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Source file src/go/build/build.go

Documentation: go/build

  // 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/doc"
  	"go/parser"
  	"go/token"
  	"io"
  	"io/ioutil"
  	"log"
  	"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
  	CgoEnabled  bool   // whether cgo can be used
  	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.
  	// In addition to the BuildTags and ReleaseTags, build constraints
  	// consider the values of GOARCH and GOOS as satisfied tags.
  	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 os.FileInfo, sorted by Name,
  	// describing the content of the named directory.
  	// If ReadDir is nil, Import uses ioutil.ReadDir.
  	ReadDir func(dir string) ([]os.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) ([]os.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 != "" {
  		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 ""
  }
  
  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 should add a tag here.
  	// Old tags should not be removed. 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".
  	// NOTE: If you add to this list, also update the doc comment in doc.go.
  	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3", "go1.4", "go1.5", "go1.6", "go1.7", "go1.8", "go1.9"}
  
  	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
  	InvalidGoFiles []string // .go source files with detected problems (parse error, wrong package name, and so on)
  	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
  
  	// Dependency information
  	Imports   []string                    // import paths from GoFiles, CgoFiles
  	ImportPos map[string][]token.Position // line information for Imports
  
  	// Test information
  	TestGoFiles    []string                    // _test.go files in package
  	TestImports    []string                    // import paths from TestGoFiles
  	TestImportPos  map[string][]token.Position // line information for TestImports
  	XTestGoFiles   []string                    // _test.go files outside package
  	XTestImports   []string                    // import paths from XTestGoFiles
  	XTestImportPos map[string][]token.Position // line information for XTestImports
  }
  
  // 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
  				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 != "" {
  					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
  				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)
  		}
  
  		// tried records the location of unsuccessful package lookups
  		var tried struct {
  			vendor []string
  			goroot string
  			gopath []string
  		}
  		gopath := ctxt.gopath()
  
  		// 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 searchVendor(ctxt.GOROOT, true) {
  				goto Found
  			}
  			for _, root := range gopath {
  				if searchVendor(root, false) {
  					goto Found
  				}
  			}
  		}
  
  		// Determine directory from import path.
  		if ctxt.GOROOT != "" {
  			dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
  			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
  		}
  		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)
  		}
  
  		// 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) {
  		// 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
  	}
  
  	dirs, err := ctxt.readDir(p.Dir)
  	if err != nil {
  		return p, err
  	}
  
  	var badGoError error
  	var Sfiles []string // files with ".S" (capital S)
  	var firstFile, firstCommentFile string
  	imported := make(map[string][]token.Position)
  	testImported := make(map[string][]token.Position)
  	xTestImported := make(map[string][]token.Position)
  	allTags := make(map[string]bool)
  	fset := token.NewFileSet()
  	for _, d := range dirs {
  		if d.IsDir() {
  			continue
  		}
  
  		name := d.Name()
  		ext := nameExt(name)
  
  		badFile := func(err error) {
  			if badGoError == nil {
  				badGoError = err
  			}
  			p.InvalidGoFiles = append(p.InvalidGoFiles, name)
  		}
  
  		match, data, filename, err := ctxt.matchFile(p.Dir, name, allTags, &p.BinaryOnly)
  		if err != nil {
  			badFile(err)
  			continue
  		}
  		if !match {
  			if ext == ".go" {
  				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
  			}
  			continue
  		}
  
  		// Going to save the file. For non-Go files, can stop here.
  		switch ext {
  		case ".c":
  			p.CFiles = append(p.CFiles, name)
  			continue
  		case ".cc", ".cpp", ".cxx":
  			p.CXXFiles = append(p.CXXFiles, name)
  			continue
  		case ".m":
  			p.MFiles = append(p.MFiles, name)
  			continue
  		case ".h", ".hh", ".hpp", ".hxx":
  			p.HFiles = append(p.HFiles, name)
  			continue
  		case ".f", ".F", ".for", ".f90":
  			p.FFiles = append(p.FFiles, name)
  			continue
  		case ".s":
  			p.SFiles = append(p.SFiles, name)
  			continue
  		case ".S":
  			Sfiles = append(Sfiles, name)
  			continue
  		case ".swig":
  			p.SwigFiles = append(p.SwigFiles, name)
  			continue
  		case ".swigcxx":
  			p.SwigCXXFiles = append(p.SwigCXXFiles, name)
  			continue
  		case ".syso":
  			// binary objects to add to package archive
  			// Likely of the form foo_windows.syso, but
  			// the name was vetted above with goodOSArchFile.
  			p.SysoFiles = append(p.SysoFiles, name)
  			continue
  		}
  
  		pf, err := parser.ParseFile(fset, filename, data, parser.ImportsOnly|parser.ParseComments)
  		if err != nil {
  			badFile(err)
  			continue
  		}
  
  		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)
  		}
  		if pf.Doc != nil && p.Doc == "" {
  			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 _, decl := range pf.Decls {
  			d, ok := decl.(*ast.GenDecl)
  			if !ok {
  				continue
  			}
  			for _, dspec := range d.Specs {
  				spec, ok := dspec.(*ast.ImportSpec)
  				if !ok {
  					continue
  				}
  				quoted := spec.Path.Value
  				path, err := strconv.Unquote(quoted)
  				if err != nil {
  					log.Panicf("%s: parser returned invalid quoted string: <%s>", filename, quoted)
  				}
  				if isXTest {
  					xTestImported[path] = append(xTestImported[path], fset.Position(spec.Pos()))
  				} else if isTest {
  					testImported[path] = append(testImported[path], fset.Position(spec.Pos()))
  				} else {
  					imported[path] = append(imported[path], fset.Position(spec.Pos()))
  				}
  				if path == "C" {
  					if isTest {
  						badFile(fmt.Errorf("use of cgo in test %s not supported", filename))
  					} else {
  						cg := spec.Doc
  						if cg == nil && len(d.Specs) == 1 {
  							cg = d.Doc
  						}
  						if cg != nil {
  							if err := ctxt.saveCgo(filename, p, cg); err != nil {
  								badFile(err)
  							}
  						}
  						isCgo = true
  					}
  				}
  			}
  		}
  		if isCgo {
  			allTags["cgo"] = true
  			if ctxt.CgoEnabled {
  				p.CgoFiles = append(p.CgoFiles, name)
  			} else {
  				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
  			}
  		} else if isXTest {
  			p.XTestGoFiles = append(p.XTestGoFiles, name)
  		} else if isTest {
  			p.TestGoFiles = append(p.TestGoFiles, name)
  		} else {
  			p.GoFiles = append(p.GoFiles, name)
  		}
  	}
  	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}
  	}
  
  	for tag := range allTags {
  		p.AllTags = append(p.AllTags, tag)
  	}
  	sort.Strings(p.AllTags)
  
  	p.Imports, p.ImportPos = cleanImports(imported)
  	p.TestImports, p.TestImportPos = cleanImports(testImported)
  	p.XTestImports, p.XTestImportPos = cleanImports(xTestImported)
  
  	// add the .S 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)
  	}
  
  	return p, pkgerr
  }
  
  // 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) {
  	match, _, _, err = ctxt.matchFile(dir, name, nil, nil)
  	return
  }
  
  // matchFile determines whether the file with the given name in the given directory
  // should be included in the package being constructed.
  // It returns the data read from the file.
  // If name denotes a Go program, matchFile reads until the end of the
  // imports (and returns that data) even though it only considers text
  // until the first non-comment.
  // 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) (match bool, data []byte, filename string, err error) {
  	if strings.HasPrefix(name, "_") ||
  		strings.HasPrefix(name, ".") {
  		return
  	}
  
  	i := strings.LastIndex(name, ".")
  	if i < 0 {
  		i = len(name)
  	}
  	ext := name[i:]
  
  	if !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
  		return
  	}
  
  	switch ext {
  	case ".go", ".c", ".cc", ".cxx", ".cpp", ".m", ".s", ".h", ".hh", ".hpp", ".hxx", ".f", ".F", ".f90", ".S", ".swig", ".swigcxx":
  		// tentatively okay - read to make sure
  	case ".syso":
  		// binary, no reading
  		match = true
  		return
  	default:
  		// skip
  		return
  	}
  
  	filename = ctxt.joinPath(dir, name)
  	f, err := ctxt.openFile(filename)
  	if err != nil {
  		return
  	}
  
  	if strings.HasSuffix(filename, ".go") {
  		data, err = readImports(f, false, nil)
  		if strings.HasSuffix(filename, "_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
  		data, err = readComments(f)
  	}
  	f.Close()
  	if err != nil {
  		err = fmt.Errorf("read %s: %v", filename, err)
  		return
  	}
  
  	// Look for +build comments to accept or reject the file.
  	var sawBinaryOnly bool
  	if !ctxt.shouldBuild(data, allTags, &sawBinaryOnly) && !ctxt.UseAllFiles {
  		return
  	}
  
  	if binaryOnly != nil && sawBinaryOnly {
  		*binaryOnly = true
  	}
  	match = true
  	return
  }
  
  func cleanImports(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 slashslash = []byte("//")
  
  // 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.
  //
  // If shouldBuild finds a //go:binary-only-package comment in the file,
  // it sets *binaryOnly to true. Otherwise it does not change *binaryOnly.
  //
  func (ctxt *Context) shouldBuild(content []byte, allTags map[string]bool, binaryOnly *bool) bool {
  	sawBinaryOnly := false
  
  	// Pass 1. Identify leading run of // comments and blank lines,
  	// which must be followed by a blank line.
  	end := 0
  	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 len(line) == 0 { // Blank line
  			end = len(content) - len(p)
  			continue
  		}
  		if !bytes.HasPrefix(line, slashslash) { // Not comment line
  			break
  		}
  	}
  	content = content[:end]
  
  	// Pass 2.  Process each line in the run.
  	p = content
  	allok := true
  	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, slashslash) {
  			if bytes.Equal(line, binaryOnlyComment) {
  				sawBinaryOnly = true
  			}
  			line = bytes.TrimSpace(line[len(slashslash):])
  			if len(line) > 0 && line[0] == '+' {
  				// Looks like a comment +line.
  				f := strings.Fields(string(line))
  				if f[0] == "+build" {
  					ok := false
  					for _, tok := range f[1:] {
  						if ctxt.match(tok, allTags) {
  							ok = true
  						}
  					}
  					if !ok {
  						allok = false
  					}
  				}
  			}
  		}
  	}
  
  	if binaryOnly != nil && sawBinaryOnly {
  		*binaryOnly = true
  	}
  
  	return allok
  }
  
  // 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.match(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,$ORIGIN.
  // We never pass these arguments to a shell (just to programs we construct argv for), so this should be okay.
  // See golang.org/issue/6038.
  // The @ is for OS X. See golang.org/issue/13720.
  // The % is for Jenkins. See golang.org/issue/16959.
  const safeString = "+-.,/0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz:$@% "
  
  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
  }
  
  // match reports whether the name is one of:
  //
  //	$GOOS
  //	$GOARCH
  //	cgo (if cgo is enabled)
  //	!cgo (if cgo is disabled)
  //	ctxt.Compiler
  //	!ctxt.Compiler
  //	tag (if tag is listed in ctxt.BuildTags or ctxt.ReleaseTags)
  //	!tag (if tag is not listed in ctxt.BuildTags or ctxt.ReleaseTags)
  //	a comma-separated list of any of these
  //
  func (ctxt *Context) match(name string, allTags map[string]bool) bool {
  	if name == "" {
  		if allTags != nil {
  			allTags[name] = true
  		}
  		return false
  	}
  	if i := strings.Index(name, ","); i >= 0 {
  		// comma-separated list
  		ok1 := ctxt.match(name[:i], allTags)
  		ok2 := ctxt.match(name[i+1:], allTags)
  		return ok1 && ok2
  	}
  	if strings.HasPrefix(name, "!!") { // bad syntax, reject always
  		return false
  	}
  	if strings.HasPrefix(name, "!") { // negation
  		return len(name) > 1 && !ctxt.match(name[1:], allTags)
  	}
  
  	if allTags != nil {
  		allTags[name] = true
  	}
  
  	// Tags must be letters, digits, underscores or dots.
  	// Unlike in Go identifiers, all digits are fine (e.g., "386").
  	for _, c := range name {
  		if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
  			return false
  		}
  	}
  
  	// 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
  	}
  
  	// 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.*
  //
  // An exception: if GOOS=android, then files with GOOS=linux 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]] {
  		if allTags != nil {
  			allTags[l[n-2]] = true
  			allTags[l[n-1]] = true
  		}
  		if l[n-1] != ctxt.GOARCH {
  			return false
  		}
  		if ctxt.GOOS == "android" && l[n-2] == "linux" {
  			return true
  		}
  		return l[n-2] == ctxt.GOOS
  	}
  	if n >= 1 && knownOS[l[n-1]] {
  		if allTags != nil {
  			allTags[l[n-1]] = true
  		}
  		if ctxt.GOOS == "android" && l[n-1] == "linux" {
  			return true
  		}
  		return l[n-1] == ctxt.GOOS
  	}
  	if n >= 1 && knownArch[l[n-1]] {
  		if allTags != nil {
  			allTags[l[n-1]] = true
  		}
  		return l[n-1] == ctxt.GOARCH
  	}
  	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 = filepath.Join(runtime.GOROOT(), "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
  
  // 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")
  }
  

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