[dev.boringcrypto] all: merge master (5b76343) into dev.boringcrypto

[gostls13.git] / src / go / build / build.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.

package build

import (
        "bytes"
        "errors"
        "fmt"
        "go/ast"
        "go/build/constraint"
        "go/doc"
        "go/token"
        exec "internal/execabs"
        "internal/goroot"
        "internal/goversion"
        "io"
        "io/fs"
        "io/ioutil"
        "os"
        pathpkg "path"
        "path/filepath"
        "runtime"
        "sort"
        "strconv"
        "strings"
        "unicode"
        "unicode/utf8"
)

// A Context specifies the supporting context for a build.
type Context struct {
        GOARCH string // target architecture
        GOOS   string // target operating system
        GOROOT string // Go root
        GOPATH string // Go path

        // Dir is the caller's working directory, or the empty string to use
        // the current directory of the running process. In module mode, this is used
        // to locate the main module.
        //
        // If Dir is non-empty, directories passed to Import and ImportDir must
        // be absolute.
        Dir string

        CgoEnabled  bool   // whether cgo files are included
        UseAllFiles bool   // use files regardless of +build lines, file names
        Compiler    string // compiler to assume when computing target paths

        // The build and release tags specify build constraints
        // that should be considered satisfied when processing +build lines.
        // Clients creating a new context may customize BuildTags, which
        // defaults to empty, but it is usually an error to customize ReleaseTags,
        // which defaults to the list of Go releases the current release is compatible with.
        // BuildTags is not set for the Default build Context.
        // In addition to the BuildTags and ReleaseTags, build constraints
        // consider the values of GOARCH and GOOS as satisfied tags.
        // The last element in ReleaseTags is assumed to be the current release.
        BuildTags   []string
        ReleaseTags []string

        // The install suffix specifies a suffix to use in the name of the installation
        // directory. By default it is empty, but custom builds that need to keep
        // their outputs separate can set InstallSuffix to do so. For example, when
        // using the race detector, the go command uses InstallSuffix = "race", so
        // that on a Linux/386 system, packages are written to a directory named
        // "linux_386_race" instead of the usual "linux_386".
        InstallSuffix string

        // By default, Import uses the operating system's file system calls
        // to read directories and files. To read from other sources,
        // callers can set the following functions. They all have default
        // behaviors that use the local file system, so clients need only set
        // the functions whose behaviors they wish to change.

        // JoinPath joins the sequence of path fragments into a single path.
        // If JoinPath is nil, Import uses filepath.Join.
        JoinPath func(elem ...string) string

        // SplitPathList splits the path list into a slice of individual paths.
        // If SplitPathList is nil, Import uses filepath.SplitList.
        SplitPathList func(list string) []string

        // IsAbsPath reports whether path is an absolute path.
        // If IsAbsPath is nil, Import uses filepath.IsAbs.
        IsAbsPath func(path string) bool

        // IsDir reports whether the path names a directory.
        // If IsDir is nil, Import calls os.Stat and uses the result's IsDir method.
        IsDir func(path string) bool

        // HasSubdir reports whether dir is lexically a subdirectory of
        // root, perhaps multiple levels below. It does not try to check
        // whether dir exists.
        // If so, HasSubdir sets rel to a slash-separated path that
        // can be joined to root to produce a path equivalent to dir.
        // If HasSubdir is nil, Import uses an implementation built on
        // filepath.EvalSymlinks.
        HasSubdir func(root, dir string) (rel string, ok bool)

        // ReadDir returns a slice of fs.FileInfo, sorted by Name,
        // describing the content of the named directory.
        // If ReadDir is nil, Import uses ioutil.ReadDir.
        ReadDir func(dir string) ([]fs.FileInfo, error)

        // OpenFile opens a file (not a directory) for reading.
        // If OpenFile is nil, Import uses os.Open.
        OpenFile func(path string) (io.ReadCloser, error)
}

// joinPath calls ctxt.JoinPath (if not nil) or else filepath.Join.
func (ctxt *Context) joinPath(elem ...string) string {
        if f := ctxt.JoinPath; f != nil {
                return f(elem...)
        }
        return filepath.Join(elem...)
}

// splitPathList calls ctxt.SplitPathList (if not nil) or else filepath.SplitList.
func (ctxt *Context) splitPathList(s string) []string {
        if f := ctxt.SplitPathList; f != nil {
                return f(s)
        }
        return filepath.SplitList(s)
}

// isAbsPath calls ctxt.IsAbsPath (if not nil) or else filepath.IsAbs.
func (ctxt *Context) isAbsPath(path string) bool {
        if f := ctxt.IsAbsPath; f != nil {
                return f(path)
        }
        return filepath.IsAbs(path)
}

// isDir calls ctxt.IsDir (if not nil) or else uses os.Stat.
func (ctxt *Context) isDir(path string) bool {
        if f := ctxt.IsDir; f != nil {
                return f(path)
        }
        fi, err := os.Stat(path)
        return err == nil && fi.IsDir()
}

// hasSubdir calls ctxt.HasSubdir (if not nil) or else uses
// the local file system to answer the question.
func (ctxt *Context) hasSubdir(root, dir string) (rel string, ok bool) {
        if f := ctxt.HasSubdir; f != nil {
                return f(root, dir)
        }

        // Try using paths we received.
        if rel, ok = hasSubdir(root, dir); ok {
                return
        }

        // Try expanding symlinks and comparing
        // expanded against unexpanded and
        // expanded against expanded.
        rootSym, _ := filepath.EvalSymlinks(root)
        dirSym, _ := filepath.EvalSymlinks(dir)

        if rel, ok = hasSubdir(rootSym, dir); ok {
                return
        }
        if rel, ok = hasSubdir(root, dirSym); ok {
                return
        }
        return hasSubdir(rootSym, dirSym)
}

// hasSubdir reports if dir is within root by performing lexical analysis only.
func hasSubdir(root, dir string) (rel string, ok bool) {
        const sep = string(filepath.Separator)
        root = filepath.Clean(root)
        if !strings.HasSuffix(root, sep) {
                root += sep
        }
        dir = filepath.Clean(dir)
        if !strings.HasPrefix(dir, root) {
                return "", false
        }
        return filepath.ToSlash(dir[len(root):]), true
}

// readDir calls ctxt.ReadDir (if not nil) or else ioutil.ReadDir.
func (ctxt *Context) readDir(path string) ([]fs.FileInfo, error) {
        if f := ctxt.ReadDir; f != nil {
                return f(path)
        }
        return ioutil.ReadDir(path)
}

// openFile calls ctxt.OpenFile (if not nil) or else os.Open.
func (ctxt *Context) openFile(path string) (io.ReadCloser, error) {
        if fn := ctxt.OpenFile; fn != nil {
                return fn(path)
        }

        f, err := os.Open(path)
        if err != nil {
                return nil, err // nil interface
        }
        return f, nil
}

// isFile determines whether path is a file by trying to open it.
// It reuses openFile instead of adding another function to the
// list in Context.
func (ctxt *Context) isFile(path string) bool {
        f, err := ctxt.openFile(path)
        if err != nil {
                return false
        }
        f.Close()
        return true
}

// gopath returns the list of Go path directories.
func (ctxt *Context) gopath() []string {
        var all []string
        for _, p := range ctxt.splitPathList(ctxt.GOPATH) {
                if p == "" || p == ctxt.GOROOT {
                        // Empty paths are uninteresting.
                        // If the path is the GOROOT, ignore it.
                        // People sometimes set GOPATH=$GOROOT.
                        // Do not get confused by this common mistake.
                        continue
                }
                if strings.HasPrefix(p, "~") {
                        // Path segments starting with ~ on Unix are almost always
                        // users who have incorrectly quoted ~ while setting GOPATH,
                        // preventing it from expanding to $HOME.
                        // The situation is made more confusing by the fact that
                        // bash allows quoted ~ in $PATH (most shells do not).
                        // Do not get confused by this, and do not try to use the path.
                        // It does not exist, and printing errors about it confuses
                        // those users even more, because they think "sure ~ exists!".
                        // The go command diagnoses this situation and prints a
                        // useful error.
                        // On Windows, ~ is used in short names, such as c:\progra~1
                        // for c:\program files.
                        continue
                }
                all = append(all, p)
        }
        return all
}

// SrcDirs returns a list of package source root directories.
// It draws from the current Go root and Go path but omits directories
// that do not exist.
func (ctxt *Context) SrcDirs() []string {
        var all []string
        if ctxt.GOROOT != "" && ctxt.Compiler != "gccgo" {
                dir := ctxt.joinPath(ctxt.GOROOT, "src")
                if ctxt.isDir(dir) {
                        all = append(all, dir)
                }
        }
        for _, p := range ctxt.gopath() {
                dir := ctxt.joinPath(p, "src")
                if ctxt.isDir(dir) {
                        all = append(all, dir)
                }
        }
        return all
}

// Default is the default Context for builds.
// It uses the GOARCH, GOOS, GOROOT, and GOPATH environment variables
// if set, or else the compiled code's GOARCH, GOOS, and GOROOT.
var Default Context = defaultContext()

func defaultGOPATH() string {
        env := "HOME"
        if runtime.GOOS == "windows" {
                env = "USERPROFILE"
        } else if runtime.GOOS == "plan9" {
                env = "home"
        }
        if home := os.Getenv(env); home != "" {
                def := filepath.Join(home, "go")
                if filepath.Clean(def) == filepath.Clean(runtime.GOROOT()) {
                        // Don't set the default GOPATH to GOROOT,
                        // as that will trigger warnings from the go tool.
                        return ""
                }
                return def
        }
        return ""
}

var defaultReleaseTags []string

func defaultContext() Context {
        var c Context

        c.GOARCH = envOr("GOARCH", runtime.GOARCH)
        c.GOOS = envOr("GOOS", runtime.GOOS)
        c.GOROOT = pathpkg.Clean(runtime.GOROOT())
        c.GOPATH = envOr("GOPATH", defaultGOPATH())
        c.Compiler = runtime.Compiler

        // Each major Go release in the Go 1.x series adds a new
        // "go1.x" release tag. That is, the go1.x tag is present in
        // all releases >= Go 1.x. Code that requires Go 1.x or later
        // should say "+build go1.x", and code that should only be
        // built before Go 1.x (perhaps it is the stub to use in that
        // case) should say "+build !go1.x".
        // The last element in ReleaseTags is the current release.
        for i := 1; i <= goversion.Version; i++ {
                c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
        }

        defaultReleaseTags = append([]string{}, c.ReleaseTags...) // our own private copy

        env := os.Getenv("CGO_ENABLED")
        if env == "" {
                env = defaultCGO_ENABLED
        }
        switch env {
        case "1":
                c.CgoEnabled = true
        case "0":
                c.CgoEnabled = false
        default:
                // cgo must be explicitly enabled for cross compilation builds
                if runtime.GOARCH == c.GOARCH && runtime.GOOS == c.GOOS {
                        c.CgoEnabled = cgoEnabled[c.GOOS+"/"+c.GOARCH]
                        break
                }
                c.CgoEnabled = false
        }

        return c
}

func envOr(name, def string) string {
        s := os.Getenv(name)
        if s == "" {
                return def
        }
        return s
}

// An ImportMode controls the behavior of the Import method.
type ImportMode uint

const (
        // If FindOnly is set, Import stops after locating the directory
        // that should contain the sources for a package. It does not
        // read any files in the directory.
        FindOnly ImportMode = 1 << iota

        // If AllowBinary is set, Import can be satisfied by a compiled
        // package object without corresponding sources.
        //
        // Deprecated:
        // The supported way to create a compiled-only package is to
        // write source code containing a //go:binary-only-package comment at
        // the top of the file. Such a package will be recognized
        // regardless of this flag setting (because it has source code)
        // and will have BinaryOnly set to true in the returned Package.
        AllowBinary

        // If ImportComment is set, parse import comments on package statements.
        // Import returns an error if it finds a comment it cannot understand
        // or finds conflicting comments in multiple source files.
        // See golang.org/s/go14customimport for more information.
        ImportComment

        // By default, Import searches vendor directories
        // that apply in the given source directory before searching
        // the GOROOT and GOPATH roots.
        // If an Import finds and returns a package using a vendor
        // directory, the resulting ImportPath is the complete path
        // to the package, including the path elements leading up
        // to and including "vendor".
        // For example, if Import("y", "x/subdir", 0) finds
        // "x/vendor/y", the returned package's ImportPath is "x/vendor/y",
        // not plain "y".
        // See golang.org/s/go15vendor for more information.
        //
        // Setting IgnoreVendor ignores vendor directories.
        //
        // In contrast to the package's ImportPath,
        // the returned package's Imports, TestImports, and XTestImports
        // are always the exact import paths from the source files:
        // Import makes no attempt to resolve or check those paths.
        IgnoreVendor
)

// A Package describes the Go package found in a directory.
type Package struct {
        Dir           string   // directory containing package sources
        Name          string   // package name
        ImportComment string   // path in import comment on package statement
        Doc           string   // documentation synopsis
        ImportPath    string   // import path of package ("" if unknown)
        Root          string   // root of Go tree where this package lives
        SrcRoot       string   // package source root directory ("" if unknown)
        PkgRoot       string   // package install root directory ("" if unknown)
        PkgTargetRoot string   // architecture dependent install root directory ("" if unknown)
        BinDir        string   // command install directory ("" if unknown)
        Goroot        bool     // package found in Go root
        PkgObj        string   // installed .a file
        AllTags       []string // tags that can influence file selection in this directory
        ConflictDir   string   // this directory shadows Dir in $GOPATH
        BinaryOnly    bool     // cannot be rebuilt from source (has //go:binary-only-package comment)

        // Source files
        GoFiles           []string // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
        CgoFiles          []string // .go source files that import "C"
        IgnoredGoFiles    []string // .go source files ignored for this build (including ignored _test.go files)
        InvalidGoFiles    []string // .go source files with detected problems (parse error, wrong package name, and so on)
        IgnoredOtherFiles []string // non-.go source files ignored for this build
        CFiles            []string // .c source files
        CXXFiles          []string // .cc, .cpp and .cxx source files
        MFiles            []string // .m (Objective-C) source files
        HFiles            []string // .h, .hh, .hpp and .hxx source files
        FFiles            []string // .f, .F, .for and .f90 Fortran source files
        SFiles            []string // .s source files
        SwigFiles         []string // .swig files
        SwigCXXFiles      []string // .swigcxx files
        SysoFiles         []string // .syso system object files to add to archive

        // Cgo directives
        CgoCFLAGS    []string // Cgo CFLAGS directives
        CgoCPPFLAGS  []string // Cgo CPPFLAGS directives
        CgoCXXFLAGS  []string // Cgo CXXFLAGS directives
        CgoFFLAGS    []string // Cgo FFLAGS directives
        CgoLDFLAGS   []string // Cgo LDFLAGS directives
        CgoPkgConfig []string // Cgo pkg-config directives

        // Test information
        TestGoFiles  []string // _test.go files in package
        XTestGoFiles []string // _test.go files outside package

        // Dependency information
        Imports        []string                    // import paths from GoFiles, CgoFiles
        ImportPos      map[string][]token.Position // line information for Imports
        TestImports    []string                    // import paths from TestGoFiles
        TestImportPos  map[string][]token.Position // line information for TestImports
        XTestImports   []string                    // import paths from XTestGoFiles
        XTestImportPos map[string][]token.Position // line information for XTestImports

        // //go:embed patterns found in Go source files
        // For example, if a source file says
        //      //go:embed a* b.c
        // then the list will contain those two strings as separate entries.
        // (See package embed for more details about //go:embed.)
        EmbedPatterns        []string                    // patterns from GoFiles, CgoFiles
        EmbedPatternPos      map[string][]token.Position // line information for EmbedPatterns
        TestEmbedPatterns    []string                    // patterns from TestGoFiles
        TestEmbedPatternPos  map[string][]token.Position // line information for TestEmbedPatterns
        XTestEmbedPatterns   []string                    // patterns from XTestGoFiles
        XTestEmbedPatternPos map[string][]token.Position // line information for XTestEmbedPatternPos
}

// IsCommand reports whether the package is considered a
// command to be installed (not just a library).
// Packages named "main" are treated as commands.
func (p *Package) IsCommand() bool {
        return p.Name == "main"
}

// ImportDir is like Import but processes the Go package found in
// the named directory.
func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) {
        return ctxt.Import(".", dir, mode)
}

// NoGoError is the error used by Import to describe a directory
// containing no buildable Go source files. (It may still contain
// test files, files hidden by build tags, and so on.)
type NoGoError struct {
        Dir string
}

func (e *NoGoError) Error() string {
        return "no buildable Go source files in " + e.Dir
}

// MultiplePackageError describes a directory containing
// multiple buildable Go source files for multiple packages.
type MultiplePackageError struct {
        Dir      string   // directory containing files
        Packages []string // package names found
        Files    []string // corresponding files: Files[i] declares package Packages[i]
}

func (e *MultiplePackageError) Error() string {
        // Error string limited to two entries for compatibility.
        return fmt.Sprintf("found packages %s (%s) and %s (%s) in %s", e.Packages[0], e.Files[0], e.Packages[1], e.Files[1], e.Dir)
}

func nameExt(name string) string {
        i := strings.LastIndex(name, ".")
        if i < 0 {
                return ""
        }
        return name[i:]
}

// Import returns details about the Go package named by the import path,
// interpreting local import paths relative to the srcDir directory.
// If the path is a local import path naming a package that can be imported
// using a standard import path, the returned package will set p.ImportPath
// to that path.
//
// In the directory containing the package, .go, .c, .h, and .s files are
// considered part of the package except for:
//
//      - .go files in package documentation
//      - files starting with _ or . (likely editor temporary files)
//      - files with build constraints not satisfied by the context
//
// If an error occurs, Import returns a non-nil error and a non-nil
// *Package containing partial information.
//
func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Package, error) {
        p := &Package{
                ImportPath: path,
        }
        if path == "" {
                return p, fmt.Errorf("import %q: invalid import path", path)
        }

        var pkgtargetroot string
        var pkga string
        var pkgerr error
        suffix := ""
        if ctxt.InstallSuffix != "" {
                suffix = "_" + ctxt.InstallSuffix
        }
        switch ctxt.Compiler {
        case "gccgo":
                pkgtargetroot = "pkg/gccgo_" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix
        case "gc":
                pkgtargetroot = "pkg/" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix
        default:
                // Save error for end of function.
                pkgerr = fmt.Errorf("import %q: unknown compiler %q", path, ctxt.Compiler)
        }
        setPkga := func() {
                switch ctxt.Compiler {
                case "gccgo":
                        dir, elem := pathpkg.Split(p.ImportPath)
                        pkga = pkgtargetroot + "/" + dir + "lib" + elem + ".a"
                case "gc":
                        pkga = pkgtargetroot + "/" + p.ImportPath + ".a"
                }
        }
        setPkga()

        binaryOnly := false
        if IsLocalImport(path) {
                pkga = "" // local imports have no installed path
                if srcDir == "" {
                        return p, fmt.Errorf("import %q: import relative to unknown directory", path)
                }
                if !ctxt.isAbsPath(path) {
                        p.Dir = ctxt.joinPath(srcDir, path)
                }
                // p.Dir directory may or may not exist. Gather partial information first, check if it exists later.
                // Determine canonical import path, if any.
                // Exclude results where the import path would include /testdata/.
                inTestdata := func(sub string) bool {
                        return strings.Contains(sub, "/testdata/") || strings.HasSuffix(sub, "/testdata") || strings.HasPrefix(sub, "testdata/") || sub == "testdata"
                }
                if ctxt.GOROOT != "" {
                        root := ctxt.joinPath(ctxt.GOROOT, "src")
                        if sub, ok := ctxt.hasSubdir(root, p.Dir); ok && !inTestdata(sub) {
                                p.Goroot = true
                                p.ImportPath = sub
                                p.Root = ctxt.GOROOT
                                setPkga() // p.ImportPath changed
                                goto Found
                        }
                }
                all := ctxt.gopath()
                for i, root := range all {
                        rootsrc := ctxt.joinPath(root, "src")
                        if sub, ok := ctxt.hasSubdir(rootsrc, p.Dir); ok && !inTestdata(sub) {
                                // We found a potential import path for dir,
                                // but check that using it wouldn't find something
                                // else first.
                                if ctxt.GOROOT != "" && ctxt.Compiler != "gccgo" {
                                        if dir := ctxt.joinPath(ctxt.GOROOT, "src", sub); ctxt.isDir(dir) {
                                                p.ConflictDir = dir
                                                goto Found
                                        }
                                }
                                for _, earlyRoot := range all[:i] {
                                        if dir := ctxt.joinPath(earlyRoot, "src", sub); ctxt.isDir(dir) {
                                                p.ConflictDir = dir
                                                goto Found
                                        }
                                }

                                // sub would not name some other directory instead of this one.
                                // Record it.
                                p.ImportPath = sub
                                p.Root = root
                                setPkga() // p.ImportPath changed
                                goto Found
                        }
                }
                // It's okay that we didn't find a root containing dir.
                // Keep going with the information we have.
        } else {
                if strings.HasPrefix(path, "/") {
                        return p, fmt.Errorf("import %q: cannot import absolute path", path)
                }

                if err := ctxt.importGo(p, path, srcDir, mode); err == nil {
                        goto Found
                } else if err != errNoModules {
                        return p, err
                }

                gopath := ctxt.gopath() // needed twice below; avoid computing many times

                // tried records the location of unsuccessful package lookups
                var tried struct {
                        vendor []string
                        goroot string
                        gopath []string
                }

                // Vendor directories get first chance to satisfy import.
                if mode&IgnoreVendor == 0 && srcDir != "" {
                        searchVendor := func(root string, isGoroot bool) bool {
                                sub, ok := ctxt.hasSubdir(root, srcDir)
                                if !ok || !strings.HasPrefix(sub, "src/") || strings.Contains(sub, "/testdata/") {
                                        return false
                                }
                                for {
                                        vendor := ctxt.joinPath(root, sub, "vendor")
                                        if ctxt.isDir(vendor) {
                                                dir := ctxt.joinPath(vendor, path)
                                                if ctxt.isDir(dir) && hasGoFiles(ctxt, dir) {
                                                        p.Dir = dir
                                                        p.ImportPath = strings.TrimPrefix(pathpkg.Join(sub, "vendor", path), "src/")
                                                        p.Goroot = isGoroot
                                                        p.Root = root
                                                        setPkga() // p.ImportPath changed
                                                        return true
                                                }
                                                tried.vendor = append(tried.vendor, dir)
                                        }
                                        i := strings.LastIndex(sub, "/")
                                        if i < 0 {
                                                break
                                        }
                                        sub = sub[:i]
                                }
                                return false
                        }
                        if ctxt.Compiler != "gccgo" && searchVendor(ctxt.GOROOT, true) {
                                goto Found
                        }
                        for _, root := range gopath {
                                if searchVendor(root, false) {
                                        goto Found
                                }
                        }
                }

                // Determine directory from import path.
                if ctxt.GOROOT != "" {
                        // If the package path starts with "vendor/", only search GOROOT before
                        // GOPATH if the importer is also within GOROOT. That way, if the user has
                        // vendored in a package that is subsequently included in the standard
                        // distribution, they'll continue to pick up their own vendored copy.
                        gorootFirst := srcDir == "" || !strings.HasPrefix(path, "vendor/")
                        if !gorootFirst {
                                _, gorootFirst = ctxt.hasSubdir(ctxt.GOROOT, srcDir)
                        }
                        if gorootFirst {
                                dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
                                if ctxt.Compiler != "gccgo" {
                                        isDir := ctxt.isDir(dir)
                                        binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
                                        if isDir || binaryOnly {
                                                p.Dir = dir
                                                p.Goroot = true
                                                p.Root = ctxt.GOROOT
                                                goto Found
                                        }
                                }
                                tried.goroot = dir
                        }
                }
                if ctxt.Compiler == "gccgo" && goroot.IsStandardPackage(ctxt.GOROOT, ctxt.Compiler, path) {
                        p.Dir = ctxt.joinPath(ctxt.GOROOT, "src", path)
                        p.Goroot = true
                        p.Root = ctxt.GOROOT
                        goto Found
                }
                for _, root := range gopath {
                        dir := ctxt.joinPath(root, "src", path)
                        isDir := ctxt.isDir(dir)
                        binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(root, pkga))
                        if isDir || binaryOnly {
                                p.Dir = dir
                                p.Root = root
                                goto Found
                        }
                        tried.gopath = append(tried.gopath, dir)
                }

                // If we tried GOPATH first due to a "vendor/" prefix, fall back to GOPATH.
                // That way, the user can still get useful results from 'go list' for
                // standard-vendored paths passed on the command line.
                if ctxt.GOROOT != "" && tried.goroot == "" {
                        dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
                        if ctxt.Compiler != "gccgo" {
                                isDir := ctxt.isDir(dir)
                                binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
                                if isDir || binaryOnly {
                                        p.Dir = dir
                                        p.Goroot = true
                                        p.Root = ctxt.GOROOT
                                        goto Found
                                }
                        }
                        tried.goroot = dir
                }

                // package was not found
                var paths []string
                format := "\t%s (vendor tree)"
                for _, dir := range tried.vendor {
                        paths = append(paths, fmt.Sprintf(format, dir))
                        format = "\t%s"
                }
                if tried.goroot != "" {
                        paths = append(paths, fmt.Sprintf("\t%s (from $GOROOT)", tried.goroot))
                } else {
                        paths = append(paths, "\t($GOROOT not set)")
                }
                format = "\t%s (from $GOPATH)"
                for _, dir := range tried.gopath {
                        paths = append(paths, fmt.Sprintf(format, dir))
                        format = "\t%s"
                }
                if len(tried.gopath) == 0 {
                        paths = append(paths, "\t($GOPATH not set. For more details see: 'go help gopath')")
                }
                return p, fmt.Errorf("cannot find package %q in any of:\n%s", path, strings.Join(paths, "\n"))
        }

Found:
        if p.Root != "" {
                p.SrcRoot = ctxt.joinPath(p.Root, "src")
                p.PkgRoot = ctxt.joinPath(p.Root, "pkg")
                p.BinDir = ctxt.joinPath(p.Root, "bin")
                if pkga != "" {
                        p.PkgTargetRoot = ctxt.joinPath(p.Root, pkgtargetroot)
                        p.PkgObj = ctxt.joinPath(p.Root, pkga)
                }
        }

        // If it's a local import path, by the time we get here, we still haven't checked
        // that p.Dir directory exists. This is the right time to do that check.
        // We can't do it earlier, because we want to gather partial information for the
        // non-nil *Package returned when an error occurs.
        // We need to do this before we return early on FindOnly flag.
        if IsLocalImport(path) && !ctxt.isDir(p.Dir) {
                if ctxt.Compiler == "gccgo" && p.Goroot {
                        // gccgo has no sources for GOROOT packages.
                        return p, nil
                }

                // package was not found
                return p, fmt.Errorf("cannot find package %q in:\n\t%s", path, p.Dir)
        }

        if mode&FindOnly != 0 {
                return p, pkgerr
        }
        if binaryOnly && (mode&AllowBinary) != 0 {
                return p, pkgerr
        }

        if ctxt.Compiler == "gccgo" && p.Goroot {
                // gccgo has no sources for GOROOT packages.
                return p, nil
        }

        dirs, err := ctxt.readDir(p.Dir)
        if err != nil {
                return p, err
        }

        var badGoError error
        var Sfiles []string // files with ".S"(capital S)/.sx(capital s equivalent for case insensitive filesystems)
        var firstFile, firstCommentFile string
        embedPos := make(map[string][]token.Position)
        testEmbedPos := make(map[string][]token.Position)
        xTestEmbedPos := make(map[string][]token.Position)
        importPos := make(map[string][]token.Position)
        testImportPos := make(map[string][]token.Position)
        xTestImportPos := make(map[string][]token.Position)
        allTags := make(map[string]bool)
        fset := token.NewFileSet()
        for _, d := range dirs {
                if d.IsDir() {
                        continue
                }
                if d.Mode()&fs.ModeSymlink != 0 {
                        if ctxt.isDir(ctxt.joinPath(p.Dir, d.Name())) {
                                // Symlinks to directories are not source files.
                                continue
                        }
                }

                name := d.Name()
                ext := nameExt(name)

                badFile := func(err error) {
                        if badGoError == nil {
                                badGoError = err
                        }
                        p.InvalidGoFiles = append(p.InvalidGoFiles, name)
                }

                info, err := ctxt.matchFile(p.Dir, name, allTags, &p.BinaryOnly, fset)
                if err != nil {
                        badFile(err)
                        continue
                }
                if info == nil {
                        if strings.HasPrefix(name, "_") || strings.HasPrefix(name, ".") {
                                // not due to build constraints - don't report
                        } else if ext == ".go" {
                                p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
                        } else if fileListForExt(p, ext) != nil {
                                p.IgnoredOtherFiles = append(p.IgnoredOtherFiles, name)
                        }
                        continue
                }
                data, filename := info.header, info.name

                // Going to save the file. For non-Go files, can stop here.
                switch ext {
                case ".go":
                        // keep going
                case ".S", ".sx":
                        // special case for cgo, handled at end
                        Sfiles = append(Sfiles, name)
                        continue
                default:
                        if list := fileListForExt(p, ext); list != nil {
                                *list = append(*list, name)
                        }
                        continue
                }

                if info.parseErr != nil {
                        badFile(info.parseErr)
                        continue
                }
                pf := info.parsed

                pkg := pf.Name.Name
                if pkg == "documentation" {
                        p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
                        continue
                }

                isTest := strings.HasSuffix(name, "_test.go")
                isXTest := false
                if isTest && strings.HasSuffix(pkg, "_test") {
                        isXTest = true
                        pkg = pkg[:len(pkg)-len("_test")]
                }

                if p.Name == "" {
                        p.Name = pkg
                        firstFile = name
                } else if pkg != p.Name {
                        badFile(&MultiplePackageError{
                                Dir:      p.Dir,
                                Packages: []string{p.Name, pkg},
                                Files:    []string{firstFile, name},
                        })
                        p.InvalidGoFiles = append(p.InvalidGoFiles, name)
                }
                // Grab the first package comment as docs, provided it is not from a test file.
                if pf.Doc != nil && p.Doc == "" && !isTest && !isXTest {
                        p.Doc = doc.Synopsis(pf.Doc.Text())
                }

                if mode&ImportComment != 0 {
                        qcom, line := findImportComment(data)
                        if line != 0 {
                                com, err := strconv.Unquote(qcom)
                                if err != nil {
                                        badFile(fmt.Errorf("%s:%d: cannot parse import comment", filename, line))
                                } else if p.ImportComment == "" {
                                        p.ImportComment = com
                                        firstCommentFile = name
                                } else if p.ImportComment != com {
                                        badFile(fmt.Errorf("found import comments %q (%s) and %q (%s) in %s", p.ImportComment, firstCommentFile, com, name, p.Dir))
                                }
                        }
                }

                // Record imports and information about cgo.
                isCgo := false
                for _, imp := range info.imports {
                        if imp.path == "C" {
                                if isTest {
                                        badFile(fmt.Errorf("use of cgo in test %s not supported", filename))
                                        continue
                                }
                                isCgo = true
                                if imp.doc != nil {
                                        if err := ctxt.saveCgo(filename, p, imp.doc); err != nil {
                                                badFile(err)
                                        }
                                }
                        }
                }

                var fileList *[]string
                var importMap, embedMap map[string][]token.Position
                switch {
                case isCgo:
                        allTags["cgo"] = true
                        if ctxt.CgoEnabled {
                                fileList = &p.CgoFiles
                                importMap = importPos
                                embedMap = embedPos
                        } else {
                                // Ignore imports and embeds from cgo files if cgo is disabled.
                                fileList = &p.IgnoredGoFiles
                        }
                case isXTest:
                        fileList = &p.XTestGoFiles
                        importMap = xTestImportPos
                        embedMap = xTestEmbedPos
                case isTest:
                        fileList = &p.TestGoFiles
                        importMap = testImportPos
                        embedMap = testEmbedPos
                default:
                        fileList = &p.GoFiles
                        importMap = importPos
                        embedMap = embedPos
                }
                *fileList = append(*fileList, name)
                if importMap != nil {
                        for _, imp := range info.imports {
                                importMap[imp.path] = append(importMap[imp.path], fset.Position(imp.pos))
                        }
                }
                if embedMap != nil {
                        for _, emb := range info.embeds {
                                embedMap[emb.pattern] = append(embedMap[emb.pattern], emb.pos)
                        }
                }
        }

        for tag := range allTags {
                p.AllTags = append(p.AllTags, tag)
        }
        sort.Strings(p.AllTags)

        p.EmbedPatterns, p.EmbedPatternPos = cleanDecls(embedPos)
        p.TestEmbedPatterns, p.TestEmbedPatternPos = cleanDecls(testEmbedPos)
        p.XTestEmbedPatterns, p.XTestEmbedPatternPos = cleanDecls(xTestEmbedPos)

        p.Imports, p.ImportPos = cleanDecls(importPos)
        p.TestImports, p.TestImportPos = cleanDecls(testImportPos)
        p.XTestImports, p.XTestImportPos = cleanDecls(xTestImportPos)

        // add the .S/.sx files only if we are using cgo
        // (which means gcc will compile them).
        // The standard assemblers expect .s files.
        if len(p.CgoFiles) > 0 {
                p.SFiles = append(p.SFiles, Sfiles...)
                sort.Strings(p.SFiles)
        } else {
                p.IgnoredOtherFiles = append(p.IgnoredOtherFiles, Sfiles...)
                sort.Strings(p.IgnoredOtherFiles)
        }

        if badGoError != nil {
                return p, badGoError
        }
        if len(p.GoFiles)+len(p.CgoFiles)+len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
                return p, &NoGoError{p.Dir}
        }
        return p, pkgerr
}

func fileListForExt(p *Package, ext string) *[]string {
        switch ext {
        case ".c":
                return &p.CFiles
        case ".cc", ".cpp", ".cxx":
                return &p.CXXFiles
        case ".m":
                return &p.MFiles
        case ".h", ".hh", ".hpp", ".hxx":
                return &p.HFiles
        case ".f", ".F", ".for", ".f90":
                return &p.FFiles
        case ".s", ".S", ".sx":
                return &p.SFiles
        case ".swig":
                return &p.SwigFiles
        case ".swigcxx":
                return &p.SwigCXXFiles
        case ".syso":
                return &p.SysoFiles
        }
        return nil
}

func uniq(list []string) []string {
        if list == nil {
                return nil
        }
        out := make([]string, len(list))
        copy(out, list)
        sort.Strings(out)
        uniq := out[:0]
        for _, x := range out {
                if len(uniq) == 0 || uniq[len(uniq)-1] != x {
                        uniq = append(uniq, x)
                }
        }
        return uniq
}

var errNoModules = errors.New("not using modules")

// importGo checks whether it can use the go command to find the directory for path.
// If using the go command is not appropriate, importGo returns errNoModules.
// Otherwise, importGo tries using the go command and reports whether that succeeded.
// Using the go command lets build.Import and build.Context.Import find code
// in Go modules. In the long term we want tools to use go/packages (currently golang.org/x/tools/go/packages),
// which will also use the go command.
// Invoking the go command here is not very efficient in that it computes information
// about the requested package and all dependencies and then only reports about the requested package.
// Then we reinvoke it for every dependency. But this is still better than not working at all.
// See golang.org/issue/26504.
func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode) error {
        // To invoke the go command,
        // we must not being doing special things like AllowBinary or IgnoreVendor,
        // and all the file system callbacks must be nil (we're meant to use the local file system).
        if mode&AllowBinary != 0 || mode&IgnoreVendor != 0 ||
                ctxt.JoinPath != nil || ctxt.SplitPathList != nil || ctxt.IsAbsPath != nil || ctxt.IsDir != nil || ctxt.HasSubdir != nil || ctxt.ReadDir != nil || ctxt.OpenFile != nil || !equal(ctxt.ReleaseTags, defaultReleaseTags) {
                return errNoModules
        }

        // Predict whether module aware mode is enabled by checking the value of
        // GO111MODULE and looking for a go.mod file in the source directory or
        // one of its parents. Running 'go env GOMOD' in the source directory would
        // give a canonical answer, but we'd prefer not to execute another command.
        go111Module := os.Getenv("GO111MODULE")
        switch go111Module {
        case "off":
                return errNoModules
        default: // "", "on", "auto", anything else
                // Maybe use modules.
        }

        if srcDir != "" {
                var absSrcDir string
                if filepath.IsAbs(srcDir) {
                        absSrcDir = srcDir
                } else if ctxt.Dir != "" {
                        return fmt.Errorf("go/build: Dir is non-empty, so relative srcDir is not allowed: %v", srcDir)
                } else {
                        // Find the absolute source directory. hasSubdir does not handle
                        // relative paths (and can't because the callbacks don't support this).
                        var err error
                        absSrcDir, err = filepath.Abs(srcDir)
                        if err != nil {
                                return errNoModules
                        }
                }

                // If the source directory is in GOROOT, then the in-process code works fine
                // and we should keep using it. Moreover, the 'go list' approach below doesn't
                // take standard-library vendoring into account and will fail.
                if _, ok := ctxt.hasSubdir(filepath.Join(ctxt.GOROOT, "src"), absSrcDir); ok {
                        return errNoModules
                }
        }

        // For efficiency, if path is a standard library package, let the usual lookup code handle it.
        if ctxt.GOROOT != "" {
                dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
                if ctxt.isDir(dir) {
                        return errNoModules
                }
        }

        // If GO111MODULE=auto, look to see if there is a go.mod.
        // Since go1.13, it doesn't matter if we're inside GOPATH.
        if go111Module == "auto" {
                var (
                        parent string
                        err    error
                )
                if ctxt.Dir == "" {
                        parent, err = os.Getwd()
                        if err != nil {
                                // A nonexistent working directory can't be in a module.
                                return errNoModules
                        }
                } else {
                        parent, err = filepath.Abs(ctxt.Dir)
                        if err != nil {
                                // If the caller passed a bogus Dir explicitly, that's materially
                                // different from not having modules enabled.
                                return err
                        }
                }
                for {
                        if f, err := ctxt.openFile(ctxt.joinPath(parent, "go.mod")); err == nil {
                                buf := make([]byte, 100)
                                _, err := f.Read(buf)
                                f.Close()
                                if err == nil || err == io.EOF {
                                        // go.mod exists and is readable (is a file, not a directory).
                                        break
                                }
                        }
                        d := filepath.Dir(parent)
                        if len(d) >= len(parent) {
                                return errNoModules // reached top of file system, no go.mod
                        }
                        parent = d
                }
        }

        cmd := exec.Command("go", "list", "-e", "-compiler="+ctxt.Compiler, "-tags="+strings.Join(ctxt.BuildTags, ","), "-installsuffix="+ctxt.InstallSuffix, "-f={{.Dir}}\n{{.ImportPath}}\n{{.Root}}\n{{.Goroot}}\n{{if .Error}}{{.Error}}{{end}}\n", "--", path)

        if ctxt.Dir != "" {
                cmd.Dir = ctxt.Dir
        }

        var stdout, stderr strings.Builder
        cmd.Stdout = &stdout
        cmd.Stderr = &stderr

        cgo := "0"
        if ctxt.CgoEnabled {
                cgo = "1"
        }
        cmd.Env = append(os.Environ(),
                "GOOS="+ctxt.GOOS,
                "GOARCH="+ctxt.GOARCH,
                "GOROOT="+ctxt.GOROOT,
                "GOPATH="+ctxt.GOPATH,
                "CGO_ENABLED="+cgo,
        )

        if err := cmd.Run(); err != nil {
                return fmt.Errorf("go/build: go list %s: %v\n%s\n", path, err, stderr.String())
        }

        f := strings.SplitN(stdout.String(), "\n", 5)
        if len(f) != 5 {
                return fmt.Errorf("go/build: importGo %s: unexpected output:\n%s\n", path, stdout.String())
        }
        dir := f[0]
        errStr := strings.TrimSpace(f[4])
        if errStr != "" && dir == "" {
                // If 'go list' could not locate the package (dir is empty),
                // return the same error that 'go list' reported.
                return errors.New(errStr)
        }

        // If 'go list' did locate the package, ignore the error.
        // It was probably related to loading source files, and we'll
        // encounter it ourselves shortly if the FindOnly flag isn't set.
        p.Dir = dir
        p.ImportPath = f[1]
        p.Root = f[2]
        p.Goroot = f[3] == "true"
        return nil
}

func equal(x, y []string) bool {
        if len(x) != len(y) {
                return false
        }
        for i, xi := range x {
                if xi != y[i] {
                        return false
                }
        }
        return true
}

// hasGoFiles reports whether dir contains any files with names ending in .go.
// For a vendor check we must exclude directories that contain no .go files.
// Otherwise it is not possible to vendor just a/b/c and still import the
// non-vendored a/b. See golang.org/issue/13832.
func hasGoFiles(ctxt *Context, dir string) bool {
        ents, _ := ctxt.readDir(dir)
        for _, ent := range ents {
                if !ent.IsDir() && strings.HasSuffix(ent.Name(), ".go") {
                        return true
                }
        }
        return false
}

func findImportComment(data []byte) (s string, line int) {
        // expect keyword package
        word, data := parseWord(data)
        if string(word) != "package" {
                return "", 0
        }

        // expect package name
        _, data = parseWord(data)

        // now ready for import comment, a // or /* */ comment
        // beginning and ending on the current line.
        for len(data) > 0 && (data[0] == ' ' || data[0] == '\t' || data[0] == '\r') {
                data = data[1:]
        }

        var comment []byte
        switch {
        case bytes.HasPrefix(data, slashSlash):
                i := bytes.Index(data, newline)
                if i < 0 {
                        i = len(data)
                }
                comment = data[2:i]
        case bytes.HasPrefix(data, slashStar):
                data = data[2:]
                i := bytes.Index(data, starSlash)
                if i < 0 {
                        // malformed comment
                        return "", 0
                }
                comment = data[:i]
                if bytes.Contains(comment, newline) {
                        return "", 0
                }
        }
        comment = bytes.TrimSpace(comment)

        // split comment into `import`, `"pkg"`
        word, arg := parseWord(comment)
        if string(word) != "import" {
                return "", 0
        }

        line = 1 + bytes.Count(data[:cap(data)-cap(arg)], newline)
        return strings.TrimSpace(string(arg)), line
}

var (
        slashSlash = []byte("//")
        slashStar  = []byte("/*")
        starSlash  = []byte("*/")
        newline    = []byte("\n")
)

// skipSpaceOrComment returns data with any leading spaces or comments removed.
func skipSpaceOrComment(data []byte) []byte {
        for len(data) > 0 {
                switch data[0] {
                case ' ', '\t', '\r', '\n':
                        data = data[1:]
                        continue
                case '/':
                        if bytes.HasPrefix(data, slashSlash) {
                                i := bytes.Index(data, newline)
                                if i < 0 {
                                        return nil
                                }
                                data = data[i+1:]
                                continue
                        }
                        if bytes.HasPrefix(data, slashStar) {
                                data = data[2:]
                                i := bytes.Index(data, starSlash)
                                if i < 0 {
                                        return nil
                                }
                                data = data[i+2:]
                                continue
                        }
                }
                break
        }
        return data
}

// parseWord skips any leading spaces or comments in data
// and then parses the beginning of data as an identifier or keyword,
// returning that word and what remains after the word.
func parseWord(data []byte) (word, rest []byte) {
        data = skipSpaceOrComment(data)

        // Parse past leading word characters.
        rest = data
        for {
                r, size := utf8.DecodeRune(rest)
                if unicode.IsLetter(r) || '0' <= r && r <= '9' || r == '_' {
                        rest = rest[size:]
                        continue
                }
                break
        }

        word = data[:len(data)-len(rest)]
        if len(word) == 0 {
                return nil, nil
        }

        return word, rest
}

// MatchFile reports whether the file with the given name in the given directory
// matches the context and would be included in a Package created by ImportDir
// of that directory.
//
// MatchFile considers the name of the file and may use ctxt.OpenFile to
// read some or all of the file's content.
func (ctxt *Context) MatchFile(dir, name string) (match bool, err error) {
        info, err := ctxt.matchFile(dir, name, nil, nil, nil)
        return info != nil, err
}

var dummyPkg Package

// fileInfo records information learned about a file included in a build.
type fileInfo struct {
        name     string // full name including dir
        header   []byte
        fset     *token.FileSet
        parsed   *ast.File
        parseErr error
        imports  []fileImport
        embeds   []fileEmbed
        embedErr error
}

type fileImport struct {
        path string
        pos  token.Pos
        doc  *ast.CommentGroup
}

type fileEmbed struct {
        pattern string
        pos     token.Position
}

// matchFile determines whether the file with the given name in the given directory
// should be included in the package being constructed.
// If the file should be included, matchFile returns a non-nil *fileInfo (and a nil error).
// Non-nil errors are reserved for unexpected problems.
//
// If name denotes a Go program, matchFile reads until the end of the
// imports and returns that section of the file in the fileInfo's header field,
// even though it only considers text until the first non-comment
// for +build lines.
//
// If allTags is non-nil, matchFile records any encountered build tag
// by setting allTags[tag] = true.
func (ctxt *Context) matchFile(dir, name string, allTags map[string]bool, binaryOnly *bool, fset *token.FileSet) (*fileInfo, error) {
        if strings.HasPrefix(name, "_") ||
                strings.HasPrefix(name, ".") {
                return nil, nil
        }

        i := strings.LastIndex(name, ".")
        if i < 0 {
                i = len(name)
        }
        ext := name[i:]

        if !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
                return nil, nil
        }

        if ext != ".go" && fileListForExt(&dummyPkg, ext) == nil {
                // skip
                return nil, nil
        }

        info := &fileInfo{name: ctxt.joinPath(dir, name), fset: fset}
        if ext == ".syso" {
                // binary, no reading
                return info, nil
        }

        f, err := ctxt.openFile(info.name)
        if err != nil {
                return nil, err
        }

        if strings.HasSuffix(name, ".go") {
                err = readGoInfo(f, info)
                if strings.HasSuffix(name, "_test.go") {
                        binaryOnly = nil // ignore //go:binary-only-package comments in _test.go files
                }
        } else {
                binaryOnly = nil // ignore //go:binary-only-package comments in non-Go sources
                info.header, err = readComments(f)
        }
        f.Close()
        if err != nil {
                return nil, fmt.Errorf("read %s: %v", info.name, err)
        }

        // Look for +build comments to accept or reject the file.
        ok, sawBinaryOnly, err := ctxt.shouldBuild(info.header, allTags)
        if err != nil {
                return nil, fmt.Errorf("%s: %v", name, err)
        }
        if !ok && !ctxt.UseAllFiles {
                return nil, nil
        }

        if binaryOnly != nil && sawBinaryOnly {
                *binaryOnly = true
        }

        return info, nil
}

func cleanDecls(m map[string][]token.Position) ([]string, map[string][]token.Position) {
        all := make([]string, 0, len(m))
        for path := range m {
                all = append(all, path)
        }
        sort.Strings(all)
        return all, m
}

// Import is shorthand for Default.Import.
func Import(path, srcDir string, mode ImportMode) (*Package, error) {
        return Default.Import(path, srcDir, mode)
}

// ImportDir is shorthand for Default.ImportDir.
func ImportDir(dir string, mode ImportMode) (*Package, error) {
        return Default.ImportDir(dir, mode)
}

var (
        bSlashSlash = []byte(slashSlash)
        bStarSlash  = []byte(starSlash)
        bSlashStar  = []byte(slashStar)
        bPlusBuild  = []byte("+build")

        goBuildComment = []byte("//go:build")

        errGoBuildWithoutBuild = errors.New("//go:build comment without // +build comment")
        errMultipleGoBuild     = errors.New("multiple //go:build comments")
)

func isGoBuildComment(line []byte) bool {
        if !bytes.HasPrefix(line, goBuildComment) {
                return false
        }
        line = bytes.TrimSpace(line)
        rest := line[len(goBuildComment):]
        return len(rest) == 0 || len(bytes.TrimSpace(rest)) < len(rest)
}

// Special comment denoting a binary-only package.
// See https://golang.org/design/2775-binary-only-packages
// for more about the design of binary-only packages.
var binaryOnlyComment = []byte("//go:binary-only-package")

// shouldBuild reports whether it is okay to use this file,
// The rule is that in the file's leading run of // comments
// and blank lines, which must be followed by a blank line
// (to avoid including a Go package clause doc comment),
// lines beginning with '// +build' are taken as build directives.
//
// The file is accepted only if each such line lists something
// matching the file. For example:
//
//      // +build windows linux
//
// marks the file as applicable only on Windows and Linux.
//
// For each build tag it consults, shouldBuild sets allTags[tag] = true.
//
// shouldBuild reports whether the file should be built
// and whether a //go:binary-only-package comment was found.
func (ctxt *Context) shouldBuild(content []byte, allTags map[string]bool) (shouldBuild, binaryOnly bool, err error) {
        // Identify leading run of // comments and blank lines,
        // which must be followed by a blank line.
        // Also identify any //go:build comments.
        content, goBuild, sawBinaryOnly, err := parseFileHeader(content)
        if err != nil {
                return false, false, err
        }

        // If //go:build line is present, it controls.
        // Otherwise fall back to +build processing.
        switch {
        case goBuild != nil:
                x, err := constraint.Parse(string(goBuild))
                if err != nil {
                        return false, false, fmt.Errorf("parsing //go:build line: %v", err)
                }
                shouldBuild = ctxt.eval(x, allTags)

        default:
                shouldBuild = true
                p := content
                for len(p) > 0 {
                        line := p
                        if i := bytes.IndexByte(line, '\n'); i >= 0 {
                                line, p = line[:i], p[i+1:]
                        } else {
                                p = p[len(p):]
                        }
                        line = bytes.TrimSpace(line)
                        if !bytes.HasPrefix(line, bSlashSlash) || !bytes.Contains(line, bPlusBuild) {
                                continue
                        }
                        text := string(line)
                        if !constraint.IsPlusBuild(text) {
                                continue
                        }
                        if x, err := constraint.Parse(text); err == nil {
                                if !ctxt.eval(x, allTags) {
                                        shouldBuild = false
                                }
                        }
                }
        }

        return shouldBuild, sawBinaryOnly, nil
}

func parseFileHeader(content []byte) (trimmed, goBuild []byte, sawBinaryOnly bool, err error) {
        end := 0
        p := content
        ended := false       // found non-blank, non-// line, so stopped accepting // +build lines
        inSlashStar := false // in /* */ comment

Lines:
        for len(p) > 0 {
                line := p
                if i := bytes.IndexByte(line, '\n'); i >= 0 {
                        line, p = line[:i], p[i+1:]
                } else {
                        p = p[len(p):]
                }
                line = bytes.TrimSpace(line)
                if len(line) == 0 && !ended { // Blank line
                        // Remember position of most recent blank line.
                        // When we find the first non-blank, non-// line,
                        // this "end" position marks the latest file position
                        // where a // +build line can appear.
                        // (It must appear _before_ a blank line before the non-blank, non-// line.
                        // Yes, that's confusing, which is part of why we moved to //go:build lines.)
                        // Note that ended==false here means that inSlashStar==false,
                        // since seeing a /* would have set ended==true.
                        end = len(content) - len(p)
                        continue Lines
                }
                if !bytes.HasPrefix(line, slashSlash) { // Not comment line
                        ended = true
                }

                if !inSlashStar && isGoBuildComment(line) {
                        if goBuild != nil {
                                return nil, nil, false, errMultipleGoBuild
                        }
                        goBuild = line
                }
                if !inSlashStar && bytes.Equal(line, binaryOnlyComment) {
                        sawBinaryOnly = true
                }

        Comments:
                for len(line) > 0 {
                        if inSlashStar {
                                if i := bytes.Index(line, starSlash); i >= 0 {
                                        inSlashStar = false
                                        line = bytes.TrimSpace(line[i+len(starSlash):])
                                        continue Comments
                                }
                                continue Lines
                        }
                        if bytes.HasPrefix(line, bSlashSlash) {
                                continue Lines
                        }
                        if bytes.HasPrefix(line, bSlashStar) {
                                inSlashStar = true
                                line = bytes.TrimSpace(line[len(bSlashStar):])
                                continue Comments
                        }
                        // Found non-comment text.
                        break Lines
                }
        }

        return content[:end], goBuild, sawBinaryOnly, nil
}

// saveCgo saves the information from the #cgo lines in the import "C" comment.
// These lines set CFLAGS, CPPFLAGS, CXXFLAGS and LDFLAGS and pkg-config directives
// that affect the way cgo's C code is built.
func (ctxt *Context) saveCgo(filename string, di *Package, cg *ast.CommentGroup) error {
        text := cg.Text()
        for _, line := range strings.Split(text, "\n") {
                orig := line

                // Line is
                //      #cgo [GOOS/GOARCH...] LDFLAGS: stuff
                //
                line = strings.TrimSpace(line)
                if len(line) < 5 || line[:4] != "#cgo" || (line[4] != ' ' && line[4] != '\t') {
                        continue
                }

                // Split at colon.
                line = strings.TrimSpace(line[4:])
                i := strings.Index(line, ":")
                if i < 0 {
                        return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
                }
                line, argstr := line[:i], line[i+1:]

                // Parse GOOS/GOARCH stuff.
                f := strings.Fields(line)
                if len(f) < 1 {
                        return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
                }

                cond, verb := f[:len(f)-1], f[len(f)-1]
                if len(cond) > 0 {
                        ok := false
                        for _, c := range cond {
                                if ctxt.matchAuto(c, nil) {
                                        ok = true
                                        break
                                }
                        }
                        if !ok {
                                continue
                        }
                }

                args, err := splitQuoted(argstr)
                if err != nil {
                        return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
                }
                var ok bool
                for i, arg := range args {
                        if arg, ok = expandSrcDir(arg, di.Dir); !ok {
                                return fmt.Errorf("%s: malformed #cgo argument: %s", filename, arg)
                        }
                        args[i] = arg
                }

                switch verb {
                case "CFLAGS", "CPPFLAGS", "CXXFLAGS", "FFLAGS", "LDFLAGS":
                        // Change relative paths to absolute.
                        ctxt.makePathsAbsolute(args, di.Dir)
                }

                switch verb {
                case "CFLAGS":
                        di.CgoCFLAGS = append(di.CgoCFLAGS, args...)
                case "CPPFLAGS":
                        di.CgoCPPFLAGS = append(di.CgoCPPFLAGS, args...)
                case "CXXFLAGS":
                        di.CgoCXXFLAGS = append(di.CgoCXXFLAGS, args...)
                case "FFLAGS":
                        di.CgoFFLAGS = append(di.CgoFFLAGS, args...)
                case "LDFLAGS":
                        di.CgoLDFLAGS = append(di.CgoLDFLAGS, args...)
                case "pkg-config":
                        di.CgoPkgConfig = append(di.CgoPkgConfig, args...)
                default:
                        return fmt.Errorf("%s: invalid #cgo verb: %s", filename, orig)
                }
        }
        return nil
}

// expandSrcDir expands any occurrence of ${SRCDIR}, making sure
// the result is safe for the shell.
func expandSrcDir(str string, srcdir string) (string, bool) {
        // "\" delimited paths cause safeCgoName to fail
        // so convert native paths with a different delimiter
        // to "/" before starting (eg: on windows).
        srcdir = filepath.ToSlash(srcdir)

        chunks := strings.Split(str, "${SRCDIR}")
        if len(chunks) < 2 {
                return str, safeCgoName(str)
        }
        ok := true
        for _, chunk := range chunks {
                ok = ok && (chunk == "" || safeCgoName(chunk))
        }
        ok = ok && (srcdir == "" || safeCgoName(srcdir))
        res := strings.Join(chunks, srcdir)
        return res, ok && res != ""
}

// makePathsAbsolute looks for compiler options that take paths and
// makes them absolute. We do this because through the 1.8 release we
// ran the compiler in the package directory, so any relative -I or -L
// options would be relative to that directory. In 1.9 we changed to
// running the compiler in the build directory, to get consistent
// build results (issue #19964). To keep builds working, we change any
// relative -I or -L options to be absolute.
//
// Using filepath.IsAbs and filepath.Join here means the results will be
// different on different systems, but that's OK: -I and -L options are
// inherently system-dependent.
func (ctxt *Context) makePathsAbsolute(args []string, srcDir string) {
        nextPath := false
        for i, arg := range args {
                if nextPath {
                        if !filepath.IsAbs(arg) {
                                args[i] = filepath.Join(srcDir, arg)
                        }
                        nextPath = false
                } else if strings.HasPrefix(arg, "-I") || strings.HasPrefix(arg, "-L") {
                        if len(arg) == 2 {
                                nextPath = true
                        } else {
                                if !filepath.IsAbs(arg[2:]) {
                                        args[i] = arg[:2] + filepath.Join(srcDir, arg[2:])
                                }
                        }
                }
        }
}

// NOTE: $ is not safe for the shell, but it is allowed here because of linker options like -Wl,$ORIGIN.
// We never pass these arguments to a shell (just to programs we construct argv for), so this should be okay.
// See golang.org/issue/6038.
// The @ is for OS X. See golang.org/issue/13720.
// The % is for Jenkins. See golang.org/issue/16959.
// The ! is because module paths may use them. See golang.org/issue/26716.
// The ~ and ^ are for sr.ht. See golang.org/issue/32260.
const safeString = "+-.,/0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz:$@%! ~^"

func safeCgoName(s string) bool {
        if s == "" {
                return false
        }
        for i := 0; i < len(s); i++ {
                if c := s[i]; c < utf8.RuneSelf && strings.IndexByte(safeString, c) < 0 {
                        return false
                }
        }
        return true
}

// splitQuoted splits the string s around each instance of one or more consecutive
// white space characters while taking into account quotes and escaping, and
// returns an array of substrings of s or an empty list if s contains only white space.
// Single quotes and double quotes are recognized to prevent splitting within the
// quoted region, and are removed from the resulting substrings. If a quote in s
// isn't closed err will be set and r will have the unclosed argument as the
// last element. The backslash is used for escaping.
//
// For example, the following string:
//
//     a b:"c d" 'e''f'  "g\""
//
// Would be parsed as:
//
//     []string{"a", "b:c d", "ef", `g"`}
//
func splitQuoted(s string) (r []string, err error) {
        var args []string
        arg := make([]rune, len(s))
        escaped := false
        quoted := false
        quote := '\x00'
        i := 0
        for _, rune := range s {
                switch {
                case escaped:
                        escaped = false
                case rune == '\\':
                        escaped = true
                        continue
                case quote != '\x00':
                        if rune == quote {
                                quote = '\x00'
                                continue
                        }
                case rune == '"' || rune == '\'':
                        quoted = true
                        quote = rune
                        continue
                case unicode.IsSpace(rune):
                        if quoted || i > 0 {
                                quoted = false
                                args = append(args, string(arg[:i]))
                                i = 0
                        }
                        continue
                }
                arg[i] = rune
                i++
        }
        if quoted || i > 0 {
                args = append(args, string(arg[:i]))
        }
        if quote != 0 {
                err = errors.New("unclosed quote")
        } else if escaped {
                err = errors.New("unfinished escaping")
        }
        return args, err
}

// matchAuto interprets text as either a +build or //go:build expression (whichever works),
// reporting whether the expression matches the build context.
//
// matchAuto is only used for testing of tag evaluation
// and in #cgo lines, which accept either syntax.
func (ctxt *Context) matchAuto(text string, allTags map[string]bool) bool {
        if strings.ContainsAny(text, "&|()") {
                text = "//go:build " + text
        } else {
                text = "// +build " + text
        }
        x, err := constraint.Parse(text)
        if err != nil {
                return false
        }
        return ctxt.eval(x, allTags)
}

func (ctxt *Context) eval(x constraint.Expr, allTags map[string]bool) bool {
        return x.Eval(func(tag string) bool { return ctxt.matchTag(tag, allTags) })
}

// matchTag reports whether the name is one of:
//
//      cgo (if cgo is enabled)
//      $GOOS
//      $GOARCH
//      boringcrypto
//      ctxt.Compiler
//      linux (if GOOS = android)
//      solaris (if GOOS = illumos)
//      tag (if tag is listed in ctxt.BuildTags or ctxt.ReleaseTags)
//
// It records all consulted tags in allTags.
func (ctxt *Context) matchTag(name string, allTags map[string]bool) bool {
        if allTags != nil {
                allTags[name] = true
        }

        // special tags
        if ctxt.CgoEnabled && name == "cgo" {
                return true
        }
        if name == ctxt.GOOS || name == ctxt.GOARCH || name == ctxt.Compiler {
                return true
        }
        if ctxt.GOOS == "android" && name == "linux" {
                return true
        }
        if ctxt.GOOS == "illumos" && name == "solaris" {
                return true
        }
        if ctxt.GOOS == "ios" && name == "darwin" {
                return true
        }
        // Let applications know that the Go+BoringCrypto toolchain is in use.
        if name == "boringcrypto" {
                return true
        }

        // other tags
        for _, tag := range ctxt.BuildTags {
                if tag == name {
                        return true
                }
        }
        for _, tag := range ctxt.ReleaseTags {
                if tag == name {
                        return true
                }
        }

        return false
}

// goodOSArchFile returns false if the name contains a $GOOS or $GOARCH
// suffix which does not match the current system.
// The recognized name formats are:
//
//     name_$(GOOS).*
//     name_$(GOARCH).*
//     name_$(GOOS)_$(GOARCH).*
//     name_$(GOOS)_test.*
//     name_$(GOARCH)_test.*
//     name_$(GOOS)_$(GOARCH)_test.*
//
// Exceptions:
// if GOOS=android, then files with GOOS=linux are also matched.
// if GOOS=illumos, then files with GOOS=solaris are also matched.
// if GOOS=ios, then files with GOOS=darwin are also matched.
func (ctxt *Context) goodOSArchFile(name string, allTags map[string]bool) bool {
        if dot := strings.Index(name, "."); dot != -1 {
                name = name[:dot]
        }

        // Before Go 1.4, a file called "linux.go" would be equivalent to having a
        // build tag "linux" in that file. For Go 1.4 and beyond, we require this
        // auto-tagging to apply only to files with a non-empty prefix, so
        // "foo_linux.go" is tagged but "linux.go" is not. This allows new operating
        // systems, such as android, to arrive without breaking existing code with
        // innocuous source code in "android.go". The easiest fix: cut everything
        // in the name before the initial _.
        i := strings.Index(name, "_")
        if i < 0 {
                return true
        }
        name = name[i:] // ignore everything before first _

        l := strings.Split(name, "_")
        if n := len(l); n > 0 && l[n-1] == "test" {
                l = l[:n-1]
        }
        n := len(l)
        if n >= 2 && knownOS[l[n-2]] && knownArch[l[n-1]] {
                return ctxt.matchTag(l[n-1], allTags) && ctxt.matchTag(l[n-2], allTags)
        }
        if n >= 1 && (knownOS[l[n-1]] || knownArch[l[n-1]]) {
                return ctxt.matchTag(l[n-1], allTags)
        }
        return true
}

var knownOS = make(map[string]bool)
var knownArch = make(map[string]bool)

func init() {
        for _, v := range strings.Fields(goosList) {
                knownOS[v] = true
        }
        for _, v := range strings.Fields(goarchList) {
                knownArch[v] = true
        }
}

// ToolDir is the directory containing build tools.
var ToolDir = getToolDir()

// IsLocalImport reports whether the import path is
// a local import path, like ".", "..", "./foo", or "../foo".
func IsLocalImport(path string) bool {
        return path == "." || path == ".." ||
                strings.HasPrefix(path, "./") || strings.HasPrefix(path, "../")
}

// ArchChar returns "?" and an error.
// In earlier versions of Go, the returned string was used to derive
// the compiler and linker tool names, the default object file suffix,
// and the default linker output name. As of Go 1.5, those strings
// no longer vary by architecture; they are compile, link, .o, and a.out, respectively.
func ArchChar(goarch string) (string, error) {
        return "?", errors.New("architecture letter no longer used")
}