.
                // As usual shared libraries are a kludgy, abstraction-violating special case:
                // we let them use the flags specified for the command-line arguments.
                p := &load.Package{}
                p.Internal.CmdlinePkg = true
                p.Internal.Ldflags = load.BuildLdflags.For(p)
                p.Internal.Gccgoflags = load.BuildGccgoflags.For(p)

                // Add implicit dependencies to pkgs list.
                // Currently buildmode=shared forces external linking mode, and
                // external linking mode forces an import of runtime/cgo (and
                // math on arm). So if it was not passed on the command line and
                // it is not present in another shared library, add it here.
                // TODO(rsc): Maybe this should only happen if "runtime" is in the original package set.
                // TODO(rsc): This should probably be changed to use load.LinkerDeps(p).
                // TODO(rsc): We don't add standard library imports for gccgo
                // because they are all always linked in anyhow.
                // Maybe load.LinkerDeps should be used and updated.
                a := &Action{
                        Mode:    "go build -buildmode=shared",
                        Package: p,
                        Objdir:  b.NewObjdir(),
                        Func:    (*Builder).linkShared,
                        Deps:    []*Action{a1},
                }
                a.Target = filepath.Join(a.Objdir, shlib)
                if cfg.BuildToolchainName != "gccgo" {
                        add := func(a1 *Action, pkg string, force bool) {
                                for _, a2 := range a1.Deps {
                                        if a2.Package != nil && a2.Package.ImportPath == pkg {
                                                return
                                        }
                                }
                                var stk load.ImportStack
                                p := load.LoadImportWithFlags(pkg, base.Cwd(), nil, &stk, nil, 0)
                                if p.Error != nil {
                                        base.Fatalf("load %s: %v", pkg, p.Error)
                                }
                                // Assume that if pkg (runtime/cgo or math)
                                // is already accounted for in a different shared library,
                                // then that shared library also contains runtime,
                                // so that anything we do will depend on that library,
                                // so we don't need to include pkg in our shared library.
                                if force || p.Shlib == "" || filepath.Base(p.Shlib) == pkg {
                                        a1.Deps = append(a1.Deps, b.CompileAction(depMode, depMode, p))
                                }
                        }
                        add(a1, "runtime/cgo", false)
                        if cfg.Goarch == "arm" {
                                add(a1, "math", false)
                        }

                        // The linker step still needs all the usual linker deps.
                        // (For example, the linker always opens runtime.a.)
                        for _, dep := range load.LinkerDeps(nil) {
                                add(a, dep, true)
                        }
                }
                b.addTransitiveLinkDeps(a, a1, shlib)
                return a
        })

        // Install result.
        if (mode == ModeInstall || mode == ModeBuggyInstall) && a.Func != nil {
                buildAction := a

                a = b.cacheAction("install-shlib "+shlib, nil, func() *Action {
                        // Determine the eventual install target.
                        // The install target is root/pkg/shlib, where root is the source root
                        // in which all the packages lie.
                        // TODO(rsc): Perhaps this cross-root check should apply to the full
                        // transitive package dependency list, not just the ones named
                        // on the command line?
                        pkgDir := a1.Deps[0].Package.Internal.Build.PkgTargetRoot
                        for _, a2 := range a1.Deps {
                                if dir := a2.Package.Internal.Build.PkgTargetRoot; dir != pkgDir {
                                        base.Fatalf("installing shared library: cannot use packages %s and %s from different roots %s and %s",
                                                a1.Deps[0].Package.ImportPath,
                                                a2.Package.ImportPath,
                                                pkgDir,
                                                dir)
                                }
                        }
                        // TODO(rsc): Find out and explain here why gccgo is different.
                        if cfg.BuildToolchainName == "gccgo" {
                                pkgDir = filepath.Join(pkgDir, "shlibs")
                        }
                        target := filepath.Join(pkgDir, shlib)

                        a := &Action{
                                Mode:   "go install -buildmode=shared",
                                Objdir: buildAction.Objdir,
                                Func:   BuildInstallFunc,
                                Deps:   []*Action{buildAction},
                                Target: target,
                        }
                        for _, a2 := range buildAction.Deps[0].Deps {
                                p := a2.Package
                                if p.Target == "" {
                                        continue
                                }
                                a.Deps = append(a.Deps, &Action{
                                        Mode:    "shlibname",
                                        Package: p,
                                        Func:    (*Builder).installShlibname,
                                        Target:  strings.TrimSuffix(p.Target, ".a") + ".shlibname",
                                        Deps:    []*Action{a.Deps[0]},
                                })
                        }
                        return a
                })
        }

        return a
}