internal/stdlib/stdlib.go - pkgsite - Git at Google

 // Copyright 2019 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 stdlib supports special handling of the Go standard library.
 // Regardless of the how the standard library has been split into modules for
 // development and testing, the discovery site treats it as a single module
 // named "std".
 package stdlib

 import (
 	"archive/zip"
 	"bytes"
 	"fmt"
 	"io"
 	"os"
 	"path"
 	"path/filepath"
 	"regexp"
 	"strings"
 	"time"

 	"golang.org/x/mod/semver"
 	"golang.org/x/pkgsite/internal/derrors"
 	"golang.org/x/pkgsite/internal/testing/testhelper"
 	"golang.org/x/pkgsite/internal/version"

 	"github.com/go-git/go-billy/v5/osfs"
 	"github.com/go-git/go-git/v5"
 	"github.com/go-git/go-git/v5/config"
 	"github.com/go-git/go-git/v5/plumbing"
 	"github.com/go-git/go-git/v5/plumbing/filemode"
 	"github.com/go-git/go-git/v5/plumbing/object"
 	"github.com/go-git/go-git/v5/storage/memory"
 )

 // ModulePath is the name of the module for the standard library.
 const ModulePath = "std"

 var (
 	// Regexp for matching go tags. The groups are:
 	// 1  the major.minor version
 	// 2  the patch version, or empty if none
 	// 3  the entire prerelease, if present
 	// 4  the prerelease type ("beta" or "rc")
 	// 5  the prerelease number
 	tagRegexp = regexp.MustCompile(`^go(\d+\.\d+)(\.\d+|)((beta|rc)(\d+))?$`)
 )

 // VersionForTag returns the semantic version for the Go tag, or "" if
 // tag doesn't correspond to a Go release or beta tag.
 // Examples:
 //   "go1" => "v1.0.0"
 //   "go1.2" => "v1.2.0"
 //   "go1.13beta1" => "v1.13.0-beta.1"
 //   "go1.9rc2" => "v1.9.0-rc.2"
 func VersionForTag(tag string) string {
 	// Special cases for go1.
 	if tag == "go1" {
 		return "v1.0.0"
 	}
 	if tag == "go1.0" {
 		return ""
 	}
 	// Special case for latest.
 	if tag == "latest" {
 		return "latest"
 	}
 	m := tagRegexp.FindStringSubmatch(tag)
 	if m == nil {
 		return ""
 	}
 	version := "v" + m[1]
 	if m[2] != "" {
 		version += m[2]
 	} else {
 		version += ".0"
 	}
 	if m[3] != "" {
 		version += "-" + m[4] + "." + m[5]
 	}
 	return version
 }

 // TagForVersion returns the Go standard library repository tag corresponding
 // to semver. The Go tags differ from standard semantic versions in a few ways,
 // such as beginning with "go" instead of "v".
 func TagForVersion(version string) (_ string, err error) {
 	defer derrors.Wrap(&err, "TagForVersion(%q)", version)

 	// Special case: v1.0.0 => go1.
 	if version == "v1.0.0" {
 		return "go1", nil
 	}
 	if !semver.IsValid(version) {
 		return "", fmt.Errorf("%w: requested version is not a valid semantic version: %q ", derrors.InvalidArgument, version)
 	}
 	goVersion := semver.Canonical(version)
 	prerelease := semver.Prerelease(goVersion)
 	versionWithoutPrerelease := strings.TrimSuffix(goVersion, prerelease)
 	patch := strings.TrimPrefix(versionWithoutPrerelease, semver.MajorMinor(goVersion)+".")
 	if patch == "0" {
 		versionWithoutPrerelease = strings.TrimSuffix(versionWithoutPrerelease, ".0")
 	}

 	goVersion = fmt.Sprintf("go%s", strings.TrimPrefix(versionWithoutPrerelease, "v"))
 	if prerelease != "" {
 		// Go prereleases look like  "beta1" instead of "beta.1".
 		// "beta1" is bad for sorting (since beta10 comes before beta9), so
 		// require the dot form.
 		i := finalDigitsIndex(prerelease)
 		if i >= 1 {
 			if prerelease[i-1] != '.' {
 				return "", fmt.Errorf("%w: final digits in a prerelease must follow a period", derrors.InvalidArgument)
 			}
 			// Remove the dot.
 			prerelease = prerelease[:i-1] + prerelease[i:]
 		}
 		goVersion += strings.TrimPrefix(prerelease, "-")
 	}
 	return goVersion, nil
 }

 // MajorVersionForVersion returns the Go major version for version.
 // E.g. "v1.13.3" => "go1".
 func MajorVersionForVersion(version string) (_ string, err error) {
 	defer derrors.Wrap(&err, "MajorVersionForVersion(%q)", version)

 	tag, err := TagForVersion(version)
 	if err != nil {
 		return "", err
 	}
 	if tag == "go1" {
 		return tag, nil
 	}
 	i := strings.IndexRune(tag, '.')
 	if i < 0 {
 		return "", fmt.Errorf("no '.' in go tag %q", tag)
 	}
 	return tag[:i], nil
 }

 // finalDigitsIndex returns the index of the first digit in the sequence of digits ending s.
 // If s doesn't end in digits, it returns -1.
 func finalDigitsIndex(s string) int {
 	// Assume ASCII (since the semver package does anyway).
 	var i int
 	for i = len(s) - 1; i >= 0; i-- {
 		if s[i] < '0' || s[i] > '9' {
 			break
 		}
 	}
 	if i == len(s)-1 {
 		return -1
 	}
 	return i + 1
 }

 const (
 	GoRepoURL       = "https://go.googlesource.com/go"
 	GoSourceRepoURL = "https://github.com/golang/go"
 )

 // UseTestData determines whether to really clone the Go repo, or use
 // stripped-down versions of the repo from the testdata directory.
 var UseTestData = false

 // TestCommitTime is the time used for all commits when UseTestData is true.
 var TestCommitTime = time.Date(2019, 9, 4, 1, 2, 3, 0, time.UTC)

 // getGoRepo returns a repo object for the Go repo at version.
 func getGoRepo(version string) (_ *git.Repository, err error) {
 	defer derrors.Wrap(&err, "getGoRepo(%q)", version)

 	tag, err := TagForVersion(version)
 	if err != nil {
 		return nil, err
 	}
 	return git.Clone(memory.NewStorage(), nil, &git.CloneOptions{
 		URL:           GoRepoURL,
 		ReferenceName: plumbing.NewTagReferenceName(tag),
 		SingleBranch:  true,
 		Depth:         1,
 		Tags:          git.NoTags,
 	})
 }

 // getTestGoRepo gets a Go repo for testing.
 func getTestGoRepo(version string) (_ *git.Repository, err error) {
 	defer derrors.Wrap(&err, "getTestGoRepo(%q)", version)

 	fs := osfs.New(filepath.Join(testhelper.TestDataPath("testdata"), version))
 	repo, err := git.Init(memory.NewStorage(), fs)
 	if err != nil {
 		return nil, err
 	}
 	wt, err := repo.Worktree()
 	if err != nil {
 		return nil, err
 	}
 	// Add all files in the directory.
 	if _, err := wt.Add(""); err != nil {
 		return nil, err
 	}
 	_, err = wt.Commit("", &git.CommitOptions{All: true, Author: &object.Signature{
 		Name:  "Joe Random",
 		Email: "joe@example.com",
 		When:  TestCommitTime,
 	}})
 	if err != nil {
 		return nil, err
 	}
 	return repo, nil
 }

 // Versions returns all the versions of Go that are relevant to the discovery
 // site. These are all release versions (tags of the forms "goN.N" and
 // "goN.N.N", where N is a number) and beta or rc versions (tags of the forms
 // "goN.NbetaN" and "goN.N.NbetaN", and similarly for "rc" replacing "beta").
 func Versions() (_ []string, err error) {
 	defer derrors.Wrap(&err, "Versions()")

 	var refNames []plumbing.ReferenceName
 	if UseTestData {
 		refNames = testRefs
 	} else {
 		re := git.NewRemote(memory.NewStorage(), &config.RemoteConfig{
 			URLs: []string{GoRepoURL},
 		})
 		refs, err := re.List(&git.ListOptions{})
 		if err != nil {
 			return nil, err
 		}
 		for _, r := range refs {
 			refNames = append(refNames, r.Name())
 		}
 	}

 	var versions []string
 	for _, name := range refNames {
 		if !name.IsTag() {
 			continue
 		}
 		v := VersionForTag(name.Short())
 		if v != "" {
 			versions = append(versions, v)
 		}
 	}
 	return versions, nil
 }

 // Directory returns the directory of the standard library relative to the repo root.
 func Directory(version string) string {
 	// For versions older than v1.4.0-beta.1, the stdlib is in src/pkg.
 	if semver.Compare(version, "v1.4.0-beta.1") < 0 {
 		return "src/pkg"
 	}
 	return "src"
 }

 // Approximate size of Zip("v1.15.2").
 const estimatedZipSize = 16 * 1024 * 1024

 func ZipInfo(requestedVersion string) (resolvedVersion string, zipSize int64, err error) {
 	defer derrors.Wrap(&err, "stdlib.ZipInfo(%q)", requestedVersion)

 	resolvedVersion, err = semanticVersion(requestedVersion)
 	if err != nil {
 		return "", 0, err
 	}
 	return resolvedVersion, estimatedZipSize, nil
 }

 // Zip creates a module zip representing the entire Go standard library at the
 // given version (which must have been resolved with ZipInfo) and returns a
 // reader to it. It also returns the time of the commit for that version. The
 // zip file is in module form, with each path prefixed by ModuleName + "@" +
 // version.
 //
 // Zip reads the standard library at the Go repository tag corresponding to to
 // the given semantic version.
 //
 // Zip ignores go.mod files in the standard library, treating it as if it were a
 // single module named "std" at the given version.
 func Zip(resolvedVersion string) (_ *zip.Reader, commitTime time.Time, err error) {
 	// This code taken, with modifications, from
 	// https://github.com/shurcooL/play/blob/master/256/moduleproxy/std/std.go.
 	defer derrors.Wrap(&err, "stdlib.Zip(%q)", resolvedVersion)

 	var repo *git.Repository
 	if UseTestData {
 		repo, err = getTestGoRepo(resolvedVersion)
 	} else {
 		repo, err = getGoRepo(resolvedVersion)
 	}
 	if err != nil {
 		return nil, time.Time{}, err
 	}
 	var buf bytes.Buffer
 	z := zip.NewWriter(&buf)
 	head, err := repo.Head()
 	if err != nil {
 		return nil, time.Time{}, err
 	}
 	commit, err := repo.CommitObject(head.Hash())
 	if err != nil {
 		return nil, time.Time{}, err
 	}
 	root, err := repo.TreeObject(commit.TreeHash)
 	if err != nil {
 		return nil, time.Time{}, err
 	}
 	prefixPath := ModulePath + "@" + resolvedVersion
 	// Add top-level files.
 	if err := addFiles(z, repo, root, prefixPath, false); err != nil {
 		return nil, time.Time{}, err
 	}
 	// Add files from the stdlib directory.
 	libdir := root
 	for _, d := range strings.Split(Directory(resolvedVersion), "/") {
 		libdir, err = subTree(repo, libdir, d)
 		if err != nil {
 			return nil, time.Time{}, err
 		}
 	}
 	if err := addFiles(z, repo, libdir, prefixPath, true); err != nil {
 		return nil, time.Time{}, err
 	}
 	if err := z.Close(); err != nil {
 		return nil, time.Time{}, err
 	}
 	br := bytes.NewReader(buf.Bytes())
 	zr, err := zip.NewReader(br, int64(br.Len()))
 	if err != nil {
 		return nil, time.Time{}, err
 	}
 	return zr, commit.Committer.When, nil
 }

 // semanticVersion returns the semantic version corresponding to the
 // requestedVersion.
 func semanticVersion(requestedVersion string) (_ string, err error) {
 	defer derrors.Wrap(&err, "semanticVersion(%q)", requestedVersion)

 	knownVersions, err := Versions()
 	if err != nil {
 		return "", err
 	}
 	if requestedVersion == "latest" {
 		var latestVersion string
 		for _, v := range knownVersions {
 			versionType, err := version.ParseType(v)
 			if err != nil {
 				return "", err
 			}
 			if versionType != version.TypeRelease {
 				// We expect there to always be at least 1 release version.
 				continue
 			}
 			if semver.Compare(v, latestVersion) > 0 {
 				latestVersion = v
 			}
 		}
 		return latestVersion, nil
 	}

 	for _, v := range knownVersions {
 		if v == requestedVersion {
 			return requestedVersion, nil
 		}
 	}
 	return "", fmt.Errorf("%w: requested version unknown: %q", derrors.InvalidArgument, requestedVersion)
 }

 // addFiles adds the files in t to z, using dirpath as the path prefix.
 // If recursive is true, it also adds the files in all subdirectories.
 func addFiles(z *zip.Writer, r *git.Repository, t *object.Tree, dirpath string, recursive bool) (err error) {
 	defer derrors.Wrap(&err, "addFiles(zip, repository, tree, %q, %t)", dirpath, recursive)

 	for _, e := range t.Entries {
 		if strings.HasPrefix(e.Name, ".") || strings.HasPrefix(e.Name, "_") {
 			continue
 		}
 		if e.Name == "go.mod" {
 			// ignore; we'll synthesize our own
 			continue
 		}
 		if e.Name == "README.vendor" && !strings.Contains(dirpath, "/") {
 			// For versions newer than v1.4.0-beta.1, the stdlib is in src/pkg.
 			// This means that our construction of the zip files will return
 			// two READMEs at the root:
 			// https://golang.org/README.md and
 			// https://golang.org/src/README.vendor
 			// We only want to display the README.md, so ignore README.vendor.
 			// However, we do want to store the README.vendor in
 			// std@<version>/cmd.
 			continue
 		}
 		switch e.Mode {
 		case filemode.Regular, filemode.Executable:
 			blob, err := r.BlobObject(e.Hash)
 			if err != nil {
 				return err
 			}
 			src, err := blob.Reader()
 			if err != nil {
 				return err
 			}
 			if err := writeZipFile(z, path.Join(dirpath, e.Name), src); err != nil {
 				_ = src.Close()
 				return err
 			}
 			if err := src.Close(); err != nil {
 				return err
 			}
 		case filemode.Dir:
 			if !recursive || e.Name == "testdata" {
 				continue
 			}
 			t2, err := r.TreeObject(e.Hash)
 			if err != nil {
 				return err
 			}
 			if err := addFiles(z, r, t2, path.Join(dirpath, e.Name), recursive); err != nil {
 				return err
 			}
 		}
 	}
 	return nil
 }

 func writeZipFile(z *zip.Writer, pathname string, src io.Reader) (err error) {
 	defer derrors.Wrap(&err, "writeZipFile(zip, %q, src)", pathname)

 	dst, err := z.Create(pathname)
 	if err != nil {
 		return err
 	}
 	_, err = io.Copy(dst, src)
 	return err
 }

 // subTree looks non-recursively for a directory with the given name in t,
 // and returns the corresponding tree.
 // If a directory with such name doesn't exist in t, it returns os.ErrNotExist.
 func subTree(r *git.Repository, t *object.Tree, name string) (_ *object.Tree, err error) {
 	defer derrors.Wrap(&err, "subTree(repository, tree, %q)", name)

 	for _, e := range t.Entries {
 		if e.Name == name {
 			return r.TreeObject(e.Hash)
 		}
 	}
 	return nil, os.ErrNotExist
 }

 // Contains reports whether the given import path could be part of the Go standard library,
 // by reporting whether the first component lacks a '.'.
 func Contains(path string) bool {
 	if i := strings.IndexByte(path, '/'); i != -1 {
 		path = path[:i]
 	}
 	return !strings.Contains(path, ".")
 }

 // References used for Versions during testing.
 var testRefs = []plumbing.ReferenceName{
 	// stdlib versions
 	"refs/tags/go1.2.1",
 	"refs/tags/go1.3.2",
 	"refs/tags/go1.4.2",
 	"refs/tags/go1.4.3",
 	"refs/tags/go1.6",
 	"refs/tags/go1.6.3",
 	"refs/tags/go1.6beta1",
 	"refs/tags/go1.8",
 	"refs/tags/go1.8rc2",
 	"refs/tags/go1.9rc1",
 	"refs/tags/go1.11",
 	"refs/tags/go1.12",
 	"refs/tags/go1.12.1",
 	"refs/tags/go1.12.5",
 	"refs/tags/go1.12.9",
 	"refs/tags/go1.13",
 	"refs/tags/go1.13beta1",
 	"refs/tags/go1.14.6",
 	// other tags
 	"refs/changes/56/93156/13",
 	"refs/tags/release.r59",
 	"refs/tags/weekly.2011-04-13",
 }
	// Copyright 2019 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 stdlib supports special handling of the Go standard library.
	// Regardless of the how the standard library has been split into modules for
	// development and testing, the discovery site treats it as a single module
	// named "std".
	package stdlib

	import (
	"archive/zip"
	"bytes"
	"fmt"
	"io"
	"os"
	"path"
	"path/filepath"
	"regexp"
	"strings"
	"time"

	"golang.org/x/mod/semver"
	"golang.org/x/pkgsite/internal/derrors"
	"golang.org/x/pkgsite/internal/testing/testhelper"
	"golang.org/x/pkgsite/internal/version"

	"github.com/go-git/go-billy/v5/osfs"
	"github.com/go-git/go-git/v5"
	"github.com/go-git/go-git/v5/config"
	"github.com/go-git/go-git/v5/plumbing"
	"github.com/go-git/go-git/v5/plumbing/filemode"
	"github.com/go-git/go-git/v5/plumbing/object"
	"github.com/go-git/go-git/v5/storage/memory"
	)

	// ModulePath is the name of the module for the standard library.
	const ModulePath = "std"

	var (
	// Regexp for matching go tags. The groups are:
	// 1 the major.minor version
	// 2 the patch version, or empty if none
	// 3 the entire prerelease, if present
	// 4 the prerelease type ("beta" or "rc")
	// 5 the prerelease number
	tagRegexp = regexp.MustCompile(`^go(\d+\.\d+)(\.\d+\|)((beta\|rc)(\d+))?$`)
	)

	// VersionForTag returns the semantic version for the Go tag, or "" if
	// tag doesn't correspond to a Go release or beta tag.
	// Examples:
	// "go1" => "v1.0.0"
	// "go1.2" => "v1.2.0"
	// "go1.13beta1" => "v1.13.0-beta.1"
	// "go1.9rc2" => "v1.9.0-rc.2"
	func VersionForTag(tag string) string {
	// Special cases for go1.
	if tag == "go1" {
	return "v1.0.0"
	}
	if tag == "go1.0" {
	return ""
	}
	// Special case for latest.
	if tag == "latest" {
	return "latest"
	}
	m := tagRegexp.FindStringSubmatch(tag)
	if m == nil {
	return ""
	}
	version := "v" + m[1]
	if m[2] != "" {
	version += m[2]
	} else {
	version += ".0"
	}
	if m[3] != "" {
	version += "-" + m[4] + "." + m[5]
	}
	return version
	}

	// TagForVersion returns the Go standard library repository tag corresponding
	// to semver. The Go tags differ from standard semantic versions in a few ways,
	// such as beginning with "go" instead of "v".
	func TagForVersion(version string) (_ string, err error) {
	defer derrors.Wrap(&err, "TagForVersion(%q)", version)

	// Special case: v1.0.0 => go1.
	if version == "v1.0.0" {
	return "go1", nil
	}
	if !semver.IsValid(version) {
	return "", fmt.Errorf("%w: requested version is not a valid semantic version: %q ", derrors.InvalidArgument, version)
	}
	goVersion := semver.Canonical(version)
	prerelease := semver.Prerelease(goVersion)
	versionWithoutPrerelease := strings.TrimSuffix(goVersion, prerelease)
	patch := strings.TrimPrefix(versionWithoutPrerelease, semver.MajorMinor(goVersion)+".")
	if patch == "0" {
	versionWithoutPrerelease = strings.TrimSuffix(versionWithoutPrerelease, ".0")
	}

	goVersion = fmt.Sprintf("go%s", strings.TrimPrefix(versionWithoutPrerelease, "v"))
	if prerelease != "" {
	// Go prereleases look like "beta1" instead of "beta.1".
	// "beta1" is bad for sorting (since beta10 comes before beta9), so
	// require the dot form.
	i := finalDigitsIndex(prerelease)
	if i >= 1 {
	if prerelease[i-1] != '.' {
	return "", fmt.Errorf("%w: final digits in a prerelease must follow a period", derrors.InvalidArgument)
	}
	// Remove the dot.
	prerelease = prerelease[:i-1] + prerelease[i:]
	}
	goVersion += strings.TrimPrefix(prerelease, "-")
	}
	return goVersion, nil
	}

	// MajorVersionForVersion returns the Go major version for version.
	// E.g. "v1.13.3" => "go1".
	func MajorVersionForVersion(version string) (_ string, err error) {
	defer derrors.Wrap(&err, "MajorVersionForVersion(%q)", version)

	tag, err := TagForVersion(version)
	if err != nil {
	return "", err
	}
	if tag == "go1" {
	return tag, nil
	}
	i := strings.IndexRune(tag, '.')
	if i < 0 {
	return "", fmt.Errorf("no '.' in go tag %q", tag)
	}
	return tag[:i], nil
	}

	// finalDigitsIndex returns the index of the first digit in the sequence of digits ending s.
	// If s doesn't end in digits, it returns -1.
	func finalDigitsIndex(s string) int {
	// Assume ASCII (since the semver package does anyway).
	var i int
	for i = len(s) - 1; i >= 0; i-- {
	if s[i] < '0' \|\| s[i] > '9' {
	break
	}
	}
	if i == len(s)-1 {
	return -1
	}
	return i + 1
	}

	const (
	GoRepoURL = "https://go.googlesource.com/go"
	GoSourceRepoURL = "https://github.com/golang/go"
	)

	// UseTestData determines whether to really clone the Go repo, or use
	// stripped-down versions of the repo from the testdata directory.
	var UseTestData = false

	// TestCommitTime is the time used for all commits when UseTestData is true.
	var TestCommitTime = time.Date(2019, 9, 4, 1, 2, 3, 0, time.UTC)

	// getGoRepo returns a repo object for the Go repo at version.
	func getGoRepo(version string) (_ *git.Repository, err error) {
	defer derrors.Wrap(&err, "getGoRepo(%q)", version)

	tag, err := TagForVersion(version)
	if err != nil {
	return nil, err
	}
	return git.Clone(memory.NewStorage(), nil, &git.CloneOptions{
	URL: GoRepoURL,
	ReferenceName: plumbing.NewTagReferenceName(tag),
	SingleBranch: true,
	Depth: 1,
	Tags: git.NoTags,
	})
	}

	// getTestGoRepo gets a Go repo for testing.
	func getTestGoRepo(version string) (_ *git.Repository, err error) {
	defer derrors.Wrap(&err, "getTestGoRepo(%q)", version)

	fs := osfs.New(filepath.Join(testhelper.TestDataPath("testdata"), version))
	repo, err := git.Init(memory.NewStorage(), fs)
	if err != nil {
	return nil, err
	}
	wt, err := repo.Worktree()
	if err != nil {
	return nil, err
	}
	// Add all files in the directory.
	if _, err := wt.Add(""); err != nil {
	return nil, err
	}
	_, err = wt.Commit("", &git.CommitOptions{All: true, Author: &object.Signature{
	Name: "Joe Random",
	Email: "joe@example.com",
	When: TestCommitTime,
	}})
	if err != nil {
	return nil, err
	}
	return repo, nil
	}

	// Versions returns all the versions of Go that are relevant to the discovery
	// site. These are all release versions (tags of the forms "goN.N" and
	// "goN.N.N", where N is a number) and beta or rc versions (tags of the forms
	// "goN.NbetaN" and "goN.N.NbetaN", and similarly for "rc" replacing "beta").
	func Versions() (_ []string, err error) {
	defer derrors.Wrap(&err, "Versions()")

	var refNames []plumbing.ReferenceName
	if UseTestData {
	refNames = testRefs
	} else {
	re := git.NewRemote(memory.NewStorage(), &config.RemoteConfig{
	URLs: []string{GoRepoURL},
	})
	refs, err := re.List(&git.ListOptions{})
	if err != nil {
	return nil, err
	}
	for _, r := range refs {
	refNames = append(refNames, r.Name())
	}
	}

	var versions []string
	for _, name := range refNames {
	if !name.IsTag() {
	continue
	}
	v := VersionForTag(name.Short())
	if v != "" {
	versions = append(versions, v)
	}
	}
	return versions, nil
	}

	// Directory returns the directory of the standard library relative to the repo root.
	func Directory(version string) string {
	// For versions older than v1.4.0-beta.1, the stdlib is in src/pkg.
	if semver.Compare(version, "v1.4.0-beta.1") < 0 {
	return "src/pkg"
	}
	return "src"
	}

	// Approximate size of Zip("v1.15.2").
	const estimatedZipSize = 16 * 1024 * 1024

	func ZipInfo(requestedVersion string) (resolvedVersion string, zipSize int64, err error) {
	defer derrors.Wrap(&err, "stdlib.ZipInfo(%q)", requestedVersion)

	resolvedVersion, err = semanticVersion(requestedVersion)
	if err != nil {
	return "", 0, err
	}
	return resolvedVersion, estimatedZipSize, nil
	}

	// Zip creates a module zip representing the entire Go standard library at the
	// given version (which must have been resolved with ZipInfo) and returns a
	// reader to it. It also returns the time of the commit for that version. The
	// zip file is in module form, with each path prefixed by ModuleName + "@" +
	// version.
	//
	// Zip reads the standard library at the Go repository tag corresponding to to
	// the given semantic version.
	//
	// Zip ignores go.mod files in the standard library, treating it as if it were a
	// single module named "std" at the given version.
	func Zip(resolvedVersion string) (_ *zip.Reader, commitTime time.Time, err error) {
	// This code taken, with modifications, from
	// https://github.com/shurcooL/play/blob/master/256/moduleproxy/std/std.go.
	defer derrors.Wrap(&err, "stdlib.Zip(%q)", resolvedVersion)

	var repo *git.Repository
	if UseTestData {
	repo, err = getTestGoRepo(resolvedVersion)
	} else {
	repo, err = getGoRepo(resolvedVersion)
	}
	if err != nil {
	return nil, time.Time{}, err
	}
	var buf bytes.Buffer
	z := zip.NewWriter(&buf)
	head, err := repo.Head()
	if err != nil {
	return nil, time.Time{}, err
	}
	commit, err := repo.CommitObject(head.Hash())
	if err != nil {
	return nil, time.Time{}, err
	}
	root, err := repo.TreeObject(commit.TreeHash)
	if err != nil {
	return nil, time.Time{}, err
	}
	prefixPath := ModulePath + "@" + resolvedVersion
	// Add top-level files.
	if err := addFiles(z, repo, root, prefixPath, false); err != nil {
	return nil, time.Time{}, err
	}
	// Add files from the stdlib directory.
	libdir := root
	for _, d := range strings.Split(Directory(resolvedVersion), "/") {
	libdir, err = subTree(repo, libdir, d)
	if err != nil {
	return nil, time.Time{}, err
	}
	}
	if err := addFiles(z, repo, libdir, prefixPath, true); err != nil {
	return nil, time.Time{}, err
	}
	if err := z.Close(); err != nil {
	return nil, time.Time{}, err
	}
	br := bytes.NewReader(buf.Bytes())
	zr, err := zip.NewReader(br, int64(br.Len()))
	if err != nil {
	return nil, time.Time{}, err
	}
	return zr, commit.Committer.When, nil
	}

	// semanticVersion returns the semantic version corresponding to the
	// requestedVersion.
	func semanticVersion(requestedVersion string) (_ string, err error) {
	defer derrors.Wrap(&err, "semanticVersion(%q)", requestedVersion)

	knownVersions, err := Versions()
	if err != nil {
	return "", err
	}
	if requestedVersion == "latest" {
	var latestVersion string
	for _, v := range knownVersions {
	versionType, err := version.ParseType(v)
	if err != nil {
	return "", err
	}
	if versionType != version.TypeRelease {
	// We expect there to always be at least 1 release version.
	continue
	}
	if semver.Compare(v, latestVersion) > 0 {
	latestVersion = v
	}
	}
	return latestVersion, nil
	}

	for _, v := range knownVersions {
	if v == requestedVersion {
	return requestedVersion, nil
	}
	}
	return "", fmt.Errorf("%w: requested version unknown: %q", derrors.InvalidArgument, requestedVersion)
	}

	// addFiles adds the files in t to z, using dirpath as the path prefix.
	// If recursive is true, it also adds the files in all subdirectories.
	func addFiles(z zip.Writer, r git.Repository, t *object.Tree, dirpath string, recursive bool) (err error) {
	defer derrors.Wrap(&err, "addFiles(zip, repository, tree, %q, %t)", dirpath, recursive)

	for _, e := range t.Entries {
	if strings.HasPrefix(e.Name, ".") \|\| strings.HasPrefix(e.Name, "_") {
	continue
	}
	if e.Name == "go.mod" {
	// ignore; we'll synthesize our own
	continue
	}
	if e.Name == "README.vendor" && !strings.Contains(dirpath, "/") {
	// For versions newer than v1.4.0-beta.1, the stdlib is in src/pkg.
	// This means that our construction of the zip files will return
	// two READMEs at the root:
	// https://golang.org/README.md and
	// https://golang.org/src/README.vendor
	// We only want to display the README.md, so ignore README.vendor.
	// However, we do want to store the README.vendor in
	// std@<version>/cmd.
	continue
	}
	switch e.Mode {
	case filemode.Regular, filemode.Executable:
	blob, err := r.BlobObject(e.Hash)
	if err != nil {
	return err
	}
	src, err := blob.Reader()
	if err != nil {
	return err
	}
	if err := writeZipFile(z, path.Join(dirpath, e.Name), src); err != nil {
	_ = src.Close()
	return err
	}
	if err := src.Close(); err != nil {
	return err
	}
	case filemode.Dir:
	if !recursive \|\| e.Name == "testdata" {
	continue
	}
	t2, err := r.TreeObject(e.Hash)
	if err != nil {
	return err
	}
	if err := addFiles(z, r, t2, path.Join(dirpath, e.Name), recursive); err != nil {
	return err
	}
	}
	}
	return nil
	}

	func writeZipFile(z *zip.Writer, pathname string, src io.Reader) (err error) {
	defer derrors.Wrap(&err, "writeZipFile(zip, %q, src)", pathname)

	dst, err := z.Create(pathname)
	if err != nil {
	return err
	}
	_, err = io.Copy(dst, src)
	return err
	}

	// subTree looks non-recursively for a directory with the given name in t,
	// and returns the corresponding tree.
	// If a directory with such name doesn't exist in t, it returns os.ErrNotExist.
	func subTree(r git.Repository, t object.Tree, name string) (_ *object.Tree, err error) {
	defer derrors.Wrap(&err, "subTree(repository, tree, %q)", name)

	for _, e := range t.Entries {
	if e.Name == name {
	return r.TreeObject(e.Hash)
	}
	}
	return nil, os.ErrNotExist
	}

	// Contains reports whether the given import path could be part of the Go standard library,
	// by reporting whether the first component lacks a '.'.
	func Contains(path string) bool {
	if i := strings.IndexByte(path, '/'); i != -1 {
	path = path[:i]
	}
	return !strings.Contains(path, ".")
	}

	// References used for Versions during testing.
	var testRefs = []plumbing.ReferenceName{
	// stdlib versions
	"refs/tags/go1.2.1",
	"refs/tags/go1.3.2",
	"refs/tags/go1.4.2",
	"refs/tags/go1.4.3",
	"refs/tags/go1.6",
	"refs/tags/go1.6.3",
	"refs/tags/go1.6beta1",
	"refs/tags/go1.8",
	"refs/tags/go1.8rc2",
	"refs/tags/go1.9rc1",
	"refs/tags/go1.11",
	"refs/tags/go1.12",
	"refs/tags/go1.12.1",
	"refs/tags/go1.12.5",
	"refs/tags/go1.12.9",
	"refs/tags/go1.13",
	"refs/tags/go1.13beta1",
	"refs/tags/go1.14.6",
	// other tags
	"refs/changes/56/93156/13",
	"refs/tags/release.r59",
	"refs/tags/weekly.2011-04-13",
	}