gitmirror.go

// Copyright 2014 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.

// The gitmirror binary watches the specified Gerrit repositories for
// new commits and reports them to the build dashboard.
//
// It also serves tarballs over HTTP for the build system, and pushes
// new commits to Github.
package main

import (
	"bufio"
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"flag"
	"fmt"
	"io/ioutil"
	"log"
	"net"
	"net/http"
	"net/url"
	"os"
	"os/exec"
	"path"
	"path/filepath"
	"runtime"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"cloud.google.com/go/compute/metadata"
	"golang.org/x/build/gerrit"
)

const (
	goBase         = "https://go.googlesource.com/"
	watcherVersion = 3        // must match dashboard/app/build/handler.go's watcherVersion
	master         = "master" // name of the master branch
)

var (
	httpAddr = flag.String("http", "", "If non-empty, the listen address to run an HTTP server on")
	cacheDir = flag.String("cachedir", "", "git cache directory. If empty a temp directory is made.")

	dashFlag = flag.String("dash", "https://build.golang.org/", "Dashboard URL (must end in /)")
	keyFile  = flag.String("key", defaultKeyFile, "Build dashboard key file. If empty, automatic from GCE project metadata")

	pollInterval = flag.Duration("poll", 10*time.Second, "Remote repo poll interval")

	// TODO(bradfitz): these three are all kinda the same and
	// redundant. Unify after research.
	network = flag.Bool("network", true, "Enable network calls (disable for testing)")
	mirror  = flag.Bool("mirror", false, "whether to mirror to github")
	report  = flag.Bool("report", true, "Report updates to build dashboard (use false for development dry-run mode)")

	filter   = flag.String("filter", "", "If non-empty, a comma-separated list of directories or files to watch for new commits (only works on main repo). If empty, watch all files in repo.")
	branches = flag.String("branches", "", "If non-empty, a comma-separated list of branches to watch. If empty, watch changes on every branch.")
)

var (
	defaultKeyFile = filepath.Join(homeDir(), ".gobuildkey")
	dashboardKey   = ""
	networkSeen    = make(map[string]bool) // testing mode only (-network=false); known hashes
)

var httpClient = &http.Client{
	Timeout: 30 * time.Second, // overkill
}

var gerritClient = gerrit.NewClient(goBase, gerrit.NoAuth)

func main() {
	flag.Parse()
	log.Printf("gitmirror running.")

	go pollGerritAndTickle()
	err := runGitMirror()
	log.Fatalf("gitmirror exiting after failure: %v", err)
}

// runGitMirror is a little wrapper so we can use defer and return to signal
// errors. It should only return a non-nil error.
func runGitMirror() error {
	if !strings.HasSuffix(*dashFlag, "/") {
		return errors.New("dashboard URL (-dashboard) must end in /")
	}

	if *mirror {
		sshDir := filepath.Join(homeDir(), ".ssh")
		sshKey := filepath.Join(sshDir, "id_ed25519")
		if _, err := os.Stat(sshKey); err == nil {
			log.Printf("Using github ssh key at %v", sshKey)
		} else {
			if privKey, err := metadata.ProjectAttributeValue("github-ssh"); err == nil && len(privKey) > 0 {
				if err := os.MkdirAll(sshDir, 0700); err != nil {
					return err
				}
				if err := ioutil.WriteFile(sshKey, []byte(privKey+"\n"), 0600); err != nil {
					return err
				}
				log.Printf("Wrote %s from GCE metadata.", sshKey)
			} else {
				return fmt.Errorf("Can't mirror to github without 'github-ssh' GCE metadata or file %v", sshKey)
			}
		}
	}

	if *cacheDir == "" {
		dir, err := ioutil.TempDir("", "gitmirror")
		if err != nil {
			log.Fatal(err)
		}
		defer os.RemoveAll(dir)
		*cacheDir = dir
	} else {
		fi, err := os.Stat(*cacheDir)
		if os.IsNotExist(err) {
			if err := os.MkdirAll(*cacheDir, 0755); err != nil {
				return fmt.Errorf("failed to create watcher's git cache dir: %v", err)
			}
		} else {
			if err != nil {
				return fmt.Errorf("invalid -cachedir: %v", err)
			}
			if !fi.IsDir() {
				return fmt.Errorf("invalid -cachedir=%q; not a directory", *cacheDir)
			}
		}
	}

	if *report {
		if k, err := readKey(); err != nil {
			return err
		} else {
			dashboardKey = k
		}
	}

	if *httpAddr != "" {
		http.HandleFunc("/debug/env", handleDebugEnv)
		http.HandleFunc("/debug/goroutines", handleDebugGoroutines)
		ln, err := net.Listen("tcp", *httpAddr)
		if err != nil {
			return err
		}
		go http.Serve(ln, nil)
	}

	errc := make(chan error)

	subrepos, err := subrepoList()
	if err != nil {
		return err
	}

	startRepo := func(name, path string, dash bool) {
		log.Printf("Starting watch of repo %s", name)
		url := goBase + name
		var dst string
		if *mirror {
			if shouldMirror(name) {
				log.Printf("Starting mirror of subrepo %s", name)
				dst = "git@github.com:golang/" + name + ".git"
			} else {
				log.Printf("Not mirroring repo %s", name)
			}
		}
		r, err := NewRepo(url, dst, path, dash)
		if err != nil {
			errc <- err
			return
		}
		http.Handle("/"+name+".tar.gz", r)
		reposMu.Lock()
		repos = append(repos, r)
		sort.Slice(repos, func(i, j int) bool { return repos[i].name() < repos[j].name() })
		reposMu.Unlock()
		r.Loop()
	}

	go startRepo("go", "", true)

	seen := map[string]bool{"go": true}
	for _, path := range subrepos {
		name := strings.TrimPrefix(path, "golang.org/x/")
		seen[name] = true
		go startRepo(name, path, true)
	}
	if *mirror {
		for name := range gerritMetaMap() {
			if seen[name] {
				// Repo already picked up by dashboard list.
				continue
			}
			go startRepo(name, "golang.org/x/"+name, false)
		}
	}

	http.HandleFunc("/", handleRoot)

	// Blocks forever if all the NewRepo calls succeed:
	return <-errc
}

var (
	reposMu sync.Mutex
	repos   []*Repo
)

// GET /
func handleRoot(w http.ResponseWriter, r *http.Request) {
	if r.URL.Path != "/" {
		http.NotFound(w, r)
		return
	}
	reposMu.Lock()
	defer reposMu.Unlock()
	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	fmt.Fprintf(w, "<html><body><pre>\a")
	for _, r := range repos {
		fmt.Fprintf(w, "<a href='/debug/watcher/%s'>%s</a> - %s\n", r.name(), r.name(), r.statusLine())
	}
}

// shouldMirror reports whether the named repo should be mirrored from
// Gerrit to Github.
func shouldMirror(name string) bool {
	switch name {
	case
		"arch",
		"benchmarks",
		"blog",
		"build",
		"crypto",
		"debug",
		"example",
		"exp",
		"gddo",
		"go",
		"gofrontend",
		"image",
		"mobile",
		"net",
		"oauth2",
		"playground",
		"proposal",
		"review",
		"sync",
		"sys",
		"talks",
		"term",
		"text",
		"time",
		"tools",
		"tour":
		return true
	}
	// Else, see if it appears to be a subrepo:
	r, err := httpClient.Get("https://golang.org/x/" + name)
	if err != nil {
		log.Printf("repo %v doesn't seem to exist: %v", name, err)
		return false
	}
	r.Body.Close()
	return r.StatusCode/100 == 2
}

// a statusEntry is a status string at a specific time.
type statusEntry struct {
	status string
	t      time.Time
}

// statusRing is a ring buffer of timestamped status messages.
type statusRing struct {
	mu   sync.Mutex      // guards rest
	head int             // next position to fill
	ent  [50]statusEntry // ring buffer of entries; zero time means unpopulated
}

func (r *statusRing) add(status string) {
	r.mu.Lock()
	defer r.mu.Unlock()

	r.ent[r.head] = statusEntry{status, time.Now()}
	r.head++
	if r.head == len(r.ent) {
		r.head = 0
	}
}

func (r *statusRing) foreachDesc(fn func(statusEntry)) {
	r.mu.Lock()
	defer r.mu.Unlock()

	i := r.head
	for {
		i--
		if i < 0 {
			i = len(r.ent) - 1
		}
		if i == r.head || r.ent[i].t.IsZero() {
			return
		}
		fn(r.ent[i])
	}
}

// Repo represents a repository to be watched.
type Repo struct {
	root     string             // on-disk location of the git repo, *cacheDir/name
	path     string             // base import path for repo (blank for main repo)
	commits  map[string]*Commit // keyed by full commit hash (40 lowercase hex digits)
	branches map[string]*Branch // keyed by branch name, eg "release-branch.go1.3" (or empty for default)
	dash     bool               // push new commits to the dashboard
	mirror   bool               // push new commits to 'dest' remote
	status   statusRing

	mu        sync.Mutex
	err       error
	firstBad  time.Time
	lastBad   time.Time
	firstGood time.Time
	lastGood  time.Time
}

// NewRepo checks out a new instance of the git repository
// specified by srcURL.
//
// If dstURL is not empty, changes from the source repository will
// be mirrored to the specified destination repository.
// The importPath argument is the base import path of the repository,
// and should be empty for the main Go repo.
// The dash argument should be set true if commits to this
// repo should be reported to the build dashboard.
func NewRepo(srcURL, dstURL, importPath string, dash bool) (*Repo, error) {
	name := path.Base(srcURL) // "go", "net", etc
	root := filepath.Join(*cacheDir, name)
	r := &Repo{
		path:     importPath,
		root:     root,
		commits:  make(map[string]*Commit),
		branches: make(map[string]*Branch),
		mirror:   dstURL != "",
		dash:     dash,
	}

	http.Handle("/debug/watcher/"+r.name(), r)

	needClone := true
	if r.shouldTryReuseGitDir(dstURL) {
		r.setStatus("reusing git dir; running git fetch")
		cmd := exec.Command("git", "fetch", "origin")
		cmd.Dir = r.root
		r.logf("running git fetch")
		t0 := time.Now()
		var stderr bytes.Buffer
		cmd.Stderr = &stderr
		err := cmd.Run()
		if err != nil {
			r.logf("git fetch failed; proceeding to wipe + clone instead; err: %v, stderr: %s", err, stderr.Bytes())
		} else {
			needClone = false
			r.logf("ran git fetch in %v", time.Since(t0))
		}
	}
	if needClone {
		r.setStatus("need clone; removing cache root")
		os.RemoveAll(r.root)
		t0 := time.Now()
		r.setStatus("running fresh git clone --mirror")
		r.logf("cloning %v into %s", srcURL, r.root)
		cmd := exec.Command("git", "clone", "--mirror", srcURL, r.root)
		if out, err := cmd.CombinedOutput(); err != nil {
			return nil, fmt.Errorf("cloning %s: %v\n\n%s", srcURL, err, out)
		}
		r.setStatus("cloned")
		r.logf("cloned in %v", time.Since(t0))
	}

	if r.mirror {
		r.setStatus("adding dest remote")
		if err := r.addRemote("dest", dstURL); err != nil {
			r.setStatus("failed to add dest")
			return nil, fmt.Errorf("adding remote: %v", err)
		}
		r.setStatus("added dest remote")
	}

	if r.dash {
		r.logf("loading commit log")
		if err := r.update(false); err != nil {
			return nil, err
		}
		r.logf("found %v branches among %v commits\n", len(r.branches), len(r.commits))
	}

	return r, nil
}

func (r *Repo) setErr(err error) {
	r.mu.Lock()
	defer r.mu.Unlock()
	change := (r.err != nil) != (err != nil)
	now := time.Now()
	if err != nil {
		if change {
			r.firstBad = now
		}
		r.lastBad = now
	} else {
		if change {
			r.firstGood = now
		}
		r.lastGood = now
	}
	r.err = err
}

var startTime = time.Now()

func (r *Repo) statusLine() string {
	r.mu.Lock()
	defer r.mu.Unlock()

	if r.lastGood.IsZero() {
		if r.err != nil {
			return fmt.Sprintf("broken; permanently? always failing, for %v", time.Since(r.firstBad))
		}
		if time.Since(startTime) < 5*time.Minute {
			return "ok; starting up, no report yet"
		}
		return fmt.Sprintf("hung; hang at start-up? no report since start %v ago", time.Since(startTime))
	}
	if r.err == nil {
		if sinceGood := time.Since(r.lastGood); sinceGood > 6*time.Minute {
			return fmt.Sprintf("hung? no activity since last success %v ago", sinceGood)
		}
		if r.lastBad.After(time.Now().Add(-1 * time.Hour)) {
			return fmt.Sprintf("ok; recent failure %v ago", time.Since(r.lastBad))
		}
		return "ok"
	}
	return fmt.Sprintf("broken for %v", time.Since(r.lastGood))
}

func (r *Repo) setStatus(status string) {
	r.status.add(status)
}

// shouldTryReuseGitDir reports whether we should try to reuse r.root as the git
// directory. (The directory may be corrupt, though.)
// dstURL is optional, and is the desired remote URL for a remote named "dest".
func (r *Repo) shouldTryReuseGitDir(dstURL string) bool {
	if _, err := os.Stat(filepath.Join(r.root, "FETCH_HEAD")); err != nil {
		if os.IsNotExist(err) {
			r.logf("not reusing git dir; no FETCH_HEAD at %s", r.root)
		} else {
			r.logf("not reusing git dir; %v", err)
		}
		return false
	}
	if dstURL == "" {
		r.logf("not reusing git dir because dstURL is empty")
		return true
	}

	// Does the "dest" remote match? If not, we return false and nuke
	// the world and re-clone out of laziness.
	cmd := exec.Command("git", "remote", "-v")
	cmd.Dir = r.root
	out, err := cmd.Output()
	if err != nil {
		log.Printf("git remote -v: %v", err)
	}
	foundWrong := false
	for _, ln := range strings.Split(string(out), "\n") {
		if !strings.HasPrefix(ln, "dest") {
			continue
		}
		f := strings.Fields(ln)
		if len(f) < 2 {
			continue
		}
		if f[0] == "dest" {
			if f[1] == dstURL {
				return true
			}
			if !foundWrong {
				foundWrong = true
				r.logf("found dest of %q, which doesn't equal sought %q", f[1], dstURL)
			}
		}
	}
	r.logf("not reusing old repo: remote \"dest\" URL doesn't match")
	return false
}

func (r *Repo) addRemote(name, url string) error {
	gitConfig := filepath.Join(r.root, "config")
	f, err := os.OpenFile(gitConfig, os.O_WRONLY|os.O_APPEND, os.ModePerm)
	if err != nil {
		return err
	}
	_, err = fmt.Fprintf(f, "\n[remote %q]\n\turl = %v\n", name, url)
	if err != nil {
		f.Close()
		return err
	}
	return f.Close()
}

// Loop continuously runs "git fetch" in the repo, checks for
// new commits, posts any new commits to the dashboard (if enabled),
// and mirrors commits to a destination repo (if enabled).
func (r *Repo) Loop() {
	tickler := repoTickler(r.name())
	for {
		if err := r.fetch(); err != nil {
			r.setErr(err)
			time.Sleep(10 * time.Second)
			continue
		}
		if r.mirror {
			if err := r.push(); err != nil {
				r.setErr(err)
				time.Sleep(10 * time.Second)
				continue
			}
		}
		if r.dash {
			if err := r.updateDashboard(); err != nil {
				r.setErr(err)
				time.Sleep(10 * time.Second)
				continue
			}
		}

		r.setErr(nil)
		r.setStatus("waiting")
		// We still run a timer but a very slow one, just
		// in case the mechanism updating the repo tickler
		// breaks for some reason.
		timer := time.NewTimer(5 * time.Minute)
		select {
		case <-tickler:
			r.setStatus("got update tickle")
			timer.Stop()
		case <-timer.C:
			r.setStatus("poll timer fired")
		}
	}
}

func (r *Repo) updateDashboard() (err error) {
	r.setStatus("updating dashboard")
	defer func() {
		if err == nil {
			r.setStatus("updated dashboard")
		}
	}()
	if err := r.update(true); err != nil {
		return err
	}
	remotes, err := r.remotes()
	if err != nil {
		return err
	}
	for _, name := range remotes {
		b, ok := r.branches[name]
		if !ok {
			// skip branch; must be already merged
			continue
		}
		if err := r.postNewCommits(b); err != nil {
			return err
		}
	}
	return nil
}

func (r *Repo) name() string {
	if r.path == "" {
		return "go"
	}
	return path.Base(r.path)
}

func (r *Repo) logf(format string, args ...interface{}) {
	log.Printf(r.name()+": "+format, args...)
}

// postNewCommits looks for unseen commits on the specified branch and
// posts them to the dashboard.
func (r *Repo) postNewCommits(b *Branch) error {
	if b.Head == b.LastSeen {
		return nil
	}
	c := b.LastSeen
	if c == nil {
		// Haven't seen anything on this branch yet:
		if b.Name == master {
			// For the master branch, bootstrap by creating a dummy
			// commit with a lone child that is the initial commit.
			c = &Commit{}
			for _, c2 := range r.commits {
				if c2.Parent == "" {
					c.children = []*Commit{c2}
					break
				}
			}
			if c.children == nil {
				return fmt.Errorf("couldn't find initial commit")
			}
		} else {
			// Find the commit that this branch forked from.
			base, err := r.mergeBase("heads/"+b.Name, master)
			if err != nil {
				return err
			}
			var ok bool
			c, ok = r.commits[base]
			if !ok {
				return fmt.Errorf("couldn't find base commit: %v", base)
			}
		}
	}
	if err := r.postChildren(b, c); err != nil {
		return err
	}
	b.LastSeen = b.Head
	return nil
}

// postChildren posts to the dashboard all descendants of the given parent.
// It ignores descendants that are not on the given branch.
func (r *Repo) postChildren(b *Branch, parent *Commit) error {
	for _, c := range parent.children {
		if c.Branch != b.Name {
			continue
		}
		if err := r.postCommit(c); err != nil {
			if strings.Contains(err.Error(), "this package already has a first commit; aborting") {
				return nil
			}
			return err
		}
	}
	for _, c := range parent.children {
		if err := r.postChildren(b, c); err != nil {
			return err
		}
	}
	return nil
}

// postCommit sends a commit to the build dashboard.
func (r *Repo) postCommit(c *Commit) error {
	if !*report {
		r.logf("dry-run mode; NOT posting commit to dashboard: %v", c)
		return nil
	}
	r.logf("sending commit to dashboard: %v", c)

	t, err := time.Parse("Mon, 2 Jan 2006 15:04:05 -0700", c.Date)
	if err != nil {
		return fmt.Errorf("postCommit: parsing date %q for commit %v: %v", c.Date, c, err)
	}
	dc := struct {
		PackagePath string // (empty for main repo commits)
		Hash        string
		ParentHash  string

		User   string
		Desc   string
		Time   time.Time
		Branch string

		NeedsBenchmarking bool
	}{
		PackagePath: r.path,
		Hash:        c.Hash,
		ParentHash:  c.Parent,

		User:   c.Author,
		Desc:   c.Desc,
		Time:   t,
		Branch: c.Branch,

		NeedsBenchmarking: c.NeedsBenchmarking(),
	}
	b, err := json.Marshal(dc)
	if err != nil {
		return fmt.Errorf("postCommit: marshaling request body: %v", err)
	}

	if !*network {
		if c.Parent != "" {
			if !networkSeen[c.Parent] {
				r.logf("%v: %v", c.Parent, r.commits[c.Parent])
				return fmt.Errorf("postCommit: no parent %v found on dashboard for %v", c.Parent, c)
			}
		}
		if networkSeen[c.Hash] {
			return fmt.Errorf("postCommit: already seen %v", c)
		}
		networkSeen[c.Hash] = true
		return nil
	}

	v := url.Values{"version": {fmt.Sprint(watcherVersion)}, "key": {dashboardKey}}
	u := *dashFlag + "commit?" + v.Encode()
	resp, err := http.Post(u, "text/json", bytes.NewReader(b))
	if err != nil {
		return err
	}
	body, err := ioutil.ReadAll(resp.Body)
	resp.Body.Close()
	if err != nil {
		return fmt.Errorf("postCommit: reading body: %v", err)
	}
	if resp.StatusCode != 200 {
		return fmt.Errorf("postCommit: status: %v\nbody: %s", resp.Status, body)
	}

	var s struct {
		Error string
	}
	if err := json.Unmarshal(body, &s); err != nil {
		return fmt.Errorf("postCommit: decoding response: %v", err)
	}
	if s.Error != "" {
		return fmt.Errorf("postCommit: error: %v", s.Error)
	}
	return nil
}

// update looks for new commits and branches,
// and updates the commits and branches maps.
func (r *Repo) update(noisy bool) error {
	remotes, err := r.remotes()
	if err != nil {
		return err
	}
	for _, name := range remotes {
		b := r.branches[name]

		// Find all unseen commits on this branch.
		revspec := "heads/" + name
		if b != nil {
			// If we know about this branch,
			// only log commits down to the known head.
			revspec = b.Head.Hash + ".." + revspec
		}
		log, err := r.log("--topo-order", revspec)
		if err != nil {
			return err
		}
		if len(log) == 0 {
			// No commits to handle; carry on.
			continue
		}

		var nDups, nDrops int

		// Add unknown commits to r.commits.
		var added []*Commit
		for _, c := range log {
			if noisy {
				r.logf("found new commit %v", c)
			}
			// If we've already seen this commit,
			// only store the master one in r.commits.
			if _, ok := r.commits[c.Hash]; ok {
				nDups++
				if name != master {
					nDrops++
					continue
				}
			}
			c.Branch = name
			r.commits[c.Hash] = c
			added = append(added, c)
		}

		if nDups > 0 {
			r.logf("saw %v duplicate commits; dropped %v of them", nDups, nDrops)
		}

		// Link added commits.
		for _, c := range added {
			if c.Parent == "" {
				// This is the initial commit; no parent.
				r.logf("no parents for initial commit %v", c)
				continue
			}
			// Find parent commit.
			p, ok := r.commits[c.Parent]
			if !ok {
				return fmt.Errorf("can't find parent %q for %v", c.Parent, c)
			}
			// Link parent Commit.
			c.parent = p
			// Link child Commits.
			p.children = append(p.children, c)
		}

		// Update branch head, or add newly discovered branch.
		head := log[0]
		if b != nil {
			// Known branch; update head.
			b.Head = head
			r.logf("updated branch head: %v", b)
		} else {
			// It's a new branch; add it.
			seen, err := r.lastSeen(head.Hash)
			if err != nil {
				return err
			}
			b = &Branch{Name: name, Head: head, LastSeen: seen}
			r.branches[name] = b
			r.logf("found branch: %v", b)
		}
	}

	return nil
}

// lastSeen finds the most recent commit the dashboard has seen,
// starting at the specified head. If the dashboard hasn't seen
// any of the commits from head to the beginning, it returns nil.
func (r *Repo) lastSeen(head string) (*Commit, error) {
	h, ok := r.commits[head]
	if !ok {
		return nil, fmt.Errorf("lastSeen: can't find %q in commits", head)
	}

	var s []*Commit
	for c := h; c != nil; c = c.parent {
		s = append(s, c)
	}

	var err error
	i := sort.Search(len(s), func(i int) bool {
		if err != nil {
			return false
		}
		ok, err = r.dashSeen(s[i].Hash)
		return ok
	})
	switch {
	case err != nil:
		return nil, fmt.Errorf("lastSeen: %v", err)
	case i < len(s):
		return s[i], nil
	default:
		// Dashboard saw no commits.
		return nil, nil
	}
}

// dashSeen reports whether the build dashboard knows the specified commit.
func (r *Repo) dashSeen(hash string) (bool, error) {
	if !*network {
		return networkSeen[hash], nil
	}
	v := url.Values{"hash": {hash}, "packagePath": {r.path}}
	u := *dashFlag + "commit?" + v.Encode()
	resp, err := httpClient.Get(u)
	if err != nil {
		return false, err
	}
	defer resp.Body.Close()
	if resp.StatusCode != 200 {
		return false, fmt.Errorf("status: %v", resp.Status)
	}
	var s struct {
		Error string
	}
	err = json.NewDecoder(resp.Body).Decode(&s)
	if err != nil {
		return false, err
	}
	switch s.Error {
	case "":
		// Found one.
		return true, nil
	case "Commit not found":
		// Commit not found, keep looking for earlier commits.
		return false, nil
	default:
		return false, fmt.Errorf("dashboard: %v", s.Error)
	}
}

// mergeBase returns the hash of the merge base for revspecs a and b.
func (r *Repo) mergeBase(a, b string) (string, error) {
	cmd := exec.Command("git", "merge-base", a, b)
	cmd.Dir = r.root
	out, err := cmd.CombinedOutput()
	if err != nil {
		return "", fmt.Errorf("git merge-base %s..%s: %v", a, b, err)
	}
	return string(bytes.TrimSpace(out)), nil
}

// remotes returns a slice of remote branches known to the git repo.
// It always puts "origin/master" first.
func (r *Repo) remotes() ([]string, error) {
	if *branches != "" {
		return strings.Split(*branches, ","), nil
	}

	cmd := exec.Command("git", "branch")
	cmd.Dir = r.root
	out, err := cmd.CombinedOutput()
	if err != nil {
		return nil, fmt.Errorf("git branch: %v", err)
	}
	bs := []string{master}
	for _, b := range strings.Split(string(out), "\n") {
		b = strings.TrimPrefix(b, "* ")
		b = strings.TrimSpace(b)
		// Ignore aliases, blank lines, and master (it's already in bs).
		if b == "" || strings.Contains(b, "->") || b == master {
			continue
		}
		// Ignore pre-go1 release branches; they are just noise.
		if strings.HasPrefix(b, "release-branch.r") {
			continue
		}
		bs = append(bs, b)
	}
	return bs, nil
}

const logFormat = `--format=format:` + logBoundary + `%H
%P
%an <%ae>
%cD
%B
` + fileBoundary

const logBoundary = `_-_- magic boundary -_-_`
const fileBoundary = `_-_- file boundary -_-_`

// log runs "git log" with the supplied arguments
// and parses the output into Commit values.
func (r *Repo) log(dir string, args ...string) ([]*Commit, error) {
	args = append([]string{"log", "--date=rfc", "--name-only", "--parents", logFormat}, args...)
	if r.path == "" && *filter != "" {
		paths := strings.Split(*filter, ",")
		args = append(args, "--")
		args = append(args, paths...)
	}
	cmd := exec.Command("git", args...)
	cmd.Dir = r.root
	out, err := cmd.CombinedOutput()
	if err != nil {
		return nil, fmt.Errorf("git %v: %v\n%s", strings.Join(args, " "), err, out)
	}

	// We have a commit with description that contains 0x1b byte.
	// Mercurial does not escape it, but xml.Unmarshal does not accept it.
	// TODO(adg): do we still need to scrub this? Probably.
	out = bytes.Replace(out, []byte{0x1b}, []byte{'?'}, -1)

	var cs []*Commit
	for _, text := range strings.Split(string(out), logBoundary) {
		text = strings.TrimSpace(text)
		if text == "" {
			continue
		}
		p := strings.SplitN(text, "\n", 5)
		if len(p) != 5 {
			return nil, fmt.Errorf("git log %v: malformed commit: %q", strings.Join(args, " "), text)
		}

		// The change summary contains the change description and files
		// modified in this commit.  There is no way to directly refer
		// to the modified files in the log formatting string, so we look
		// for the file boundary after the description.
		changeSummary := p[4]
		descAndFiles := strings.SplitN(changeSummary, fileBoundary, 2)
		desc := strings.TrimSpace(descAndFiles[0])

		// For branch merges, the list of files can still be empty
		// because there are no changed files.
		files := strings.Replace(strings.TrimSpace(descAndFiles[1]), "\n", " ", -1)

		cs = append(cs, &Commit{
			Hash: p[0],
			// TODO(adg): This may break with branch merges.
			Parent: strings.Split(p[1], " ")[0],
			Author: p[2],
			Date:   p[3],
			Desc:   desc,
			Files:  files,
		})
	}
	return cs, nil
}

// fetch runs "git fetch" in the repository root.
// It tries three times, just in case it failed because of a transient error.
func (r *Repo) fetch() (err error) {
	n := 0
	r.setStatus("running git fetch origin")
	defer func() {
		if err != nil {
			r.setStatus("git fetch failed")
		} else {
			r.setStatus("ran git fetch")
		}
	}()
	return try(3, func() error {
		n++
		if n > 1 {
			r.setStatus(fmt.Sprintf("running git fetch origin, attempt %d", n))
		}
		cmd := exec.Command("git", "fetch", "origin")
		cmd.Dir = r.root
		if out, err := cmd.CombinedOutput(); err != nil {
			err = fmt.Errorf("%v\n\n%s", err, out)
			r.logf("git fetch: %v", err)
			return err
		}
		return nil
	})
}

// push runs "git push -f --mirror dest" in the repository root.
// It tries three times, just in case it failed because of a transient error.
func (r *Repo) push() (err error) {
	n := 0
	r.setStatus("syncing to github")
	defer func() {
		if err != nil {
			r.setStatus("sync to github failed")
		} else {
			r.setStatus("did sync to github")
		}
	}()
	return try(3, func() error {
		n++
		if n > 1 {
			r.setStatus(fmt.Sprintf("syncing to github, attempt %d", n))
		}
		r.setStatus("sync: fetching local refs")
		local, err := r.getLocalRefs()
		if err != nil {
			r.logf("failed to get local refs: %v", err)
			return err
		}
		r.setStatus(fmt.Sprintf("sync: got %d local refs", len(local)))

		r.setStatus("sync: fetching remote refs")
		remote, err := r.getRemoteRefs("dest")
		if err != nil {
			r.logf("failed to get remote refs: %v", err)
			return err
		}
		r.setStatus(fmt.Sprintf("sync: got %d remote refs", len(remote)))

		var pushRefs []string
		for ref, hash := range local {
			if strings.Contains(ref, "refs/users/") ||
				strings.Contains(ref, "refs/cache-auto") ||
				strings.Contains(ref, "refs/changes/") {
				continue
			}
			if remote[ref] != hash {
				pushRefs = append(pushRefs, ref)
			}
		}
		sort.Slice(pushRefs, func(i, j int) bool {
			p1 := priority[refType(pushRefs[i])]
			p2 := priority[refType(pushRefs[j])]
			if p1 != p2 {
				return p1 > p2
			}
			return pushRefs[i] <= pushRefs[j]
		})
		if len(pushRefs) == 0 {
			r.setStatus("nothing to sync")
			return nil
		}
		for len(pushRefs) > 0 {
			r.setStatus(fmt.Sprintf("%d refs to push; pushing batch", len(pushRefs)))
			r.logf("%d refs remain to sync to github", len(pushRefs))
			args := []string{"push", "-f", "dest"}
			n := 0
			for _, ref := range pushRefs {
				args = append(args, "+"+local[ref]+":"+ref)
				n++
				if n == 200 {
					break
				}
			}
			pushRefs = pushRefs[n:]
			cmd := exec.Command("git", args...)
			cmd.Dir = r.root
			cmd.Stderr = os.Stderr
			out, err := cmd.Output()
			if err != nil {
				r.logf("git push failed, running git %s: %s", args, out)
				r.setStatus("git push failure")
				return err
			}
		}
		r.setStatus("sync complete")
		return nil
	})
}

// hasRev returns true if the repo contains the commit-ish rev.
func (r *Repo) hasRev(ctx context.Context, rev string) bool {
	cmd := exec.CommandContext(ctx, "git", "cat-file", "-t", rev)
	cmd.Dir = r.root
	return cmd.Run() == nil
}

// if non-nil, used by r.archive to create a "git archive" command.
var testHookArchiveCmd func(context.Context, string, ...string) *exec.Cmd

// if non-nil, used by r.archive to create a "git fetch" command.
var testHookFetchCmd func(context.Context, string, ...string) *exec.Cmd

// archive exports the git repository at the given rev and returns the
// compressed repository.
func (r *Repo) archive(ctx context.Context, rev string) ([]byte, error) {
	var cmd *exec.Cmd
	if testHookArchiveCmd == nil {
		cmd = exec.CommandContext(ctx, "git", "archive", "--format=tgz", rev)
	} else {
		cmd = testHookArchiveCmd(ctx, "git", "archive", "--format=tgz", rev)
	}
	cmd.Dir = r.root
	return cmd.Output()
}

// fetchRev attempts to fetch rev from remote.
func (r *Repo) fetchRev(ctx context.Context, remote, rev string) error {
	var cmd *exec.Cmd
	if testHookFetchCmd == nil {
		cmd = exec.CommandContext(ctx, "git", "fetch", remote, rev)
	} else {
		cmd = testHookFetchCmd(ctx, "git", "fetch", remote, rev)
	}
	cmd.Dir = r.root
	return cmd.Run()
}

func (r *Repo) fetchRevIfNeeded(ctx context.Context, rev string) error {
	if r.hasRev(ctx, rev) {
		return nil
	}
	r.logf("attempting to fetch missing revision %s from origin", rev)
	return r.fetchRev(ctx, "origin", rev)
}

// GET /<name>.tar.gz
// GET /debug/watcher/<name>
func (r *Repo) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	if req.Method != "GET" && req.Method != "HEAD" {
		w.WriteHeader(http.StatusBadRequest)
		return
	}
	if strings.HasPrefix(req.URL.Path, "/debug/watcher/") {
		r.serveStatus(w, req)
		return
	}
	rev := req.FormValue("rev")
	if rev == "" {
		w.WriteHeader(http.StatusBadRequest)
		return
	}
	ctx, cancel := context.WithTimeout(req.Context(), 30*time.Second)
	defer cancel()
	if err := r.fetchRevIfNeeded(ctx, rev); err != nil {
		// Try the archive anyway, it might work
		r.logf("error fetching revision %s: %v", rev, err)
	}
	tgz, err := r.archive(ctx, rev)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	w.Header().Set("Content-Length", strconv.Itoa(len(tgz)))
	w.Header().Set("Content-Type", "application/x-compressed")
	w.Write(tgz)
}

func (r *Repo) serveStatus(w http.ResponseWriter, req *http.Request) {
	w.Header().Set("Content-Type", "text/html")
	fmt.Fprintf(w, "<html><head><title>watcher: %s</title><body><h1>watcher status for repo: %q</h1>\n",
		r.name(), r.name())
	fmt.Fprintf(w, "<pre>\n")
	nowRound := time.Now().Round(time.Second)
	r.status.foreachDesc(func(ent statusEntry) {
		fmt.Fprintf(w, "%v   %-20s %v\n",
			ent.t.In(time.UTC).Format(time.RFC3339),
			nowRound.Sub(ent.t.Round(time.Second)).String()+" ago",
			ent.status)
	})
	fmt.Fprintf(w, "\n</pre></body></html>")
}

func try(n int, fn func() error) error {
	var err error
	for tries := 0; tries < n; tries++ {
		time.Sleep(time.Duration(tries) * 5 * time.Second) // Linear back-off.
		if err = fn(); err == nil {
			break
		}
	}
	return err
}

// Branch represents a Mercurial branch.
type Branch struct {
	Name     string
	Head     *Commit
	LastSeen *Commit // the last commit posted to the dashboard
}

func (b *Branch) String() string {
	return fmt.Sprintf("%q(Head: %v LastSeen: %v)", b.Name, b.Head, b.LastSeen)
}

// Commit represents a single Git commit.
type Commit struct {
	Hash   string
	Author string
	Date   string // Format: "Mon, 2 Jan 2006 15:04:05 -0700"
	Desc   string // Plain text, first line is a short description.
	Parent string
	Branch string
	Files  string

	// For walking the graph.
	parent   *Commit
	children []*Commit
}

func (c *Commit) String() string {
	s := c.Hash
	if c.Branch != "" {
		s += fmt.Sprintf("[%v]", c.Branch)
	}
	s += fmt.Sprintf("(%q)", strings.SplitN(c.Desc, "\n", 2)[0])
	return s
}

// NeedsBenchmarking reports whether the Commit needs benchmarking.
func (c *Commit) NeedsBenchmarking() bool {
	// Do not benchmark branch commits, they are usually not interesting
	// and fall out of the trunk succession.
	if c.Branch != master {
		return false
	}
	// Do not benchmark commits that do not touch source files (e.g. CONTRIBUTORS).
	for _, f := range strings.Split(c.Files, " ") {
		if (strings.HasPrefix(f, "include") || strings.HasPrefix(f, "src")) &&
			!strings.HasSuffix(f, "_test.go") && !strings.Contains(f, "testdata") {
			return true
		}
	}
	return false
}

func homeDir() string {
	switch runtime.GOOS {
	case "plan9":
		return os.Getenv("home")
	case "windows":
		return os.Getenv("HOMEDRIVE") + os.Getenv("HOMEPATH")
	}
	return os.Getenv("HOME")
}

func readKey() (string, error) {
	c, err := ioutil.ReadFile(*keyFile)
	if os.IsNotExist(err) && metadata.OnGCE() {
		key, err := metadata.ProjectAttributeValue("builder-master-key")
		if err != nil {
			return "", fmt.Errorf("-key=%s doesn't exist, and key can't be loaded from GCE metadata: %v", *keyFile, err)
		}
		return strings.TrimSpace(key), nil
	}
	if err != nil {
		return "", err
	}
	return string(bytes.TrimSpace(bytes.SplitN(c, []byte("\n"), 2)[0])), nil
}

// subrepoList fetches a list of sub-repositories from the dashboard
// and returns them as a slice of base import paths.
// Eg, []string{"golang.org/x/tools", "golang.org/x/net"}.
func subrepoList() ([]string, error) {
	if !*network {
		return nil, nil
	}

	r, err := httpClient.Get(*dashFlag + "packages?kind=subrepo")
	if err != nil {
		return nil, fmt.Errorf("subrepo list: %v", err)
	}
	defer r.Body.Close()
	if r.StatusCode != 200 {
		return nil, fmt.Errorf("subrepo list: got status %v", r.Status)
	}
	var resp struct {
		Response []struct {
			Path string
		}
		Error string
	}
	err = json.NewDecoder(r.Body).Decode(&resp)
	if err != nil {
		return nil, fmt.Errorf("subrepo list: %v", err)
	}
	if resp.Error != "" {
		return nil, fmt.Errorf("subrepo list: %v", resp.Error)
	}
	var pkgs []string
	for _, r := range resp.Response {
		pkgs = append(pkgs, r.Path)
	}
	return pkgs, nil
}

var (
	ticklerMu sync.Mutex
	ticklers  = make(map[string]chan bool)
)

// repo is the gerrit repo: e.g. "go", "net", "crypto", ...
func repoTickler(repo string) chan bool {
	ticklerMu.Lock()
	defer ticklerMu.Unlock()
	if c, ok := ticklers[repo]; ok {
		return c
	}
	c := make(chan bool, 1)
	ticklers[repo] = c
	return c
}

// pollGerritAndTickle polls Gerrit's JSON meta URL of all its URLs
// and their current branch heads.  When this sees that one has
// changed, it tickles the channel for that repo and wakes up its
// poller, if its poller is in a sleep.
func pollGerritAndTickle() {
	last := map[string]string{} // repo -> last seen hash
	for {
		for repo, hash := range gerritMetaMap() {
			if hash != last[repo] {
				last[repo] = hash
				select {
				case repoTickler(repo) <- true:
				default:
				}
			}
		}
		time.Sleep(*pollInterval)
	}
}

// gerritMetaMap returns the map from repo name (e.g. "go") to its
// latest master hash.
// The returned map is nil on any transient error.
func gerritMetaMap() map[string]string {
	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
	defer cancel()
	meta, err := gerritClient.GetProjects(ctx, "master")
	if err != nil {
		return nil
	}
	m := map[string]string{}
	for repo, v := range meta {
		if master, ok := v.Branches["master"]; ok {
			m[repo] = master
		}
	}
	return m
}

func (r *Repo) getLocalRefs() (map[string]string, error) {
	cmd := exec.Command("git", "show-ref")
	cmd.Dir = r.root
	return parseRefs(cmd)
}

// getRemoteRefs tells you which references are available in the remote
// named dest, and returns a map of refs to the matching commit, e.g.
// "refs/heads/master" => "6b27048ae5e6ad1ef927e72e437531493de612fe".
func (r *Repo) getRemoteRefs(dest string) (map[string]string, error) {
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	cmd := exec.CommandContext(ctx, "git", "ls-remote", dest)
	cmd.Dir = r.root
	return parseRefs(cmd)
}

func parseRefs(cmd *exec.Cmd) (map[string]string, error) {
	refHash := map[string]string{}
	var errBuf bytes.Buffer
	if cmd.Stderr == nil {
		cmd.Stderr = &errBuf
	}
	out, err := cmd.StdoutPipe()
	if err != nil {
		return nil, err
	}
	bs := bufio.NewScanner(out)
	if err := cmd.Start(); err != nil {
		return nil, err
	}
	for bs.Scan() {
		f := strings.Fields(bs.Text())
		refHash[f[1]] = f[0]
	}
	if err := bs.Err(); err != nil {
		go cmd.Wait() // prevent zombies
		return nil, err
	}
	if err := cmd.Wait(); err != nil {
		return nil, fmt.Errorf("wait err: %v, stderr: %s", err, errBuf.Bytes())
	}
	return refHash, nil
}

func refType(s string) string {
	s = strings.TrimPrefix(s, "refs/")
	if i := strings.IndexByte(s, '/'); i != -1 {
		return s[:i]
	}
	return s
}

var priority = map[string]int{
	"heads":   5,
	"tags":    4,
	"changes": 3,
}

// GET /debug/goroutines
func handleDebugGoroutines(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	buf := make([]byte, 1<<20)
	w.Write(buf[:runtime.Stack(buf, true)])
}

// GET /debug/env
func handleDebugEnv(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	for _, kv := range os.Environ() {
		fmt.Fprintf(w, "%s\n", kv)
	}
}