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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 modfetch
import (
web "cmd/go/internal/web"
const traceRepo = false // trace all repo actions, for debugging
// A Repo represents a repository storing all versions of a single module.
// It must be safe for simultaneous use by multiple goroutines.
type Repo interface {
// ModulePath returns the module path.
ModulePath() string
// Versions lists all known versions with the given prefix.
// Pseudo-versions are not included.
// Versions should be returned sorted in semver order
// (implementations can use semver.Sort).
// Versions returns a non-nil error only if there was a problem
// fetching the list of versions: it may return an empty list
// along with a nil error if the list of matching versions
// is known to be empty.
// If the underlying repository does not exist,
// Versions returns an error matching errors.Is(_, os.NotExist).
Versions(prefix string) ([]string, error)
// Stat returns information about the revision rev.
// A revision can be any identifier known to the underlying service:
// commit hash, branch, tag, and so on.
Stat(rev string) (*RevInfo, error)
// Latest returns the latest revision on the default branch,
// whatever that means in the underlying source code repository.
// It is only used when there are no tagged versions.
Latest() (*RevInfo, error)
// GoMod returns the go.mod file for the given version.
GoMod(version string) (data []byte, err error)
// Zip writes a zip file for the given version to dst.
Zip(dst io.Writer, version string) error
// A Rev describes a single revision in a module repository.
type RevInfo struct {
Version string // suggested version string for this revision
Time time.Time // commit time
// These fields are used for Stat of arbitrary rev,
// but they are not recorded when talking about module versions.
Name string `json:"-"` // complete ID in underlying repository
Short string `json:"-"` // shortened ID, for use in pseudo-version
// Re: module paths, import paths, repository roots, and lookups
// A module is a collection of Go packages stored in a file tree
// with a go.mod file at the root of the tree.
// The go.mod defines the module path, which is the import path
// corresponding to the root of the file tree.
// The import path of a directory within that file tree is the module path
// joined with the name of the subdirectory relative to the root.
// For example, the module with path corresponds to the
// file tree in the repository
// That file tree has a go.mod that says "module".
// The package in the root directory has import path "".
// The package in the gf256 subdirectory has import path "".
// In this example, "" is both a module path and an import path.
// But "" is only an import path, not a module path:
// it names an importable package, but not a module.
// As a special case to incorporate code written before modules were
// introduced, if a path p resolves using the pre-module "go get" lookup
// to the root of a source code repository without a go.mod file,
// that repository is treated as if it had a go.mod in its root directory
// declaring module path p. (The go.mod is further considered to
// contain requirements corresponding to any legacy version
// tracking format such as Gopkg.lock, vendor/vendor.conf, and so on.)
// The presentation so far ignores the fact that a source code repository
// has many different versions of a file tree, and those versions may
// differ in whether a particular go.mod exists and what it contains.
// In fact there is a well-defined mapping only from a module path, version
// pair - often written path@version - to a particular file tree.
// For example depends on the "implicit go.mod at root of
// repository" rule, while has an explicit go.mod.
// Because the "go get" import paths and
// both redirect to the Git repository,
// is the same file tree as
// but a different module (a different name). In contrast, since v0.2.0
// of that repository has an explicit go.mod that declares path,
// is an invalid module path, version pair.
// Before modules, import comments would have had the same effect.
// The set of import paths associated with a given module path is
// clearly not fixed: at the least, new directories with new import paths
// can always be added. But another potential operation is to split a
// subtree out of a module into its own module. If done carefully,
// this operation can be done while preserving compatibility for clients.
// For example, suppose that we want to split into its
// own module, so that there would be two modules and
// Then we can simultaneously issue v0.3.0 (dropping the gf256 subdirectory)
// and v0.1.0, including in their respective go.mod
// cyclic requirements pointing at each other: v0.3.0 requires
// v0.1.0 and vice versa. Then a build can be
// using an older module that includes the gf256 package, but if
// it adds a requirement on either the newer or the newer
// module, it will automatically add the requirement
// on the complementary half, ensuring both that is
// available for importing by the build and also that it is only defined
// by a single module. The gf256 package could move back into the
// original by another simultaneous release of v0.4.0 including
// the gf256 subdirectory and an v0.2.0 with no code
// in its root directory, along with a new requirement cycle.
// The ability to shift module boundaries in this way is expected to be
// important in large-scale program refactorings, similar to the ones
// described in
// The possibility of shifting module boundaries reemphasizes
// that you must know both the module path and its version
// to determine the set of packages provided directly by that module.
// On top of all this, it is possible for a single code repository
// to contain multiple modules, either in branches or subdirectories,
// as a limited kind of monorepo. For example,
// the v2.x.x continuation of, is expected to be found
// in v2-tagged commits in, either
// in the root or in a v2 subdirectory, disambiguated by go.mod.
// Again the precise file tree corresponding to a module
// depends on which version we are considering.
// It is also possible for the underlying repository to change over time,
// without changing the module path. If I copy the github repo over
// to and update,
// then clients of all versions should start fetching from bitbucket
// instead of github. That is, in contrast to the exact file tree,
// the location of the source code repository associated with a module path
// does not depend on the module version. (This is by design, as the whole
// point of these redirects is to allow package authors to establish a stable
// name that can be updated as code moves from one service to another.)
// All of this is important background for the lookup APIs defined in this
// file.
// The Lookup function takes a module path and returns a Repo representing
// that module path. Lookup can do only a little with the path alone.
// It can check that the path is well-formed (see semver.CheckPath)
// and it can check that the path can be resolved to a target repository.
// To avoid version control access except when absolutely necessary,
// Lookup does not attempt to connect to the repository itself.
var lookupCache par.Cache
type lookupCacheKey struct {
proxy, path string
// Lookup returns the module with the given module path,
// fetched through the given proxy.
// The distinguished proxy "direct" indicates that the path should be fetched
// from its origin, and "noproxy" indicates that the patch should be fetched
// directly only if GONOPROXY matches the given path.
// For the distinguished proxy "off", Lookup always returns a Repo that returns
// a non-nil error for every method call.
// A successful return does not guarantee that the module
// has any defined versions.
func Lookup(proxy, path string) Repo {
if traceRepo {
defer logCall("Lookup(%q, %q)", proxy, path)()
type cached struct {
r Repo
c := lookupCache.Do(lookupCacheKey{proxy, path}, func() interface{} {
r := newCachingRepo(path, func() (Repo, error) {
r, err := lookup(proxy, path)
if err == nil && traceRepo {
r = newLoggingRepo(r)
return r, err
return cached{r}
return c.r
// lookup returns the module with the given module path.
func lookup(proxy, path string) (r Repo, err error) {
if cfg.BuildMod == "vendor" {
return nil, errLookupDisabled
if module.MatchPrefixPatterns(cfg.GONOPROXY, path) {
switch proxy {
case "noproxy", "direct":
return lookupDirect(path)
return nil, errNoproxy
switch proxy {
case "off":
return errRepo{path, errProxyOff}, nil
case "direct":
return lookupDirect(path)
case "noproxy":
return nil, errUseProxy
return newProxyRepo(proxy, path)
type lookupDisabledError struct{}
func (lookupDisabledError) Error() string {
if cfg.BuildModReason == "" {
return fmt.Sprintf("module lookup disabled by -mod=%s", cfg.BuildMod)
return fmt.Sprintf("module lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
var errLookupDisabled error = lookupDisabledError{}
var (
errProxyOff = notExistErrorf("module lookup disabled by GOPROXY=off")
errNoproxy error = notExistErrorf("disabled by GOPRIVATE/GONOPROXY")
errUseProxy error = notExistErrorf("path does not match GOPRIVATE/GONOPROXY")
func lookupDirect(path string) (Repo, error) {
security := web.SecureOnly
if module.MatchPrefixPatterns(cfg.GOINSECURE, path) {
security = web.Insecure
rr, err := vcs.RepoRootForImportPath(path, vcs.PreferMod, security)
if err != nil {
// We don't know where to find code for a module with this path.
return nil, notExistError{err: err}
if rr.VCS.Name == "mod" {
// Fetch module from proxy with base URL rr.Repo.
return newProxyRepo(rr.Repo, path)
code, err := lookupCodeRepo(rr)
if err != nil {
return nil, err
return newCodeRepo(code, rr.Root, path)
func lookupCodeRepo(rr *vcs.RepoRoot) (codehost.Repo, error) {
code, err := codehost.NewRepo(rr.VCS.Cmd, rr.Repo)
if err != nil {
if _, ok := err.(*codehost.VCSError); ok {
return nil, err
return nil, fmt.Errorf("lookup %s: %v", rr.Root, err)
return code, nil
// A loggingRepo is a wrapper around an underlying Repo
// that prints a log message at the start and end of each call.
// It can be inserted when debugging.
type loggingRepo struct {
r Repo
func newLoggingRepo(r Repo) *loggingRepo {
return &loggingRepo{r}
// logCall prints a log message using format and args and then
// also returns a function that will print the same message again,
// along with the elapsed time.
// Typical usage is:
// defer logCall("hello %s", arg)()
// Note the final ().
func logCall(format string, args ...interface{}) func() {
start := time.Now()
fmt.Fprintf(os.Stderr, "+++ %s\n", fmt.Sprintf(format, args...))
return func() {
fmt.Fprintf(os.Stderr, "%.3fs %s\n", time.Since(start).Seconds(), fmt.Sprintf(format, args...))
func (l *loggingRepo) ModulePath() string {
return l.r.ModulePath()
func (l *loggingRepo) Versions(prefix string) (tags []string, err error) {
defer logCall("Repo[%s]: Versions(%q)", l.r.ModulePath(), prefix)()
return l.r.Versions(prefix)
func (l *loggingRepo) Stat(rev string) (*RevInfo, error) {
defer logCall("Repo[%s]: Stat(%q)", l.r.ModulePath(), rev)()
return l.r.Stat(rev)
func (l *loggingRepo) Latest() (*RevInfo, error) {
defer logCall("Repo[%s]: Latest()", l.r.ModulePath())()
return l.r.Latest()
func (l *loggingRepo) GoMod(version string) ([]byte, error) {
defer logCall("Repo[%s]: GoMod(%q)", l.r.ModulePath(), version)()
return l.r.GoMod(version)
func (l *loggingRepo) Zip(dst io.Writer, version string) error {
dstName := "_"
if dst, ok := dst.(interface{ Name() string }); ok {
dstName = strconv.Quote(dst.Name())
defer logCall("Repo[%s]: Zip(%s, %q)", l.r.ModulePath(), dstName, version)()
return l.r.Zip(dst, version)
// errRepo is a Repo that returns the same error for all operations.
// It is useful in conjunction with caching, since cache hits will not attempt
// the prohibited operations.
type errRepo struct {
modulePath string
err error
func (r errRepo) ModulePath() string { return r.modulePath }
func (r errRepo) Versions(prefix string) (tags []string, err error) { return nil, r.err }
func (r errRepo) Stat(rev string) (*RevInfo, error) { return nil, r.err }
func (r errRepo) Latest() (*RevInfo, error) { return nil, r.err }
func (r errRepo) GoMod(version string) ([]byte, error) { return nil, r.err }
func (r errRepo) Zip(dst io.Writer, version string) error { return r.err }
// A notExistError is like fs.ErrNotExist, but with a custom message
type notExistError struct {
err error
func notExistErrorf(format string, args ...interface{}) error {
return notExistError{fmt.Errorf(format, args...)}
func (e notExistError) Error() string {
return e.err.Error()
func (notExistError) Is(target error) bool {
return target == fs.ErrNotExist
func (e notExistError) Unwrap() error {
return e.err