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// Copyright 2013 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 loader
// See doc.go for package documentation and implementation notes.
import (
var ignoreVendor build.ImportMode
const trace = false // show timing info for type-checking
// Config specifies the configuration for loading a whole program from
// Go source code.
// The zero value for Config is a ready-to-use default configuration.
type Config struct {
// Fset is the file set for the parser to use when loading the
// program. If nil, it may be lazily initialized by any
// method of Config.
Fset *token.FileSet
// ParserMode specifies the mode to be used by the parser when
// loading source packages.
ParserMode parser.Mode
// TypeChecker contains options relating to the type checker.
// The supplied IgnoreFuncBodies is not used; the effective
// value comes from the TypeCheckFuncBodies func below.
// The supplied Import function is not used either.
TypeChecker types.Config
// TypeCheckFuncBodies is a predicate over package paths.
// A package for which the predicate is false will
// have its package-level declarations type checked, but not
// its function bodies; this can be used to quickly load
// dependencies from source. If nil, all func bodies are type
// checked.
TypeCheckFuncBodies func(path string) bool
// If Build is non-nil, it is used to locate source packages.
// Otherwise &build.Default is used.
// By default, cgo is invoked to preprocess Go files that
// import the fake package "C". This behaviour can be
// disabled by setting CGO_ENABLED=0 in the environment prior
// to startup, or by setting Build.CgoEnabled=false.
Build *build.Context
// The current directory, used for resolving relative package
// references such as "./go/loader". If empty, os.Getwd will be
// used instead.
Cwd string
// If DisplayPath is non-nil, it is used to transform each
// file name obtained from Build.Import(). This can be used
// to prevent a virtualized build.Config's file names from
// leaking into the user interface.
DisplayPath func(path string) string
// If AllowErrors is true, Load will return a Program even
// if some of the its packages contained I/O, parser or type
// errors; such errors are accessible via PackageInfo.Errors. If
// false, Load will fail if any package had an error.
AllowErrors bool
// CreatePkgs specifies a list of non-importable initial
// packages to create. The resulting packages will appear in
// the corresponding elements of the Program.Created slice.
CreatePkgs []PkgSpec
// ImportPkgs specifies a set of initial packages to load.
// The map keys are package paths.
// The map value indicates whether to load tests. If true, Load
// will add and type-check two lists of files to the package:
// non-test files followed by in-package *_test.go files. In
// addition, it will append the external test package (if any)
// to Program.Created.
ImportPkgs map[string]bool
// FindPackage is called during Load to create the build.Package
// for a given import path from a given directory.
// If FindPackage is nil, (*build.Context).Import is used.
// A client may use this hook to adapt to a proprietary build
// system that does not follow the "go build" layout
// conventions, for example.
// It must be safe to call concurrently from multiple goroutines.
FindPackage func(ctxt *build.Context, importPath, fromDir string, mode build.ImportMode) (*build.Package, error)
// AfterTypeCheck is called immediately after a list of files
// has been type-checked and appended to info.Files.
// This optional hook function is the earliest opportunity for
// the client to observe the output of the type checker,
// which may be useful to reduce analysis latency when loading
// a large program.
// The function is permitted to modify info.Info, for instance
// to clear data structures that are no longer needed, which can
// dramatically reduce peak memory consumption.
// The function may be called twice for the same PackageInfo:
// once for the files of the package and again for the
// in-package test files.
// It must be safe to call concurrently from multiple goroutines.
AfterTypeCheck func(info *PackageInfo, files []*ast.File)
// A PkgSpec specifies a non-importable package to be created by Load.
// Files are processed first, but typically only one of Files and
// Filenames is provided. The path needn't be globally unique.
// For vendoring purposes, the package's directory is the one that
// contains the first file.
type PkgSpec struct {
Path string // package path ("" => use package declaration)
Files []*ast.File // ASTs of already-parsed files
Filenames []string // names of files to be parsed
// A Program is a Go program loaded from source as specified by a Config.
type Program struct {
Fset *token.FileSet // the file set for this program
// Created[i] contains the initial package whose ASTs or
// filenames were supplied by Config.CreatePkgs[i], followed by
// the external test package, if any, of each package in
// Config.ImportPkgs ordered by ImportPath.
// NOTE: these files must not import "C". Cgo preprocessing is
// only performed on imported packages, not ad hoc packages.
// TODO(adonovan): we need to copy and adapt the logic of
// goFilesPackage (from $GOROOT/src/cmd/go/build.go) and make
// Config.Import and Config.Create methods return the same kind
// of entity, essentially a build.Package.
// Perhaps we can even reuse that type directly.
Created []*PackageInfo
// Imported contains the initially imported packages,
// as specified by Config.ImportPkgs.
Imported map[string]*PackageInfo
// AllPackages contains the PackageInfo of every package
// encountered by Load: all initial packages and all
// dependencies, including incomplete ones.
AllPackages map[*types.Package]*PackageInfo
// importMap is the canonical mapping of package paths to
// packages. It contains all Imported initial packages, but not
// Created ones, and all imported dependencies.
importMap map[string]*types.Package
// PackageInfo holds the ASTs and facts derived by the type-checker
// for a single package.
// Not mutated once exposed via the API.
type PackageInfo struct {
Pkg *types.Package
Importable bool // true if 'import "Pkg.Path()"' would resolve to this
TransitivelyErrorFree bool // true if Pkg and all its dependencies are free of errors
Files []*ast.File // syntax trees for the package's files
Errors []error // non-nil if the package had errors
types.Info // type-checker deductions.
dir string // package directory
checker *types.Checker // transient type-checker state
errorFunc func(error)
func (info *PackageInfo) String() string { return info.Pkg.Path() }
func (info *PackageInfo) appendError(err error) {
if info.errorFunc != nil {
} else {
fmt.Fprintln(os.Stderr, err)
info.Errors = append(info.Errors, err)
func (conf *Config) fset() *token.FileSet {
if conf.Fset == nil {
conf.Fset = token.NewFileSet()
return conf.Fset
// ParseFile is a convenience function (intended for testing) that invokes
// the parser using the Config's FileSet, which is initialized if nil.
// src specifies the parser input as a string, []byte, or io.Reader, and
// filename is its apparent name. If src is nil, the contents of
// filename are read from the file system.
func (conf *Config) ParseFile(filename string, src interface{}) (*ast.File, error) {
// TODO(adonovan): use etc like parseFiles does.
return parser.ParseFile(conf.fset(), filename, src, conf.ParserMode)
// FromArgsUsage is a partial usage message that applications calling
// FromArgs may wish to include in their -help output.
const FromArgsUsage = `
<args> is a list of arguments denoting a set of initial packages.
It may take one of two forms:
1. A list of *.go source files.
All of the specified files are loaded, parsed and type-checked
as a single package. All the files must belong to the same directory.
2. A list of import paths, each denoting a package.
The package's directory is found relative to the $GOROOT and
$GOPATH using similar logic to 'go build', and the *.go files in
that directory are loaded, parsed and type-checked as a single
In addition, all *_test.go files in the directory are then loaded
and parsed. Those files whose package declaration equals that of
the non-*_test.go files are included in the primary package. Test
files whose package declaration ends with "_test" are type-checked
as another package, the 'external' test package, so that a single
import path may denote two packages. (Whether this behaviour is
enabled is tool-specific, and may depend on additional flags.)
A '--' argument terminates the list of packages.
// FromArgs interprets args as a set of initial packages to load from
// source and updates the configuration. It returns the list of
// unconsumed arguments.
// It is intended for use in command-line interfaces that require a
// set of initial packages to be specified; see FromArgsUsage message
// for details.
// Only superficial errors are reported at this stage; errors dependent
// on I/O are detected during Load.
func (conf *Config) FromArgs(args []string, xtest bool) ([]string, error) {
var rest []string
for i, arg := range args {
if arg == "--" {
rest = args[i+1:]
args = args[:i]
break // consume "--" and return the remaining args
if len(args) > 0 && strings.HasSuffix(args[0], ".go") {
// Assume args is a list of a *.go files
// denoting a single ad hoc package.
for _, arg := range args {
if !strings.HasSuffix(arg, ".go") {
return nil, fmt.Errorf("named files must be .go files: %s", arg)
conf.CreateFromFilenames("", args...)
} else {
// Assume args are directories each denoting a
// package and (perhaps) an external test, iff xtest.
for _, arg := range args {
if xtest {
} else {
return rest, nil
// CreateFromFilenames is a convenience function that adds
// a conf.CreatePkgs entry to create a package of the specified *.go
// files.
func (conf *Config) CreateFromFilenames(path string, filenames ...string) {
conf.CreatePkgs = append(conf.CreatePkgs, PkgSpec{Path: path, Filenames: filenames})
// CreateFromFiles is a convenience function that adds a conf.CreatePkgs
// entry to create package of the specified path and parsed files.
func (conf *Config) CreateFromFiles(path string, files ...*ast.File) {
conf.CreatePkgs = append(conf.CreatePkgs, PkgSpec{Path: path, Files: files})
// ImportWithTests is a convenience function that adds path to
// ImportPkgs, the set of initial source packages located relative to
// $GOPATH. The package will be augmented by any *_test.go files in
// its directory that contain a "package x" (not "package x_test")
// declaration.
// In addition, if any *_test.go files contain a "package x_test"
// declaration, an additional package comprising just those files will
// be added to CreatePkgs.
func (conf *Config) ImportWithTests(path string) { conf.addImport(path, true) }
// Import is a convenience function that adds path to ImportPkgs, the
// set of initial packages that will be imported from source.
func (conf *Config) Import(path string) { conf.addImport(path, false) }
func (conf *Config) addImport(path string, tests bool) {
if path == "C" {
return // ignore; not a real package
if conf.ImportPkgs == nil {
conf.ImportPkgs = make(map[string]bool)
conf.ImportPkgs[path] = conf.ImportPkgs[path] || tests
// PathEnclosingInterval returns the PackageInfo and ast.Node that
// contain source interval [start, end), and all the node's ancestors
// up to the AST root. It searches all ast.Files of all packages in prog.
// exact is defined as for astutil.PathEnclosingInterval.
// The zero value is returned if not found.
func (prog *Program) PathEnclosingInterval(start, end token.Pos) (pkg *PackageInfo, path []ast.Node, exact bool) {
for _, info := range prog.AllPackages {
for _, f := range info.Files {
if f.Pos() == token.NoPos {
// This can happen if the parser saw
// too many errors and bailed out.
// (Use parser.AllErrors to prevent that.)
if !tokenFileContainsPos(prog.Fset.File(f.Pos()), start) {
if path, exact := astutil.PathEnclosingInterval(f, start, end); path != nil {
return info, path, exact
return nil, nil, false
// InitialPackages returns a new slice containing the set of initial
// packages (Created + Imported) in unspecified order.
func (prog *Program) InitialPackages() []*PackageInfo {
infos := make([]*PackageInfo, 0, len(prog.Created)+len(prog.Imported))
infos = append(infos, prog.Created...)
for _, info := range prog.Imported {
infos = append(infos, info)
return infos
// Package returns the ASTs and results of type checking for the
// specified package.
func (prog *Program) Package(path string) *PackageInfo {
if info, ok := prog.AllPackages[prog.importMap[path]]; ok {
return info
for _, info := range prog.Created {
if path == info.Pkg.Path() {
return info
return nil
// ---------- Implementation ----------
// importer holds the working state of the algorithm.
type importer struct {
conf *Config // the client configuration
start time.Time // for logging
progMu sync.Mutex // guards prog
prog *Program // the resulting program
// findpkg is a memoization of FindPackage.
findpkgMu sync.Mutex // guards findpkg
findpkg map[findpkgKey]*findpkgValue
importedMu sync.Mutex // guards imported
imported map[string]*importInfo // all imported packages (incl. failures) by import path
// import dependency graph: graph[x][y] => x imports y
// Since non-importable packages cannot be cyclic, we ignore
// their imports, thus we only need the subgraph over importable
// packages. Nodes are identified by their import paths.
graphMu sync.Mutex
graph map[string]map[string]bool
type findpkgKey struct {
importPath string
fromDir string
mode build.ImportMode
type findpkgValue struct {
ready chan struct{} // closed to broadcast readiness
bp *build.Package
err error
// importInfo tracks the success or failure of a single import.
// Upon completion, exactly one of info and err is non-nil:
// info on successful creation of a package, err otherwise.
// A successful package may still contain type errors.
type importInfo struct {
path string // import path
info *PackageInfo // results of typechecking (including errors)
complete chan struct{} // closed to broadcast that info is set.
// awaitCompletion blocks until ii is complete,
// i.e. the info field is safe to inspect.
func (ii *importInfo) awaitCompletion() {
<-ii.complete // wait for close
// Complete marks ii as complete.
// Its info and err fields will not be subsequently updated.
func (ii *importInfo) Complete(info *PackageInfo) {
if info == nil {
panic("info == nil")
} = info
type importError struct {
path string // import path
err error // reason for failure to create a package
// Load creates the initial packages specified by conf.{Create,Import}Pkgs,
// loading their dependencies packages as needed.
// On success, Load returns a Program containing a PackageInfo for
// each package. On failure, it returns an error.
// If AllowErrors is true, Load will return a Program even if some
// packages contained I/O, parser or type errors, or if dependencies
// were missing. (Such errors are accessible via PackageInfo.Errors. If
// false, Load will fail if any package had an error.
// It is an error if no packages were loaded.
func (conf *Config) Load() (*Program, error) {
// Create a simple default error handler for parse/type errors.
if conf.TypeChecker.Error == nil {
conf.TypeChecker.Error = func(e error) { fmt.Fprintln(os.Stderr, e) }
// Set default working directory for relative package references.
if conf.Cwd == "" {
var err error
conf.Cwd, err = os.Getwd()
if err != nil {
return nil, err
// Install default FindPackage hook using go/build logic.
if conf.FindPackage == nil {
conf.FindPackage = (*build.Context).Import
prog := &Program{
Fset: conf.fset(),
Imported: make(map[string]*PackageInfo),
importMap: make(map[string]*types.Package),
AllPackages: make(map[*types.Package]*PackageInfo),
imp := importer{
conf: conf,
prog: prog,
findpkg: make(map[findpkgKey]*findpkgValue),
imported: make(map[string]*importInfo),
start: time.Now(),
graph: make(map[string]map[string]bool),
// -- loading proper (concurrent phase) --------------------------------
var errpkgs []string // packages that contained errors
// Load the initially imported packages and their dependencies,
// in parallel.
// No vendor check on packages imported from the command line.
infos, importErrors := imp.importAll("", conf.Cwd, conf.ImportPkgs, ignoreVendor)
for _, ie := range importErrors {
conf.TypeChecker.Error(ie.err) // failed to create package
errpkgs = append(errpkgs, ie.path)
for _, info := range infos {
prog.Imported[info.Pkg.Path()] = info
// Augment the designated initial packages by their tests.
// Dependencies are loaded in parallel.
var xtestPkgs []*build.Package
for importPath, augment := range conf.ImportPkgs {
if !augment {
// No vendor check on packages imported from command line.
bp, err := imp.findPackage(importPath, conf.Cwd, ignoreVendor)
if err != nil {
// Package not found, or can't even parse package declaration.
// Already reported by previous loop; ignore it.
// Needs external test package?
if len(bp.XTestGoFiles) > 0 {
xtestPkgs = append(xtestPkgs, bp)
// Consult the cache using the canonical package path.
path := bp.ImportPath
imp.importedMu.Lock() // (unnecessary, we're sequential here)
ii, ok := imp.imported[path]
// Paranoid checks added due to issue #11012.
if !ok {
// Unreachable.
// The previous loop called importAll and thus
// startLoad for each path in ImportPkgs, which
// populates imp.imported[path] with a non-zero value.
panic(fmt.Sprintf("imported[%q] not found", path))
if ii == nil {
// Unreachable.
// The ii values in this loop are the same as in
// the previous loop, which enforced the invariant
// that at least one of ii.err and is non-nil.
panic(fmt.Sprintf("imported[%q] == nil", path))
if == nil {
// Unreachable.
// awaitCompletion has the postcondition
// != nil.
panic(fmt.Sprintf("imported[%q].info = nil", path))
info :=
// Parse the in-package test files.
files, errs := imp.conf.parsePackageFiles(bp, 't')
for _, err := range errs {
// The test files augmenting package P cannot be imported,
// but may import packages that import P,
// so we must disable the cycle check.
imp.addFiles(info, files, false)
createPkg := func(path, dir string, files []*ast.File, errs []error) {
info := imp.newPackageInfo(path, dir)
for _, err := range errs {
// Ad hoc packages are non-importable,
// so no cycle check is needed.
// addFiles loads dependencies in parallel.
imp.addFiles(info, files, false)
prog.Created = append(prog.Created, info)
// Create packages specified by conf.CreatePkgs.
for _, cp := range conf.CreatePkgs {
files, errs := parseFiles(conf.fset(),, nil, conf.Cwd, cp.Filenames, conf.ParserMode)
files = append(files, cp.Files...)
path := cp.Path
if path == "" {
if len(files) > 0 {
path = files[0].Name.Name
} else {
path = "(unnamed)"
dir := conf.Cwd
if len(files) > 0 && files[0].Pos().IsValid() {
dir = filepath.Dir(conf.fset().File(files[0].Pos()).Name())
createPkg(path, dir, files, errs)
// Create external test packages.
for _, bp := range xtestPkgs {
files, errs := imp.conf.parsePackageFiles(bp, 'x')
createPkg(bp.ImportPath+"_test", bp.Dir, files, errs)
// -- finishing up (sequential) ----------------------------------------
if len(prog.Imported)+len(prog.Created) == 0 {
return nil, errors.New("no initial packages were loaded")
// Create infos for indirectly imported packages.
// e.g. incomplete packages without syntax, loaded from export data.
for _, obj := range prog.importMap {
info := prog.AllPackages[obj]
if info == nil {
prog.AllPackages[obj] = &PackageInfo{Pkg: obj, Importable: true}
} else {
// finished
info.checker = nil
info.errorFunc = nil
if !conf.AllowErrors {
// Report errors in indirectly imported packages.
for _, info := range prog.AllPackages {
if len(info.Errors) > 0 {
errpkgs = append(errpkgs, info.Pkg.Path())
if errpkgs != nil {
var more string
if len(errpkgs) > 3 {
more = fmt.Sprintf(" and %d more", len(errpkgs)-3)
errpkgs = errpkgs[:3]
return nil, fmt.Errorf("couldn't load packages due to errors: %s%s",
strings.Join(errpkgs, ", "), more)
return prog, nil
type byImportPath []*build.Package
func (b byImportPath) Len() int { return len(b) }
func (b byImportPath) Less(i, j int) bool { return b[i].ImportPath < b[j].ImportPath }
func (b byImportPath) Swap(i, j int) { b[i], b[j] = b[j], b[i] }
// markErrorFreePackages sets the TransitivelyErrorFree flag on all
// applicable packages.
func markErrorFreePackages(allPackages map[*types.Package]*PackageInfo) {
// Build the transpose of the import graph.
importedBy := make(map[*types.Package]map[*types.Package]bool)
for P := range allPackages {
for _, Q := range P.Imports() {
clients, ok := importedBy[Q]
if !ok {
clients = make(map[*types.Package]bool)
importedBy[Q] = clients
clients[P] = true
// Find all packages reachable from some error package.
reachable := make(map[*types.Package]bool)
var visit func(*types.Package)
visit = func(p *types.Package) {
if !reachable[p] {
reachable[p] = true
for q := range importedBy[p] {
for _, info := range allPackages {
if len(info.Errors) > 0 {
// Mark the others as "transitively error-free".
for _, info := range allPackages {
if !reachable[info.Pkg] {
info.TransitivelyErrorFree = true
// build returns the effective build context.
func (conf *Config) build() *build.Context {
if conf.Build != nil {
return conf.Build
return &build.Default
// parsePackageFiles enumerates the files belonging to package path,
// then loads, parses and returns them, plus a list of I/O or parse
// errors that were encountered.
// 'which' indicates which files to include:
// 'g': include non-test *.go source files (GoFiles + processed CgoFiles)
// 't': include in-package *_test.go source files (TestGoFiles)
// 'x': include external *_test.go source files. (XTestGoFiles)
func (conf *Config) parsePackageFiles(bp *build.Package, which rune) ([]*ast.File, []error) {
if bp.ImportPath == "unsafe" {
return nil, nil
var filenames []string
switch which {
case 'g':
filenames = bp.GoFiles
case 't':
filenames = bp.TestGoFiles
case 'x':
filenames = bp.XTestGoFiles
files, errs := parseFiles(conf.fset(),, conf.DisplayPath, bp.Dir, filenames, conf.ParserMode)
// Preprocess CgoFiles and parse the outputs (sequentially).
if which == 'g' && bp.CgoFiles != nil {
cgofiles, err := cgo.ProcessFiles(bp, conf.fset(), conf.DisplayPath, conf.ParserMode)
if err != nil {
errs = append(errs, err)
} else {
files = append(files, cgofiles...)
return files, errs
// doImport imports the package denoted by path.
// It implements the types.Importer signature.
// It returns an error if a package could not be created
// (e.g. go/build or parse error), but type errors are reported via
// the types.Config.Error callback (the first of which is also saved
// in the package's PackageInfo).
// Idempotent.
func (imp *importer) doImport(from *PackageInfo, to string) (*types.Package, error) {
if to == "C" {
// This should be unreachable, but ad hoc packages are
// not currently subject to cgo preprocessing.
// See
return nil, fmt.Errorf(`the loader doesn't cgo-process ad hoc packages like %q; see Go issue 11627`,
bp, err := imp.findPackage(to, from.dir, 0)
if err != nil {
return nil, err
// The standard unsafe package is handled specially,
// and has no PackageInfo.
if bp.ImportPath == "unsafe" {
return types.Unsafe, nil
// Look for the package in the cache using its canonical path.
path := bp.ImportPath
ii := imp.imported[path]
if ii == nil {
panic("internal error: unexpected import: " + path)
if != nil {
return, nil
// Import of incomplete package: this indicates a cycle.
fromPath := from.Pkg.Path()
if cycle := imp.findPath(path, fromPath); cycle != nil {
// Normalize cycle: start from alphabetically largest node.
pos, start := -1, ""
for i, s := range cycle {
if pos < 0 || s > start {
pos, start = i, s
cycle = append(cycle, cycle[:pos]...)[pos:] // rotate cycle to start from largest
cycle = append(cycle, cycle[0]) // add start node to end to show cycliness
return nil, fmt.Errorf("import cycle: %s", strings.Join(cycle, " -> "))
panic("internal error: import of incomplete (yet acyclic) package: " + fromPath)
// findPackage locates the package denoted by the importPath in the
// specified directory.
func (imp *importer) findPackage(importPath, fromDir string, mode build.ImportMode) (*build.Package, error) {
// We use a non-blocking duplicate-suppressing cache ( ยง9.7)
// to avoid holding the lock around FindPackage.
key := findpkgKey{importPath, fromDir, mode}
v, ok := imp.findpkg[key]
if ok {
// cache hit
<-v.ready // wait for entry to become ready
} else {
// Cache miss: this goroutine becomes responsible for
// populating the map entry and broadcasting its readiness.
v = &findpkgValue{ready: make(chan struct{})}
imp.findpkg[key] = v
ioLimit <- true
v.bp, v.err = imp.conf.FindPackage(, importPath, fromDir, mode)
if _, ok := v.err.(*build.NoGoError); ok {
v.err = nil // empty directory is not an error
close(v.ready) // broadcast ready condition
return v.bp, v.err
// importAll loads, parses, and type-checks the specified packages in
// parallel and returns their completed importInfos in unspecified order.
// fromPath is the package path of the importing package, if it is
// importable, "" otherwise. It is used for cycle detection.
// fromDir is the directory containing the import declaration that
// caused these imports.
func (imp *importer) importAll(fromPath, fromDir string, imports map[string]bool, mode build.ImportMode) (infos []*PackageInfo, errors []importError) {
if fromPath != "" {
// We're loading a set of imports.
// We must record graph edges from the importing package
// to its dependencies, and check for cycles.
deps, ok := imp.graph[fromPath]
if !ok {
deps = make(map[string]bool)
imp.graph[fromPath] = deps
for importPath := range imports {
deps[importPath] = true
var pending []*importInfo
for importPath := range imports {
if fromPath != "" {
if cycle := imp.findPath(importPath, fromPath); cycle != nil {
// Cycle-forming import: we must not check it
// since it would deadlock.
if trace {
fmt.Fprintf(os.Stderr, "import cycle: %q\n", cycle)
bp, err := imp.findPackage(importPath, fromDir, mode)
if err != nil {
errors = append(errors, importError{
path: importPath,
err: err,
pending = append(pending, imp.startLoad(bp))
for _, ii := range pending {
infos = append(infos,
return infos, errors
// findPath returns an arbitrary path from 'from' to 'to' in the import
// graph, or nil if there was none.
func (imp *importer) findPath(from, to string) []string {
defer imp.graphMu.Unlock()
seen := make(map[string]bool)
var search func(stack []string, importPath string) []string
search = func(stack []string, importPath string) []string {
if !seen[importPath] {
seen[importPath] = true
stack = append(stack, importPath)
if importPath == to {
return stack
for x := range imp.graph[importPath] {
if p := search(stack, x); p != nil {
return p
return nil
return search(make([]string, 0, 20), from)
// startLoad initiates the loading, parsing and type-checking of the
// specified package and its dependencies, if it has not already begun.
// It returns an importInfo, not necessarily in a completed state. The
// caller must call awaitCompletion() before accessing its info field.
// startLoad is concurrency-safe and idempotent.
func (imp *importer) startLoad(bp *build.Package) *importInfo {
path := bp.ImportPath
ii, ok := imp.imported[path]
if !ok {
ii = &importInfo{path: path, complete: make(chan struct{})}
imp.imported[path] = ii
go func() {
info := imp.load(bp)
return ii
// load implements package loading by parsing Go source files
// located by go/build.
func (imp *importer) load(bp *build.Package) *PackageInfo {
info := imp.newPackageInfo(bp.ImportPath, bp.Dir)
info.Importable = true
files, errs := imp.conf.parsePackageFiles(bp, 'g')
for _, err := range errs {
imp.addFiles(info, files, true)
imp.prog.importMap[bp.ImportPath] = info.Pkg
return info
// addFiles adds and type-checks the specified files to info, loading
// their dependencies if needed. The order of files determines the
// package initialization order. It may be called multiple times on the
// same package. Errors are appended to the info.Errors field.
// cycleCheck determines whether the imports within files create
// dependency edges that should be checked for potential cycles.
func (imp *importer) addFiles(info *PackageInfo, files []*ast.File, cycleCheck bool) {
// Ensure the dependencies are loaded, in parallel.
var fromPath string
if cycleCheck {
fromPath = info.Pkg.Path()
// TODO(adonovan): opt: make the caller do scanImports.
// Callers with a build.Package can skip it.
imp.importAll(fromPath, info.dir, scanImports(files), 0)
if trace {
fmt.Fprintf(os.Stderr, "%s: start %q (%d)\n",
time.Since(imp.start), info.Pkg.Path(), len(files))
// Don't call checker.Files on Unsafe, even with zero files,
// because it would mutate the package, which is a global.
if info.Pkg == types.Unsafe {
if len(files) > 0 {
panic(`"unsafe" package contains unexpected files`)
} else {
// Ignore the returned (first) error since we
// already collect them all in the PackageInfo.
info.Files = append(info.Files, files...)
if imp.conf.AfterTypeCheck != nil {
imp.conf.AfterTypeCheck(info, files)
if trace {
fmt.Fprintf(os.Stderr, "%s: stop %q\n",
time.Since(imp.start), info.Pkg.Path())
func (imp *importer) newPackageInfo(path, dir string) *PackageInfo {
var pkg *types.Package
if path == "unsafe" {
pkg = types.Unsafe
} else {
pkg = types.NewPackage(path, "")
info := &PackageInfo{
Pkg: pkg,
Info: types.Info{
Types: make(map[ast.Expr]types.TypeAndValue),
Defs: make(map[*ast.Ident]types.Object),
Uses: make(map[*ast.Ident]types.Object),
Implicits: make(map[ast.Node]types.Object),
Scopes: make(map[ast.Node]*types.Scope),
Selections: make(map[*ast.SelectorExpr]*types.Selection),
errorFunc: imp.conf.TypeChecker.Error,
dir: dir,
// Copy the types.Config so we can vary it across PackageInfos.
tc := imp.conf.TypeChecker
tc.IgnoreFuncBodies = false
if f := imp.conf.TypeCheckFuncBodies; f != nil {
tc.IgnoreFuncBodies = !f(path)
tc.Importer = closure{imp, info}
tc.Error = info.appendError // appendError wraps the user's Error function
info.checker = types.NewChecker(&tc, imp.conf.fset(), pkg, &info.Info)
imp.prog.AllPackages[pkg] = info
return info
type closure struct {
imp *importer
info *PackageInfo
func (c closure) Import(to string) (*types.Package, error) { return c.imp.doImport(, to) }