vendor/cmd/go/internal/modfetch/codehost/git.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 codehost

 import (
 	"bytes"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"sort"
 	"strconv"
 	"strings"
 	"sync"
 	"time"

 	"cmd/go/internal/par"
 )

 // GitRepo returns the code repository at the given Git remote reference.
 func GitRepo(remote string) (Repo, error) {
 	return newGitRepoCached(remote, false)
 }

 // LocalGitRepo is like Repo but accepts both Git remote references
 // and paths to repositories on the local file system.
 func LocalGitRepo(remote string) (Repo, error) {
 	return newGitRepoCached(remote, true)
 }

 const gitWorkDirType = "git2"

 var gitRepoCache par.Cache

 func newGitRepoCached(remote string, localOK bool) (Repo, error) {
 	type key struct {
 		remote  string
 		localOK bool
 	}
 	type cached struct {
 		repo Repo
 		err  error
 	}

 	c := gitRepoCache.Do(key{remote, localOK}, func() interface{} {
 		repo, err := newGitRepo(remote, localOK)
 		return cached{repo, err}
 	}).(cached)

 	return c.repo, c.err
 }

 func newGitRepo(remote string, localOK bool) (Repo, error) {
 	r := &gitRepo{remote: remote}
 	if strings.Contains(remote, "://") {
 		// This is a remote path.
 		dir, err := WorkDir(gitWorkDirType, r.remote)
 		if err != nil {
 			return nil, err
 		}
 		r.dir = dir
 		if _, err := os.Stat(filepath.Join(dir, "objects")); err != nil {
 			if _, err := Run(dir, "git", "init", "--bare"); err != nil {
 				os.RemoveAll(dir)
 				return nil, err
 			}
 			// We could just say git fetch https://whatever later,
 			// but this lets us say git fetch origin instead, which
 			// is a little nicer. More importantly, using a named remote
 			// avoids a problem with Git LFS. See golang.org/issue/25605.
 			if _, err := Run(dir, "git", "remote", "add", "origin", r.remote); err != nil {
 				os.RemoveAll(dir)
 				return nil, err
 			}
 			r.remote = "origin"
 		}
 	} else {
 		// Local path.
 		// Disallow colon (not in ://) because sometimes
 		// that's rcp-style host:path syntax and sometimes it's not (c:\work).
 		// The go command has always insisted on URL syntax for ssh.
 		if strings.Contains(remote, ":") {
 			return nil, fmt.Errorf("git remote cannot use host:path syntax")
 		}
 		if !localOK {
 			return nil, fmt.Errorf("git remote must not be local directory")
 		}
 		r.local = true
 		info, err := os.Stat(remote)
 		if err != nil {
 			return nil, err
 		}
 		if !info.IsDir() {
 			return nil, fmt.Errorf("%s exists but is not a directory", remote)
 		}
 		r.dir = remote
 	}
 	return r, nil
 }

 type gitRepo struct {
 	remote string
 	local  bool
 	dir    string

 	mu         sync.Mutex // protects fetchLevel, some git repo state
 	fetchLevel int

 	statCache par.Cache

 	refsOnce sync.Once
 	refs     map[string]string
 	refsErr  error

 	localTagsOnce sync.Once
 	localTags     map[string]bool
 }

 const (
 	// How much have we fetched into the git repo (in this process)?
 	fetchNone = iota // nothing yet
 	fetchSome        // shallow fetches of individual hashes
 	fetchAll         // "fetch -t origin": get all remote branches and tags
 )

 // loadLocalTags loads tag references from the local git cache
 // into the map r.localTags.
 // Should only be called as r.localTagsOnce.Do(r.loadLocalTags).
 func (r *gitRepo) loadLocalTags() {
 	// The git protocol sends all known refs and ls-remote filters them on the client side,
 	// so we might as well record both heads and tags in one shot.
 	// Most of the time we only care about tags but sometimes we care about heads too.
 	out, err := Run(r.dir, "git", "tag", "-l")
 	if err != nil {
 		return
 	}

 	r.localTags = make(map[string]bool)
 	for _, line := range strings.Split(string(out), "\n") {
 		if line != "" {
 			r.localTags[line] = true
 		}
 	}
 }

 // loadRefs loads heads and tags references from the remote into the map r.refs.
 // Should only be called as r.refsOnce.Do(r.loadRefs).
 func (r *gitRepo) loadRefs() {
 	// The git protocol sends all known refs and ls-remote filters them on the client side,
 	// so we might as well record both heads and tags in one shot.
 	// Most of the time we only care about tags but sometimes we care about heads too.
 	out, err := Run(r.dir, "git", "ls-remote", "-q", r.remote)
 	if err != nil {
 		r.refsErr = err
 		return
 	}

 	r.refs = make(map[string]string)
 	for _, line := range strings.Split(string(out), "\n") {
 		f := strings.Fields(line)
 		if len(f) != 2 {
 			continue
 		}
 		if f[1] == "HEAD" || strings.HasPrefix(f[1], "refs/heads/") || strings.HasPrefix(f[1], "refs/tags/") {
 			r.refs[f[1]] = f[0]
 		}
 	}
 	for ref, hash := range r.refs {
 		if strings.HasSuffix(ref, "^{}") { // record unwrapped annotated tag as value of tag
 			r.refs[strings.TrimSuffix(ref, "^{}")] = hash
 			delete(r.refs, ref)
 		}
 	}
 }

 func (r *gitRepo) Tags(prefix string) ([]string, error) {
 	r.refsOnce.Do(r.loadRefs)
 	if r.refsErr != nil {
 		return nil, r.refsErr
 	}

 	tags := []string{}
 	for ref := range r.refs {
 		if !strings.HasPrefix(ref, "refs/tags/") {
 			continue
 		}
 		tag := ref[len("refs/tags/"):]
 		if !strings.HasPrefix(tag, prefix) {
 			continue
 		}
 		tags = append(tags, tag)
 	}
 	sort.Strings(tags)
 	return tags, nil
 }

 func (r *gitRepo) Latest() (*RevInfo, error) {
 	r.refsOnce.Do(r.loadRefs)
 	if r.refsErr != nil {
 		return nil, r.refsErr
 	}
 	if r.refs["HEAD"] == "" {
 		return nil, fmt.Errorf("no commits")
 	}
 	return r.Stat(r.refs["HEAD"])
 }

 // findRef finds some ref name for the given hash,
 // for use when the server requires giving a ref instead of a hash.
 // There may be multiple ref names for a given hash,
 // in which case this returns some name - it doesn't matter which.
 func (r *gitRepo) findRef(hash string) (ref string, ok bool) {
 	r.refsOnce.Do(r.loadRefs)
 	for ref, h := range r.refs {
 		if h == hash {
 			return ref, true
 		}
 	}
 	return "", false
 }

 func unshallow(gitDir string) []string {
 	if _, err := os.Stat(filepath.Join(gitDir, "shallow")); err == nil {
 		return []string{"--unshallow"}
 	}
 	return []string{}
 }

 // minHashDigits is the minimum number of digits to require
 // before accepting a hex digit sequence as potentially identifying
 // a specific commit in a git repo. (Of course, users can always
 // specify more digits, and many will paste in all 40 digits,
 // but many of git's commands default to printing short hashes
 // as 7 digits.)
 const minHashDigits = 7

 // stat stats the given rev in the local repository,
 // or else it fetches more info from the remote repository and tries again.
 func (r *gitRepo) stat(rev string) (*RevInfo, error) {
 	if r.local {
 		return r.statLocal(rev, rev)
 	}

 	// Fast path: maybe rev is a hash we already have locally.
 	if len(rev) >= minHashDigits && len(rev) <= 40 && AllHex(rev) {
 		if info, err := r.statLocal(rev, rev); err == nil {
 			return info, nil
 		}
 	}

 	// Maybe rev is a tag we already have locally.
 	// (Note that we're excluding branches, which can be stale.)
 	r.localTagsOnce.Do(r.loadLocalTags)
 	if r.localTags[rev] {
 		return r.statLocal(rev, "refs/tags/"+rev)
 	}

 	// Maybe rev is the name of a tag or branch on the remote server.
 	// Or maybe it's the prefix of a hash of a named ref.
 	// Try to resolve to both a ref (git name) and full (40-hex-digit) commit hash.
 	r.refsOnce.Do(r.loadRefs)
 	var ref, hash string
 	if r.refs["refs/tags/"+rev] != "" {
 		ref = "refs/tags/" + rev
 		hash = r.refs[ref]
 		// Keep rev as is: tags are assumed not to change meaning.
 	} else if r.refs["refs/heads/"+rev] != "" {
 		ref = "refs/heads/" + rev
 		hash = r.refs[ref]
 		rev = hash // Replace rev, because meaning of refs/heads/foo can change.
 	} else if rev == "HEAD" && r.refs["HEAD"] != "" {
 		ref = "HEAD"
 		hash = r.refs[ref]
 		rev = hash // Replace rev, because meaning of HEAD can change.
 	} else if len(rev) >= minHashDigits && len(rev) <= 40 && AllHex(rev) {
 		// At the least, we have a hash prefix we can look up after the fetch below.
 		// Maybe we can map it to a full hash using the known refs.
 		prefix := rev
 		// Check whether rev is prefix of known ref hash.
 		for k, h := range r.refs {
 			if strings.HasPrefix(h, prefix) {
 				if hash != "" && hash != h {
 					// Hash is an ambiguous hash prefix.
 					// More information will not change that.
 					return nil, fmt.Errorf("ambiguous revision %s", rev)
 				}
 				if ref == "" || ref > k { // Break ties deterministically when multiple refs point at same hash.
 					ref = k
 				}
 				rev = h
 				hash = h
 			}
 		}
 		if hash == "" && len(rev) == 40 { // Didn't find a ref, but rev is a full hash.
 			hash = rev
 		}
 	} else {
 		return nil, fmt.Errorf("unknown revision %s", rev)
 	}

 	// Protect r.fetchLevel and the "fetch more and more" sequence.
 	// TODO(rsc): Add LockDir and use it for protecting that
 	// sequence, so that multiple processes don't collide in their
 	// git commands.
 	r.mu.Lock()
 	defer r.mu.Unlock()

 	// If we know a specific commit we need, fetch it.
 	if r.fetchLevel <= fetchSome && hash != "" && !r.local {
 		r.fetchLevel = fetchSome
 		var refspec string
 		if ref != "" && ref != "head" {
 			// If we do know the ref name, save the mapping locally
 			// so that (if it is a tag) it can show up in localTags
 			// on a future call. Also, some servers refuse to allow
 			// full hashes in ref specs, so prefer a ref name if known.
 			refspec = ref + ":" + ref
 		} else {
 			// Fetch the hash but give it a local name (refs/dummy),
 			// because that triggers the fetch behavior of creating any
 			// other known remote tags for the hash. We never use
 			// refs/dummy (it's not refs/tags/dummy) and it will be
 			// overwritten in the next command, and that's fine.
 			ref = hash
 			refspec = hash + ":refs/dummy"
 		}
 		_, err := Run(r.dir, "git", "fetch", "-f", "--depth=1", r.remote, refspec)
 		if err == nil {
 			return r.statLocal(rev, ref)
 		}
 		if !strings.Contains(err.Error(), "unadvertised object") && !strings.Contains(err.Error(), "no such remote ref") && !strings.Contains(err.Error(), "does not support shallow") {
 			return nil, err
 		}
 	}

 	// Last resort.
 	// Fetch all heads and tags and hope the hash we want is in the history.
 	if r.fetchLevel < fetchAll {
 		r.fetchLevel = fetchAll

 		// To work around a protocol version 2 bug that breaks --unshallow,
 		// add -c protocol.version=0.
 		// TODO(rsc): The bug is believed to be server-side, meaning only
 		// on Google's Git servers. Once the servers are fixed, drop the
 		// protocol.version=0. See Google-internal bug b/110495752.
 		var protoFlag []string
 		unshallowFlag := unshallow(r.dir)
 		if len(unshallowFlag) > 0 {
 			protoFlag = []string{"-c", "protocol.version=0"}
 		}
 		if _, err := Run(r.dir, "git", protoFlag, "fetch", unshallowFlag, "-f", r.remote, "refs/heads/*:refs/heads/*", "refs/tags/*:refs/tags/*"); err != nil {
 			return nil, err
 		}
 	}

 	return r.statLocal(rev, rev)
 }

 // statLocal returns a RevInfo describing rev in the local git repository.
 // It uses version as info.Version.
 func (r *gitRepo) statLocal(version, rev string) (*RevInfo, error) {
 	out, err := Run(r.dir, "git", "log", "-n1", "--format=format:%H %ct %D", rev)
 	if err != nil {
 		return nil, fmt.Errorf("unknown revision %s", rev)
 	}
 	f := strings.Fields(string(out))
 	if len(f) < 2 {
 		return nil, fmt.Errorf("unexpected response from git log: %q", out)
 	}
 	hash := f[0]
 	if strings.HasPrefix(hash, version) {
 		version = hash // extend to full hash
 	}
 	t, err := strconv.ParseInt(f[1], 10, 64)
 	if err != nil {
 		return nil, fmt.Errorf("invalid time from git log: %q", out)
 	}

 	info := &RevInfo{
 		Name:    hash,
 		Short:   ShortenSHA1(hash),
 		Time:    time.Unix(t, 0).UTC(),
 		Version: hash,
 	}

 	// Add tags. Output looks like:
 	//	ede458df7cd0fdca520df19a33158086a8a68e81 1523994202 HEAD -> master, tag: v1.2.4-annotated, tag: v1.2.3, origin/master, origin/HEAD
 	for i := 2; i < len(f); i++ {
 		if f[i] == "tag:" {
 			i++
 			if i < len(f) {
 				info.Tags = append(info.Tags, strings.TrimSuffix(f[i], ","))
 			}
 		}
 	}
 	sort.Strings(info.Tags)

 	// Used hash as info.Version above.
 	// Use caller's suggested version if it appears in the tag list
 	// (filters out branch names, HEAD).
 	for _, tag := range info.Tags {
 		if version == tag {
 			info.Version = version
 		}
 	}

 	return info, nil
 }

 func (r *gitRepo) Stat(rev string) (*RevInfo, error) {
 	if rev == "latest" {
 		return r.Latest()
 	}
 	type cached struct {
 		info *RevInfo
 		err  error
 	}
 	c := r.statCache.Do(rev, func() interface{} {
 		info, err := r.stat(rev)
 		return cached{info, err}
 	}).(cached)
 	return c.info, c.err
 }

 func (r *gitRepo) ReadFile(rev, file string, maxSize int64) ([]byte, error) {
 	// TODO: Could use git cat-file --batch.
 	info, err := r.Stat(rev) // download rev into local git repo
 	if err != nil {
 		return nil, err
 	}
 	out, err := Run(r.dir, "git", "cat-file", "blob", info.Name+":"+file)
 	if err != nil {
 		return nil, os.ErrNotExist
 	}
 	return out, nil
 }

 func (r *gitRepo) ReadZip(rev, subdir string, maxSize int64) (zip io.ReadCloser, actualSubdir string, err error) {
 	// TODO: Use maxSize or drop it.
 	args := []string{}
 	if subdir != "" {
 		args = append(args, "--", subdir)
 	}
 	info, err := r.Stat(rev) // download rev into local git repo
 	if err != nil {
 		return nil, "", err
 	}

 	// Incredibly, git produces different archives depending on whether
 	// it is running on a Windows system or not, in an attempt to normalize
 	// text file line endings. Setting -c core.autocrlf=input means only
 	// translate files on the way into the repo, not on the way out (archive).
 	// The -c core.eol=lf should be unnecessary but set it anyway.
 	archive, err := Run(r.dir, "git", "-c", "core.autocrlf=input", "-c", "core.eol=lf", "archive", "--format=zip", "--prefix=prefix/", info.Name, args)
 	if err != nil {
 		if bytes.Contains(err.(*RunError).Stderr, []byte("did not match any files")) {
 			return nil, "", os.ErrNotExist
 		}
 		return nil, "", err
 	}

 	return ioutil.NopCloser(bytes.NewReader(archive)), "", nil
 }
	// 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 codehost

	import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"cmd/go/internal/par"
	)

	// GitRepo returns the code repository at the given Git remote reference.
	func GitRepo(remote string) (Repo, error) {
	return newGitRepoCached(remote, false)
	}

	// LocalGitRepo is like Repo but accepts both Git remote references
	// and paths to repositories on the local file system.
	func LocalGitRepo(remote string) (Repo, error) {
	return newGitRepoCached(remote, true)
	}

	const gitWorkDirType = "git2"

	var gitRepoCache par.Cache

	func newGitRepoCached(remote string, localOK bool) (Repo, error) {
	type key struct {
	remote string
	localOK bool
	}
	type cached struct {
	repo Repo
	err error
	}

	c := gitRepoCache.Do(key{remote, localOK}, func() interface{} {
	repo, err := newGitRepo(remote, localOK)
	return cached{repo, err}
	}).(cached)

	return c.repo, c.err
	}

	func newGitRepo(remote string, localOK bool) (Repo, error) {
	r := &gitRepo{remote: remote}
	if strings.Contains(remote, "://") {
	// This is a remote path.
	dir, err := WorkDir(gitWorkDirType, r.remote)
	if err != nil {
	return nil, err
	}
	r.dir = dir
	if _, err := os.Stat(filepath.Join(dir, "objects")); err != nil {
	if _, err := Run(dir, "git", "init", "--bare"); err != nil {
	os.RemoveAll(dir)
	return nil, err
	}
	// We could just say git fetch https://whatever later,
	// but this lets us say git fetch origin instead, which
	// is a little nicer. More importantly, using a named remote
	// avoids a problem with Git LFS. See golang.org/issue/25605.
	if _, err := Run(dir, "git", "remote", "add", "origin", r.remote); err != nil {
	os.RemoveAll(dir)
	return nil, err
	}
	r.remote = "origin"
	}
	} else {
	// Local path.
	// Disallow colon (not in ://) because sometimes
	// that's rcp-style host:path syntax and sometimes it's not (c:\work).
	// The go command has always insisted on URL syntax for ssh.
	if strings.Contains(remote, ":") {
	return nil, fmt.Errorf("git remote cannot use host:path syntax")
	}
	if !localOK {
	return nil, fmt.Errorf("git remote must not be local directory")
	}
	r.local = true
	info, err := os.Stat(remote)
	if err != nil {
	return nil, err
	}
	if !info.IsDir() {
	return nil, fmt.Errorf("%s exists but is not a directory", remote)
	}
	r.dir = remote
	}
	return r, nil
	}

	type gitRepo struct {
	remote string
	local bool
	dir string

	mu sync.Mutex // protects fetchLevel, some git repo state
	fetchLevel int

	statCache par.Cache

	refsOnce sync.Once
	refs map[string]string
	refsErr error

	localTagsOnce sync.Once
	localTags map[string]bool
	}

	const (
	// How much have we fetched into the git repo (in this process)?
	fetchNone = iota // nothing yet
	fetchSome // shallow fetches of individual hashes
	fetchAll // "fetch -t origin": get all remote branches and tags
	)

	// loadLocalTags loads tag references from the local git cache
	// into the map r.localTags.
	// Should only be called as r.localTagsOnce.Do(r.loadLocalTags).
	func (r *gitRepo) loadLocalTags() {
	// The git protocol sends all known refs and ls-remote filters them on the client side,
	// so we might as well record both heads and tags in one shot.
	// Most of the time we only care about tags but sometimes we care about heads too.
	out, err := Run(r.dir, "git", "tag", "-l")
	if err != nil {
	return
	}

	r.localTags = make(map[string]bool)
	for _, line := range strings.Split(string(out), "\n") {
	if line != "" {
	r.localTags[line] = true
	}
	}
	}

	// loadRefs loads heads and tags references from the remote into the map r.refs.
	// Should only be called as r.refsOnce.Do(r.loadRefs).
	func (r *gitRepo) loadRefs() {
	// The git protocol sends all known refs and ls-remote filters them on the client side,
	// so we might as well record both heads and tags in one shot.
	// Most of the time we only care about tags but sometimes we care about heads too.
	out, err := Run(r.dir, "git", "ls-remote", "-q", r.remote)
	if err != nil {
	r.refsErr = err
	return
	}

	r.refs = make(map[string]string)
	for _, line := range strings.Split(string(out), "\n") {
	f := strings.Fields(line)
	if len(f) != 2 {
	continue
	}
	if f[1] == "HEAD" \|\| strings.HasPrefix(f[1], "refs/heads/") \|\| strings.HasPrefix(f[1], "refs/tags/") {
	r.refs[f[1]] = f[0]
	}
	}
	for ref, hash := range r.refs {
	if strings.HasSuffix(ref, "^{}") { // record unwrapped annotated tag as value of tag
	r.refs[strings.TrimSuffix(ref, "^{}")] = hash
	delete(r.refs, ref)
	}
	}
	}

	func (r *gitRepo) Tags(prefix string) ([]string, error) {
	r.refsOnce.Do(r.loadRefs)
	if r.refsErr != nil {
	return nil, r.refsErr
	}

	tags := []string{}
	for ref := range r.refs {
	if !strings.HasPrefix(ref, "refs/tags/") {
	continue
	}
	tag := ref[len("refs/tags/"):]
	if !strings.HasPrefix(tag, prefix) {
	continue
	}
	tags = append(tags, tag)
	}
	sort.Strings(tags)
	return tags, nil
	}

	func (r gitRepo) Latest() (RevInfo, error) {
	r.refsOnce.Do(r.loadRefs)
	if r.refsErr != nil {
	return nil, r.refsErr
	}
	if r.refs["HEAD"] == "" {
	return nil, fmt.Errorf("no commits")
	}
	return r.Stat(r.refs["HEAD"])
	}

	// findRef finds some ref name for the given hash,
	// for use when the server requires giving a ref instead of a hash.
	// There may be multiple ref names for a given hash,
	// in which case this returns some name - it doesn't matter which.
	func (r *gitRepo) findRef(hash string) (ref string, ok bool) {
	r.refsOnce.Do(r.loadRefs)
	for ref, h := range r.refs {
	if h == hash {
	return ref, true
	}
	}
	return "", false
	}

	func unshallow(gitDir string) []string {
	if _, err := os.Stat(filepath.Join(gitDir, "shallow")); err == nil {
	return []string{"--unshallow"}
	}
	return []string{}
	}

	// minHashDigits is the minimum number of digits to require
	// before accepting a hex digit sequence as potentially identifying
	// a specific commit in a git repo. (Of course, users can always
	// specify more digits, and many will paste in all 40 digits,
	// but many of git's commands default to printing short hashes
	// as 7 digits.)
	const minHashDigits = 7

	// stat stats the given rev in the local repository,
	// or else it fetches more info from the remote repository and tries again.
	func (r gitRepo) stat(rev string) (RevInfo, error) {
	if r.local {
	return r.statLocal(rev, rev)
	}

	// Fast path: maybe rev is a hash we already have locally.
	if len(rev) >= minHashDigits && len(rev) <= 40 && AllHex(rev) {
	if info, err := r.statLocal(rev, rev); err == nil {
	return info, nil
	}
	}

	// Maybe rev is a tag we already have locally.
	// (Note that we're excluding branches, which can be stale.)
	r.localTagsOnce.Do(r.loadLocalTags)
	if r.localTags[rev] {
	return r.statLocal(rev, "refs/tags/"+rev)
	}

	// Maybe rev is the name of a tag or branch on the remote server.
	// Or maybe it's the prefix of a hash of a named ref.
	// Try to resolve to both a ref (git name) and full (40-hex-digit) commit hash.
	r.refsOnce.Do(r.loadRefs)
	var ref, hash string
	if r.refs["refs/tags/"+rev] != "" {
	ref = "refs/tags/" + rev
	hash = r.refs[ref]
	// Keep rev as is: tags are assumed not to change meaning.
	} else if r.refs["refs/heads/"+rev] != "" {
	ref = "refs/heads/" + rev
	hash = r.refs[ref]
	rev = hash // Replace rev, because meaning of refs/heads/foo can change.
	} else if rev == "HEAD" && r.refs["HEAD"] != "" {
	ref = "HEAD"
	hash = r.refs[ref]
	rev = hash // Replace rev, because meaning of HEAD can change.
	} else if len(rev) >= minHashDigits && len(rev) <= 40 && AllHex(rev) {
	// At the least, we have a hash prefix we can look up after the fetch below.
	// Maybe we can map it to a full hash using the known refs.
	prefix := rev
	// Check whether rev is prefix of known ref hash.
	for k, h := range r.refs {
	if strings.HasPrefix(h, prefix) {
	if hash != "" && hash != h {
	// Hash is an ambiguous hash prefix.
	// More information will not change that.
	return nil, fmt.Errorf("ambiguous revision %s", rev)
	}
	if ref == "" \|\| ref > k { // Break ties deterministically when multiple refs point at same hash.
	ref = k
	}
	rev = h
	hash = h
	}
	}
	if hash == "" && len(rev) == 40 { // Didn't find a ref, but rev is a full hash.
	hash = rev
	}
	} else {
	return nil, fmt.Errorf("unknown revision %s", rev)
	}

	// Protect r.fetchLevel and the "fetch more and more" sequence.
	// TODO(rsc): Add LockDir and use it for protecting that
	// sequence, so that multiple processes don't collide in their
	// git commands.
	r.mu.Lock()
	defer r.mu.Unlock()

	// If we know a specific commit we need, fetch it.
	if r.fetchLevel <= fetchSome && hash != "" && !r.local {
	r.fetchLevel = fetchSome
	var refspec string
	if ref != "" && ref != "head" {
	// If we do know the ref name, save the mapping locally
	// so that (if it is a tag) it can show up in localTags
	// on a future call. Also, some servers refuse to allow
	// full hashes in ref specs, so prefer a ref name if known.
	refspec = ref + ":" + ref
	} else {
	// Fetch the hash but give it a local name (refs/dummy),
	// because that triggers the fetch behavior of creating any
	// other known remote tags for the hash. We never use
	// refs/dummy (it's not refs/tags/dummy) and it will be
	// overwritten in the next command, and that's fine.
	ref = hash
	refspec = hash + ":refs/dummy"
	}
	_, err := Run(r.dir, "git", "fetch", "-f", "--depth=1", r.remote, refspec)
	if err == nil {
	return r.statLocal(rev, ref)
	}
	if !strings.Contains(err.Error(), "unadvertised object") && !strings.Contains(err.Error(), "no such remote ref") && !strings.Contains(err.Error(), "does not support shallow") {
	return nil, err
	}
	}

	// Last resort.
	// Fetch all heads and tags and hope the hash we want is in the history.
	if r.fetchLevel < fetchAll {
	r.fetchLevel = fetchAll

	// To work around a protocol version 2 bug that breaks --unshallow,
	// add -c protocol.version=0.
	// TODO(rsc): The bug is believed to be server-side, meaning only
	// on Google's Git servers. Once the servers are fixed, drop the
	// protocol.version=0. See Google-internal bug b/110495752.
	var protoFlag []string
	unshallowFlag := unshallow(r.dir)
	if len(unshallowFlag) > 0 {
	protoFlag = []string{"-c", "protocol.version=0"}
	}
	if _, err := Run(r.dir, "git", protoFlag, "fetch", unshallowFlag, "-f", r.remote, "refs/heads/:refs/heads/", "refs/tags/:refs/tags/"); err != nil {
	return nil, err
	}
	}

	return r.statLocal(rev, rev)
	}

	// statLocal returns a RevInfo describing rev in the local git repository.
	// It uses version as info.Version.
	func (r gitRepo) statLocal(version, rev string) (RevInfo, error) {
	out, err := Run(r.dir, "git", "log", "-n1", "--format=format:%H %ct %D", rev)
	if err != nil {
	return nil, fmt.Errorf("unknown revision %s", rev)
	}
	f := strings.Fields(string(out))
	if len(f) < 2 {
	return nil, fmt.Errorf("unexpected response from git log: %q", out)
	}
	hash := f[0]
	if strings.HasPrefix(hash, version) {
	version = hash // extend to full hash
	}
	t, err := strconv.ParseInt(f[1], 10, 64)
	if err != nil {
	return nil, fmt.Errorf("invalid time from git log: %q", out)
	}

	info := &RevInfo{
	Name: hash,
	Short: ShortenSHA1(hash),
	Time: time.Unix(t, 0).UTC(),
	Version: hash,
	}

	// Add tags. Output looks like:
	// ede458df7cd0fdca520df19a33158086a8a68e81 1523994202 HEAD -> master, tag: v1.2.4-annotated, tag: v1.2.3, origin/master, origin/HEAD
	for i := 2; i < len(f); i++ {
	if f[i] == "tag:" {
	i++
	if i < len(f) {
	info.Tags = append(info.Tags, strings.TrimSuffix(f[i], ","))
	}
	}
	}
	sort.Strings(info.Tags)

	// Used hash as info.Version above.
	// Use caller's suggested version if it appears in the tag list
	// (filters out branch names, HEAD).
	for _, tag := range info.Tags {
	if version == tag {
	info.Version = version
	}
	}

	return info, nil
	}

	func (r gitRepo) Stat(rev string) (RevInfo, error) {
	if rev == "latest" {
	return r.Latest()
	}
	type cached struct {
	info *RevInfo
	err error
	}
	c := r.statCache.Do(rev, func() interface{} {
	info, err := r.stat(rev)
	return cached{info, err}
	}).(cached)
	return c.info, c.err
	}

	func (r *gitRepo) ReadFile(rev, file string, maxSize int64) ([]byte, error) {
	// TODO: Could use git cat-file --batch.
	info, err := r.Stat(rev) // download rev into local git repo
	if err != nil {
	return nil, err
	}
	out, err := Run(r.dir, "git", "cat-file", "blob", info.Name+":"+file)
	if err != nil {
	return nil, os.ErrNotExist
	}
	return out, nil
	}

	func (r *gitRepo) ReadZip(rev, subdir string, maxSize int64) (zip io.ReadCloser, actualSubdir string, err error) {
	// TODO: Use maxSize or drop it.
	args := []string{}
	if subdir != "" {
	args = append(args, "--", subdir)
	}
	info, err := r.Stat(rev) // download rev into local git repo
	if err != nil {
	return nil, "", err
	}

	// Incredibly, git produces different archives depending on whether
	// it is running on a Windows system or not, in an attempt to normalize
	// text file line endings. Setting -c core.autocrlf=input means only
	// translate files on the way into the repo, not on the way out (archive).
	// The -c core.eol=lf should be unnecessary but set it anyway.
	archive, err := Run(r.dir, "git", "-c", "core.autocrlf=input", "-c", "core.eol=lf", "archive", "--format=zip", "--prefix=prefix/", info.Name, args)
	if err != nil {
	if bytes.Contains(err.(*RunError).Stderr, []byte("did not match any files")) {
	return nil, "", os.ErrNotExist
	}
	return nil, "", err
	}

	return ioutil.NopCloser(bytes.NewReader(archive)), "", nil
	}