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// Copyright 2018 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 modload
import (
pathpkg "path"
type ImportMissingError struct {
Path string
Module module.Version
QueryErr error
// isStd indicates whether we would expect to find the package in the standard
// library. This is normally true for all dotless import paths, but replace
// directives can cause us to treat the replaced paths as also being in
// modules.
isStd bool
// replaced the highest replaced version of the module where the replacement
// contains the package. replaced is only set if the replacement is unused.
replaced module.Version
// newMissingVersion is set to a newer version of Module if one is present
// in the build list. When set, we can't automatically upgrade.
newMissingVersion string
func (e *ImportMissingError) Error() string {
if e.Module.Path == "" {
if e.isStd {
return fmt.Sprintf("package %s is not in GOROOT (%s)", e.Path, filepath.Join(cfg.GOROOT, "src", e.Path))
if e.QueryErr != nil && e.QueryErr != ErrNoModRoot {
return fmt.Sprintf("cannot find module providing package %s: %v", e.Path, e.QueryErr)
if cfg.BuildMod == "mod" || (cfg.BuildMod == "readonly" && allowMissingModuleImports) {
return "cannot find module providing package " + e.Path
if e.replaced.Path != "" {
suggestArg := e.replaced.Path
if !module.IsZeroPseudoVersion(e.replaced.Version) {
suggestArg = e.replaced.String()
return fmt.Sprintf("module %s provides package %s and is replaced but not required; to add it:\n\tgo get %s", e.replaced.Path, e.Path, suggestArg)
message := fmt.Sprintf("no required module provides package %s", e.Path)
if e.QueryErr != nil {
return fmt.Sprintf("%s: %v", message, e.QueryErr)
return fmt.Sprintf("%s; to add it:\n\tgo get %s", message, e.Path)
if e.newMissingVersion != "" {
return fmt.Sprintf("package %s provided by %s at latest version %s but not at required version %s", e.Path, e.Module.Path, e.Module.Version, e.newMissingVersion)
return fmt.Sprintf("missing module for import: %s@%s provides %s", e.Module.Path, e.Module.Version, e.Path)
func (e *ImportMissingError) Unwrap() error {
return e.QueryErr
func (e *ImportMissingError) ImportPath() string {
return e.Path
// An AmbiguousImportError indicates an import of a package found in multiple
// modules in the build list, or found in both the main module and its vendor
// directory.
type AmbiguousImportError struct {
importPath string
Dirs []string
Modules []module.Version // Either empty or 1:1 with Dirs.
func (e *AmbiguousImportError) ImportPath() string {
return e.importPath
func (e *AmbiguousImportError) Error() string {
locType := "modules"
if len(e.Modules) == 0 {
locType = "directories"
var buf strings.Builder
fmt.Fprintf(&buf, "ambiguous import: found package %s in multiple %s:", e.importPath, locType)
for i, dir := range e.Dirs {
if i < len(e.Modules) {
m := e.Modules[i]
if m.Version != "" {
fmt.Fprintf(&buf, " %s", m.Version)
fmt.Fprintf(&buf, " (%s)", dir)
} else {
return buf.String()
// A DirectImportFromImplicitDependencyError indicates a package directly
// imported by a package or test in the main module that is satisfied by a
// dependency that is not explicit in the main module's go.mod file.
type DirectImportFromImplicitDependencyError struct {
ImporterPath string
ImportedPath string
Module module.Version
func (e *DirectImportFromImplicitDependencyError) Error() string {
return fmt.Sprintf("package %s imports %s from implicitly required module; to add missing requirements, run:\n\tgo get %s@%s", e.ImporterPath, e.ImportedPath, e.Module.Path, e.Module.Version)
func (e *DirectImportFromImplicitDependencyError) ImportPath() string {
return e.ImporterPath
// ImportMissingSumError is reported in readonly mode when we need to check
// if a module contains a package, but we don't have a sum for its .zip file.
// We might need sums for multiple modules to verify the package is unique.
// TODO(#43653): consolidate multiple errors of this type into a single error
// that suggests a 'go get' command for root packages that transtively import
// packages from modules with missing sums. load.CheckPackageErrors would be
// a good place to consolidate errors, but we'll need to attach the import
// stack here.
type ImportMissingSumError struct {
importPath string
found bool
mods []module.Version
importer, importerVersion string // optional, but used for additional context
importerIsTest bool
func (e *ImportMissingSumError) Error() string {
var importParen string
if e.importer != "" {
importParen = fmt.Sprintf(" (imported by %s)", e.importer)
var message string
if e.found {
message = fmt.Sprintf("missing go.sum entry needed to verify package %s%s is provided by exactly one module", e.importPath, importParen)
} else {
message = fmt.Sprintf("missing go.sum entry for module providing package %s%s", e.importPath, importParen)
var hint string
if e.importer == "" {
// Importing package is unknown, or the missing package was named on the
// command line. Recommend 'go mod download' for the modules that could
// provide the package, since that shouldn't change go.mod.
if len(e.mods) > 0 {
args := make([]string, len(e.mods))
for i, mod := range e.mods {
args[i] = mod.Path
hint = fmt.Sprintf("; to add:\n\tgo mod download %s", strings.Join(args, " "))
} else {
// Importing package is known (common case). Recommend 'go get' on the
// current version of the importing package.
tFlag := ""
if e.importerIsTest {
tFlag = " -t"
version := ""
if e.importerVersion != "" {
version = "@" + e.importerVersion
hint = fmt.Sprintf("; to add:\n\tgo get%s %s%s", tFlag, e.importer, version)
return message + hint
func (e *ImportMissingSumError) ImportPath() string {
return e.importPath
type invalidImportError struct {
importPath string
err error
func (e *invalidImportError) ImportPath() string {
return e.importPath
func (e *invalidImportError) Error() string {
return e.err.Error()
func (e *invalidImportError) Unwrap() error {
return e.err
// importFromModules finds the module and directory in the dependency graph of
// rs containing the package with the given import path. If mg is nil,
// importFromModules attempts to locate the module using only the main module
// and the roots of rs before it loads the full graph.
// The answer must be unique: importFromModules returns an error if multiple
// modules are observed to provide the same package.
// importFromModules can return a module with an empty m.Path, for packages in
// the standard library.
// importFromModules can return an empty directory string, for fake packages
// like "C" and "unsafe".
// If the package is not present in any module selected from the requirement
// graph, importFromModules returns an *ImportMissingError.
func importFromModules(ctx context.Context, path string, rs *Requirements, mg *ModuleGraph) (m module.Version, dir string, err error) {
if strings.Contains(path, "@") {
return module.Version{}, "", fmt.Errorf("import path should not have @version")
if build.IsLocalImport(path) {
return module.Version{}, "", fmt.Errorf("relative import not supported")
if path == "C" {
// There's no directory for import "C".
return module.Version{}, "", nil
// Before any further lookup, check that the path is valid.
if err := module.CheckImportPath(path); err != nil {
return module.Version{}, "", &invalidImportError{importPath: path, err: err}
// Is the package in the standard library?
pathIsStd := search.IsStandardImportPath(path)
if pathIsStd && goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
if targetInGorootSrc {
if dir, ok, err := dirInModule(path, targetPrefix, ModRoot(), true); err != nil {
return module.Version{}, dir, err
} else if ok {
return Target, dir, nil
dir := filepath.Join(cfg.GOROOT, "src", path)
return module.Version{}, dir, nil
// -mod=vendor is special.
// Everything must be in the main module or the main module's vendor directory.
if cfg.BuildMod == "vendor" {
mainDir, mainOK, mainErr := dirInModule(path, targetPrefix, ModRoot(), true)
vendorDir, vendorOK, _ := dirInModule(path, "", filepath.Join(ModRoot(), "vendor"), false)
if mainOK && vendorOK {
return module.Version{}, "", &AmbiguousImportError{importPath: path, Dirs: []string{mainDir, vendorDir}}
// Prefer to return main directory if there is one,
// Note that we're not checking that the package exists.
// We'll leave that for load.
if !vendorOK && mainDir != "" {
return Target, mainDir, nil
if mainErr != nil {
return module.Version{}, "", mainErr
return vendorPkgModule[path], vendorDir, nil
// Check each module on the build list.
var dirs []string
var mods []module.Version
// Iterate over possible modules for the path, not all selected modules.
// Iterating over selected modules would make the overall loading time
// O(M × P) for M modules providing P imported packages, whereas iterating
// over path prefixes is only O(P × k) with maximum path depth k. For
// large projects both M and P may be very large (note that M ≤ P), but k
// will tend to remain smallish (if for no other reason than filesystem
// path limitations).
// We perform this iteration either one or two times. If mg is initially nil,
// then we first attempt to load the package using only the main module and
// its root requirements. If that does not identify the package, or if mg is
// already non-nil, then we attempt to load the package using the full
// requirements in mg.
for {
var sumErrMods []module.Version
for prefix := path; prefix != "."; prefix = pathpkg.Dir(prefix) {
var (
v string
ok bool
if mg == nil {
v, ok = rs.rootSelected(prefix)
} else {
v, ok = mg.Selected(prefix), true
if !ok || v == "none" {
m := module.Version{Path: prefix, Version: v}
needSum := true
root, isLocal, err := fetch(ctx, m, needSum)
if err != nil {
if sumErr := (*sumMissingError)(nil); errors.As(err, &sumErr) {
// We are missing a sum needed to fetch a module in the build list.
// We can't verify that the package is unique, and we may not find
// the package at all. Keep checking other modules to decide which
// error to report. Multiple sums may be missing if we need to look in
// multiple nested modules to resolve the import; we'll report them all.
sumErrMods = append(sumErrMods, m)
// Report fetch error.
// Note that we don't know for sure this module is necessary,
// but it certainly _could_ provide the package, and even if we
// continue the loop and find the package in some other module,
// we need to look at this module to make sure the import is
// not ambiguous.
return module.Version{}, "", err
if dir, ok, err := dirInModule(path, m.Path, root, isLocal); err != nil {
return module.Version{}, "", err
} else if ok {
mods = append(mods, m)
dirs = append(dirs, dir)
if len(mods) > 1 {
// We produce the list of directories from longest to shortest candidate
// module path, but the AmbiguousImportError should report them from
// shortest to longest. Reverse them now.
for i := 0; i < len(mods)/2; i++ {
j := len(mods) - 1 - i
mods[i], mods[j] = mods[j], mods[i]
dirs[i], dirs[j] = dirs[j], dirs[i]
return module.Version{}, "", &AmbiguousImportError{importPath: path, Dirs: dirs, Modules: mods}
if len(sumErrMods) > 0 {
for i := 0; i < len(sumErrMods)/2; i++ {
j := len(sumErrMods) - 1 - i
sumErrMods[i], sumErrMods[j] = sumErrMods[j], sumErrMods[i]
return module.Version{}, "", &ImportMissingSumError{
importPath: path,
mods: sumErrMods,
found: len(mods) > 0,
if len(mods) == 1 {
return mods[0], dirs[0], nil
if mg != nil {
// We checked the full module graph and still didn't find the
// requested package.
var queryErr error
if !HasModRoot() {
queryErr = ErrNoModRoot
return module.Version{}, "", &ImportMissingError{Path: path, QueryErr: queryErr, isStd: pathIsStd}
// So far we've checked the root dependencies.
// Load the full module graph and try again.
mg, err = rs.Graph(ctx)
if err != nil {
// We might be missing one or more transitive (implicit) dependencies from
// the module graph, so we can't return an ImportMissingError here — one
// of the missing modules might actually contain the package in question,
// in which case we shouldn't go looking for it in some new dependency.
return module.Version{}, "", err
// queryImport attempts to locate a module that can be added to the current
// build list to provide the package with the given import path.
// Unlike QueryPattern, queryImport prefers to add a replaced version of a
// module *before* checking the proxies for a version to add.
func queryImport(ctx context.Context, path string, rs *Requirements) (module.Version, error) {
// To avoid spurious remote fetches, try the latest replacement for each
// module (
if index != nil {
var mods []module.Version
for mp, mv := range index.highestReplaced {
if !maybeInModule(path, mp) {
if mv == "" {
// The only replacement is a wildcard that doesn't specify a version, so
// synthesize a pseudo-version with an appropriate major version and a
// timestamp below any real timestamp. That way, if the main module is
// used from within some other module, the user will be able to upgrade
// the requirement to any real version they choose.
if _, pathMajor, ok := module.SplitPathVersion(mp); ok && len(pathMajor) > 0 {
mv = module.ZeroPseudoVersion(pathMajor[1:])
} else {
mv = module.ZeroPseudoVersion("v0")
mg, err := rs.Graph(ctx)
if err != nil {
return module.Version{}, err
if cmpVersion(mg.Selected(mp), mv) >= 0 {
// We can't resolve the import by adding mp@mv to the module graph,
// because the selected version of mp is already at least mv.
mods = append(mods, module.Version{Path: mp, Version: mv})
// Every module path in mods is a prefix of the import path.
// As in QueryPattern, prefer the longest prefix that satisfies the import.
sort.Slice(mods, func(i, j int) bool {
return len(mods[i].Path) > len(mods[j].Path)
for _, m := range mods {
needSum := true
root, isLocal, err := fetch(ctx, m, needSum)
if err != nil {
if sumErr := (*sumMissingError)(nil); errors.As(err, &sumErr) {
return module.Version{}, &ImportMissingSumError{importPath: path}
return module.Version{}, err
if _, ok, err := dirInModule(path, m.Path, root, isLocal); err != nil {
return m, err
} else if ok {
if cfg.BuildMod == "readonly" {
return module.Version{}, &ImportMissingError{Path: path, replaced: m}
return m, nil
if len(mods) > 0 && module.CheckPath(path) != nil {
// The package path is not valid to fetch remotely,
// so it can only exist in a replaced module,
// and we know from the above loop that it is not.
return module.Version{}, &PackageNotInModuleError{
Mod: mods[0],
Query: "latest",
Pattern: path,
Replacement: Replacement(mods[0]),
if search.IsStandardImportPath(path) {
// This package isn't in the standard library, isn't in any module already
// in the build list, and isn't in any other module that the user has
// shimmed in via a "replace" directive.
// Moreover, the import path is reserved for the standard library, so
// QueryPattern cannot possibly find a module containing this package.
// Instead of trying QueryPattern, report an ImportMissingError immediately.
return module.Version{}, &ImportMissingError{Path: path, isStd: true}
if cfg.BuildMod == "readonly" && !allowMissingModuleImports {
// In readonly mode, we can't write go.mod, so we shouldn't try to look up
// the module. If readonly mode was enabled explicitly, include that in
// the error message.
var queryErr error
if cfg.BuildModExplicit {
queryErr = fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)
} else if cfg.BuildModReason != "" {
queryErr = fmt.Errorf("import lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
return module.Version{}, &ImportMissingError{Path: path, QueryErr: queryErr}
// Look up module containing the package, for addition to the build list.
// Goal is to determine the module, download it to dir,
// and return m, dir, ImpportMissingError.
fmt.Fprintf(os.Stderr, "go: finding module for package %s\n", path)
mg, err := rs.Graph(ctx)
if err != nil {
return module.Version{}, err
candidates, err := QueryPackages(ctx, path, "latest", mg.Selected, CheckAllowed)
if err != nil {
if errors.Is(err, fs.ErrNotExist) {
// Return "cannot find module providing package […]" instead of whatever
// low-level error QueryPattern produced.
return module.Version{}, &ImportMissingError{Path: path, QueryErr: err}
} else {
return module.Version{}, err
candidate0MissingVersion := ""
for i, c := range candidates {
if v := mg.Selected(c.Mod.Path); semver.Compare(v, c.Mod.Version) > 0 {
// QueryPattern proposed that we add module c.Mod to provide the package,
// but we already depend on a newer version of that module (and that
// version doesn't have the package).
// This typically happens when a package is present at the "@latest"
// version (e.g., v1.0.0) of a module, but we have a newer version
// of the same module in the build list (e.g., v1.0.1-beta), and
// the package is not present there.
if i == 0 {
candidate0MissingVersion = v
return c.Mod, nil
return module.Version{}, &ImportMissingError{
Path: path,
Module: candidates[0].Mod,
newMissingVersion: candidate0MissingVersion,
// maybeInModule reports whether, syntactically,
// a package with the given import path could be supplied
// by a module with the given module path (mpath).
func maybeInModule(path, mpath string) bool {
return mpath == path ||
len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath
var (
haveGoModCache par.Cache // dir → bool
haveGoFilesCache par.Cache // dir → goFilesEntry
type goFilesEntry struct {
haveGoFiles bool
err error
// dirInModule locates the directory that would hold the package named by the given path,
// if it were in the module with module path mpath and root mdir.
// If path is syntactically not within mpath,
// or if mdir is a local file tree (isLocal == true) and the directory
// that would hold path is in a sub-module (covered by a go.mod below mdir),
// dirInModule returns "", false, nil.
// Otherwise, dirInModule returns the name of the directory where
// Go source files would be expected, along with a boolean indicating
// whether there are in fact Go source files in that directory.
// A non-nil error indicates that the existence of the directory and/or
// source files could not be determined, for example due to a permission error.
func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFiles bool, err error) {
// Determine where to expect the package.
if path == mpath {
dir = mdir
} else if mpath == "" { // vendor directory
dir = filepath.Join(mdir, path)
} else if len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath {
dir = filepath.Join(mdir, path[len(mpath)+1:])
} else {
return "", false, nil
// Check that there aren't other modules in the way.
// This check is unnecessary inside the module cache
// and important to skip in the vendor directory,
// where all the module trees have been overlaid.
// So we only check local module trees
// (the main module, and any directory trees pointed at by replace directives).
if isLocal {
for d := dir; d != mdir && len(d) > len(mdir); {
haveGoMod := haveGoModCache.Do(d, func() interface{} {
fi, err := fsys.Stat(filepath.Join(d, "go.mod"))
return err == nil && !fi.IsDir()
if haveGoMod {
return "", false, nil
parent := filepath.Dir(d)
if parent == d {
// Break the loop, as otherwise we'd loop
// forever if d=="." and mdir=="".
d = parent
// Now committed to returning dir (not "").
// Are there Go source files in the directory?
// We don't care about build tags, not even "+build ignore".
// We're just looking for a plausible directory.
res := haveGoFilesCache.Do(dir, func() interface{} {
ok, err := fsys.IsDirWithGoFiles(dir)
return goFilesEntry{haveGoFiles: ok, err: err}
return dir, res.haveGoFiles, res.err
// fetch downloads the given module (or its replacement)
// and returns its location.
// needSum indicates whether the module may be downloaded in readonly mode
// without a go.sum entry. It should only be false for modules fetched
// speculatively (for example, for incompatible version filtering). The sum
// will still be verified normally.
// The isLocal return value reports whether the replacement,
// if any, is local to the filesystem.
func fetch(ctx context.Context, mod module.Version, needSum bool) (dir string, isLocal bool, err error) {
if mod == Target {
return ModRoot(), true, nil
if r := Replacement(mod); r.Path != "" {
if r.Version == "" {
dir = r.Path
if !filepath.IsAbs(dir) {
dir = filepath.Join(ModRoot(), dir)
// Ensure that the replacement directory actually exists:
// dirInModule does not report errors for missing modules,
// so if we don't report the error now, later failures will be
// very mysterious.
if _, err := fsys.Stat(dir); err != nil {
if os.IsNotExist(err) {
// Semantically the module version itself “exists” — we just don't
// have its source code. Remove the equivalence to os.ErrNotExist,
// and make the message more concise while we're at it.
err = fmt.Errorf("replacement directory %s does not exist", r.Path)
} else {
err = fmt.Errorf("replacement directory %s: %w", r.Path, err)
return dir, true, module.VersionError(mod, err)
return dir, true, nil
mod = r
if HasModRoot() && cfg.BuildMod == "readonly" && needSum && !modfetch.HaveSum(mod) {
return "", false, module.VersionError(mod, &sumMissingError{})
dir, err = modfetch.Download(ctx, mod)
return dir, false, err
type sumMissingError struct {
suggestion string
func (e *sumMissingError) Error() string {
return "missing go.sum entry" + e.suggestion