vendor/cmd/go/internal/modfetch/repo.go - vgo - Git at Google

 // 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 (
 	"fmt"
 	"os"
 	pathpkg "path"
 	"sort"
 	"time"

 	"cmd/go/internal/cfg"
 	"cmd/go/internal/get"
 	"cmd/go/internal/modfetch/codehost"
 	"cmd/go/internal/modfetch/gitrepo"
 	"cmd/go/internal/module"
 	"cmd/go/internal/par"
 	"cmd/go/internal/semver"
 	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
 	Name    string    // complete ID in underlying repository
 	Short   string    // shortened ID, for use in pseudo-version
 	Time    time.Time // commit time
 }

 // 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 rsc.io/qr corresponds to the
 // file tree in the repository https://github.com/rsc/qr.
 // That file tree has a go.mod that says "module rsc.io/qr".
 // The package in the root directory has import path "rsc.io/qr".
 // The package in the gf256 subdirectory has import path "rsc.io/qr/gf256".
 // In this example, "rsc.io/qr" is both a module path and an import path.
 // But "rsc.io/qr/gf256" 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 rsc.io/qr@v0.1.0 depends on the "implicit go.mod at root of
 // repository" rule, while rsc.io/qr@v0.2.0 has an explicit go.mod.
 // Because the "go get" import paths rsc.io/qr and github.com/rsc/qr
 // both redirect to the Git repository https://github.com/rsc/qr,
 // github.com/rsc/qr@v0.1.0 is the same file tree as rsc.io/qr@v0.1.0
 // but a different module (a different name). In contrast, since v0.2.0
 // of that repository has an explicit go.mod that declares path rsc.io/qr,
 // github.com/rsc/qr@v0.2.0 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 rsc.io/qr/gf256 into its
 // own module, so that there would be two modules rsc.io/qr and rsc.io/qr/gf256.
 // Then we can simultaneously issue rsc.io/qr v0.3.0 (dropping the gf256 subdirectory)
 // and rsc.io/qr/gf256 v0.1.0, including in their respective go.mod
 // cyclic requirements pointing at each other: rsc.io/qr v0.3.0 requires
 // rsc.io/qr/gf256 v0.1.0 and vice versa. Then a build can be
 // using an older rsc.io/qr module that includes the gf256 package, but if
 // it adds a requirement on either the newer rsc.io/qr or the newer
 // rsc.io/qr/gf256 module, it will automatically add the requirement
 // on the complementary half, ensuring both that rsc.io/qr/gf256 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 rsc.io/qr v0.4.0 including
 // the gf256 subdirectory and an rsc.io/qr/gf256 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 https://talks.golang.org/2016/refactor.article.
 //
 // 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 rsc.io/qr/v2,
 // the v2.x.x continuation of rsc.io/qr, is expected to be found
 // in v2-tagged commits in https://github.com/rsc/qr, 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 https://bitbucket.org/rsc/qr and update https://rsc.io/qr?go-get=1,
 // 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}
 	}).(cached)

 	return c.r, c.err
 }

 // lookup returns the module with the given module path.
 func lookup(path string) (r Repo, err error) {
 	if cfg.BuildGetmode != "" {
 		return nil, fmt.Errorf("module lookup disabled by -getmode=%s", cfg.BuildGetmode)
 	}
 	if proxyURL != "" {
 		return lookupProxy(path)
 	}

 	rr, err := get.RepoRootForImportPath(path, get.PreferMod, web.Secure)
 	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), nil
 	}

 	code, err := lookupCodeRepo(rr)
 	if err != nil {
 		return nil, err
 	}
 	return newCodeRepo(code, rr.Root, path)
 }

 func lookupCodeRepo(rr *get.RepoRoot) (codehost.Repo, error) {
 	switch rr.VCS {
 	default:
 		return nil, fmt.Errorf("lookup %s: unknown VCS %s %s", rr.Root, rr.VCS, rr.Repo)
 	case "git":
 		return gitrepo.Repo(rr.Repo)
 		// TODO: "hg", "svn", "bzr", "fossil"
 	}
 }

 // Import returns the module repo and version to use to satisfy the given import path.
 // It considers a sequence of module paths starting with the import path and
 // removing successive path elements from the end. It stops when it finds a module
 // path for which the latest version of the module provides the expected package.
 // If non-nil, the allowed function is used to filter known versions of a given module
 // before determining which one is "latest".
 func Import(path string, allowed func(module.Version) bool) (Repo, *RevInfo, error) {
 	if cfg.BuildGetmode != "" {
 		return nil, nil, fmt.Errorf("import resolution disabled by -getmode=%s", cfg.BuildGetmode)
 	}
 	if traceRepo {
 		defer logCall("Import(%q, ...)", path)()
 	}

 	try := func(path string) (Repo, *RevInfo, error) {
 		r, err := Lookup(path)
 		if err != nil {
 			return nil, nil, err
 		}
 		info, err := Query(path, "latest", allowed)
 		if err != nil {
 			return nil, nil, err
 		}
 		_, err = r.GoMod(info.Version)
 		if err != nil {
 			return nil, nil, err
 		}
 		// TODO(rsc): Do what the docs promise: download the module
 		// source code and check that it actually contains code for the
 		// target import path. To do that efficiently we will need to move
 		// the unzipped code cache out of ../vgo into this package.
 		return r, info, nil
 	}

 	// Find enclosing module by walking up path element by element.
 	var firstErr error
 	for {
 		r, info, err := try(path)
 		if err == nil {
 			return r, info, nil
 		}
 		if firstErr == nil {
 			firstErr = err
 		}
 		p := pathpkg.Dir(path)
 		if p == "." {
 			break
 		}
 		path = p
 	}
 	return nil, nil, firstErr
 }

 // 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.BuildGetmode != "" {
 		return nil, nil, fmt.Errorf("repo version lookup disabled by -getmode=%s", cfg.BuildGetmode)
 	}

 	// 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).
 	rr, err := get.RepoRootForImportPath(path, get.IgnoreMod, web.Secure)
 	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)
 }
	// 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 (
	"fmt"
	"os"
	pathpkg "path"
	"sort"
	"time"

	"cmd/go/internal/cfg"
	"cmd/go/internal/get"
	"cmd/go/internal/modfetch/codehost"
	"cmd/go/internal/modfetch/gitrepo"
	"cmd/go/internal/module"
	"cmd/go/internal/par"
	"cmd/go/internal/semver"
	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
	Name string // complete ID in underlying repository
	Short string // shortened ID, for use in pseudo-version
	Time time.Time // commit time
	}

	// 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 rsc.io/qr corresponds to the
	// file tree in the repository https://github.com/rsc/qr.
	// That file tree has a go.mod that says "module rsc.io/qr".
	// The package in the root directory has import path "rsc.io/qr".
	// The package in the gf256 subdirectory has import path "rsc.io/qr/gf256".
	// In this example, "rsc.io/qr" is both a module path and an import path.
	// But "rsc.io/qr/gf256" 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 rsc.io/qr@v0.1.0 depends on the "implicit go.mod at root of
	// repository" rule, while rsc.io/qr@v0.2.0 has an explicit go.mod.
	// Because the "go get" import paths rsc.io/qr and github.com/rsc/qr
	// both redirect to the Git repository https://github.com/rsc/qr,
	// github.com/rsc/qr@v0.1.0 is the same file tree as rsc.io/qr@v0.1.0
	// but a different module (a different name). In contrast, since v0.2.0
	// of that repository has an explicit go.mod that declares path rsc.io/qr,
	// github.com/rsc/qr@v0.2.0 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 rsc.io/qr/gf256 into its
	// own module, so that there would be two modules rsc.io/qr and rsc.io/qr/gf256.
	// Then we can simultaneously issue rsc.io/qr v0.3.0 (dropping the gf256 subdirectory)
	// and rsc.io/qr/gf256 v0.1.0, including in their respective go.mod
	// cyclic requirements pointing at each other: rsc.io/qr v0.3.0 requires
	// rsc.io/qr/gf256 v0.1.0 and vice versa. Then a build can be
	// using an older rsc.io/qr module that includes the gf256 package, but if
	// it adds a requirement on either the newer rsc.io/qr or the newer
	// rsc.io/qr/gf256 module, it will automatically add the requirement
	// on the complementary half, ensuring both that rsc.io/qr/gf256 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 rsc.io/qr v0.4.0 including
	// the gf256 subdirectory and an rsc.io/qr/gf256 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 https://talks.golang.org/2016/refactor.article.
	//
	// 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 rsc.io/qr/v2,
	// the v2.x.x continuation of rsc.io/qr, is expected to be found
	// in v2-tagged commits in https://github.com/rsc/qr, 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 https://bitbucket.org/rsc/qr and update https://rsc.io/qr?go-get=1,
	// 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}
	}).(cached)

	return c.r, c.err
	}

	// lookup returns the module with the given module path.
	func lookup(path string) (r Repo, err error) {
	if cfg.BuildGetmode != "" {
	return nil, fmt.Errorf("module lookup disabled by -getmode=%s", cfg.BuildGetmode)
	}
	if proxyURL != "" {
	return lookupProxy(path)
	}

	rr, err := get.RepoRootForImportPath(path, get.PreferMod, web.Secure)
	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), nil
	}

	code, err := lookupCodeRepo(rr)
	if err != nil {
	return nil, err
	}
	return newCodeRepo(code, rr.Root, path)
	}

	func lookupCodeRepo(rr *get.RepoRoot) (codehost.Repo, error) {
	switch rr.VCS {
	default:
	return nil, fmt.Errorf("lookup %s: unknown VCS %s %s", rr.Root, rr.VCS, rr.Repo)
	case "git":
	return gitrepo.Repo(rr.Repo)
	// TODO: "hg", "svn", "bzr", "fossil"
	}
	}

	// Import returns the module repo and version to use to satisfy the given import path.
	// It considers a sequence of module paths starting with the import path and
	// removing successive path elements from the end. It stops when it finds a module
	// path for which the latest version of the module provides the expected package.
	// If non-nil, the allowed function is used to filter known versions of a given module
	// before determining which one is "latest".
	func Import(path string, allowed func(module.Version) bool) (Repo, *RevInfo, error) {
	if cfg.BuildGetmode != "" {
	return nil, nil, fmt.Errorf("import resolution disabled by -getmode=%s", cfg.BuildGetmode)
	}
	if traceRepo {
	defer logCall("Import(%q, ...)", path)()
	}

	try := func(path string) (Repo, *RevInfo, error) {
	r, err := Lookup(path)
	if err != nil {
	return nil, nil, err
	}
	info, err := Query(path, "latest", allowed)
	if err != nil {
	return nil, nil, err
	}
	_, err = r.GoMod(info.Version)
	if err != nil {
	return nil, nil, err
	}
	// TODO(rsc): Do what the docs promise: download the module
	// source code and check that it actually contains code for the
	// target import path. To do that efficiently we will need to move
	// the unzipped code cache out of ../vgo into this package.
	return r, info, nil
	}

	// Find enclosing module by walking up path element by element.
	var firstErr error
	for {
	r, info, err := try(path)
	if err == nil {
	return r, info, nil
	}
	if firstErr == nil {
	firstErr = err
	}
	p := pathpkg.Dir(path)
	if p == "." {
	break
	}
	path = p
	}
	return nil, nil, firstErr
	}

	// 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.BuildGetmode != "" {
	return nil, nil, fmt.Errorf("repo version lookup disabled by -getmode=%s", cfg.BuildGetmode)
	}

	// 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).
	rr, err := get.RepoRootForImportPath(path, get.IgnoreMod, web.Secure)
	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)
	}