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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 SortVersions).
Versions(prefix string) (tags []string, err 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 downloads a zip file for the given version
// to a new file in a given temporary directory.
// It returns the name of the new file.
// The caller should remove the file when finished with it.
Zip(version, tmpdir string) (tmpfile string, err error)
// A Rev describes a single revision in a module repository.
type RevInfo struct {
Version string // version string
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.
// The Import function takes an import path found in source code and
// determines which module to add to the requirement list to satisfy
// that import. It checks successive truncations of the import path
// to determine possible modules and stops when it finds a module
// in which the latest version satisfies the import path.
// The ImportRepoRev function is a variant of Import which is limited
// to code in a source code repository at a particular revision identifier
// (usually a commit hash or source code repository tag, not necessarily
// a module version).
// ImportRepoRev is used when converting legacy dependency requirements
// from older systems into go.mod files. Those older systems worked
// at either package or repository granularity, and most of the time they
// recorded commit hashes, not tagged versions.
var lookupCache par.Cache
// Lookup returns the module with the given module path.
// A successful return does not guarantee that the module
// has any defined versions.
func Lookup(path string) (Repo, error) {
if traceRepo {
defer logCall("Lookup(%q)", path)()
type cached struct {
r Repo
err error
c := lookupCache.Do(path, func() interface{} {
r, err := lookup(path)
if err == nil {
if traceRepo {
r = newLoggingRepo(r)
r = newCachingRepo(r)
return cached{r, err}
return c.r, c.err
// lookup returns the module with the given module path.
func lookup(path string) (r Repo, err error) {
if cfg.BuildMod == "vendor" {
return nil, fmt.Errorf("module lookup disabled by -mod=%s", cfg.BuildMod)
if proxyURL == "off" {
return nil, fmt.Errorf("module lookup disabled by GOPROXY=%s", proxyURL)
if proxyURL != "" && proxyURL != "direct" {
return lookupProxy(path)
security := web.Secure
if get.Insecure {
security = web.Insecure
rr, err := get.RepoRootForImportPath(path, get.PreferMod, security)
if err != nil {
// We don't know where to find code for a module with this path.
return nil, err
if rr.VCS == "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 *get.RepoRoot) (codehost.Repo, error) {
code, err := codehost.NewRepo(rr.VCS, 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
// ImportRepoRev returns the module and version to use to access
// the given import path loaded from the source code repository that
// the original "go get" would have used, at the specific repository revision
// (typically a commit hash, but possibly also a source control tag).
func ImportRepoRev(path, rev string) (Repo, *RevInfo, error) {
if cfg.BuildMod == "vendor" || cfg.BuildMod == "readonly" {
return nil, nil, fmt.Errorf("repo version lookup disabled by -mod=%s", cfg.BuildMod)
// Note: Because we are converting a code reference from a legacy
// version control system, we ignore meta tags about modules
// and use only direct source control entries (get.IgnoreMod).
security := web.Secure
if get.Insecure {
security = web.Insecure
rr, err := get.RepoRootForImportPath(path, get.IgnoreMod, security)
if err != nil {
return nil, nil, err
code, err := lookupCodeRepo(rr)
if err != nil {
return nil, nil, err
revInfo, err := code.Stat(rev)
if err != nil {
return nil, nil, err
// TODO: Look in repo to find path, check for go.mod files.
// For now we're just assuming rr.Root is the module path,
// which is true in the absence of go.mod files.
repo, err := newCodeRepo(code, rr.Root, rr.Root)
if err != nil {
return nil, nil, err
info, err := repo.(*codeRepo).convert(revInfo, "")
if err != nil {
return nil, nil, err
return repo, info, nil
func SortVersions(list []string) {
sort.Slice(list, func(i, j int) bool {
cmp := semver.Compare(list[i], list[j])
if cmp != 0 {
return cmp < 0
return list[i] < list[j]
// 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(version, tmpdir string) (string, error) {
defer logCall("Repo[%s]: Zip(%q, %q)", l.r.ModulePath(), version, tmpdir)()
return l.r.Zip(version, tmpdir)