dashboard/builder: don't use vcs TagSync to update a path to a hash

Change-Id: Ic4bbcb294995483482f51e3539b9ba8a741d1a98
Reviewed-on: https://go-review.googlesource.com/1245
Reviewed-by: Andrew Gerrand <adg@golang.org>

dashboard/builder/vcs.go

@@ -95,6 +95,18 @@ func (r *Repo) UpdateTo(hash string) error {
 	r.Lock()
 	defer r.Unlock()
 
+	if r.Master.VCS.Cmd == "git" {
+		cmd := exec.Command("git", "reset", "--hard", hash)
+		var log bytes.Buffer
+		err := run(cmd, runTimeout(*cmdTimeout), runDir(r.Path), allOutput(&log))
+		if err != nil {
+			return fmt.Errorf("Error running git update -C %v: %v ; output=%s", hash, err, log.Bytes())
+		}
+		return nil
+	}
+
+	// Else go down three more levels of abstractions, at
+	// least two of which are broken for git.
 	return timeout(*cmdTimeout, func() error {
 		return r.Master.VCS.TagSync(r.Path, hash)
 	})
@@ -159,23 +171,38 @@ func (r *Repo) Log() ([]HgLog, error) {
 	return logStruct.Log, nil
 }
 
-// FullHash returns the full hash for the given Mercurial revision.
+// FullHash returns the full hash for the given Git or Mercurial revision.
 func (r *Repo) FullHash(rev string) (string, error) {
 	r.Lock()
 	defer r.Unlock()
 
 	var hash string
 	err := timeout(*cmdTimeout, func() error {
-		data, err := r.Master.VCS.LogAtRev(r.Path, rev, "{node}")
+		var data []byte
-		if err != nil {
+		// Avoid the vcs package for git, since it's broken
-			return err
+		// for git, and and we're trying to remove levels of
+		// abstraction which are increasingly getting
+		// difficult to navigate.
+		if r.Master.VCS.Cmd == "git" {
+			cmd := exec.Command("git", "rev-parse", rev)
+			var out bytes.Buffer
+			err := run(cmd, runTimeout(*cmdTimeout), runDir(r.Path), allOutput(&out))
+			data = out.Bytes()
+			if err != nil {
+				return fmt.Errorf("Failed to find FullHash of %q; git rev-parse: %v, %s", rev, err, data)
+			}
+		} else {
+			var err error
+			data, err = r.Master.VCS.LogAtRev(r.Path, rev, "{node}")
+			if err != nil {
+				return err
+			}
 		}
-
 		s := strings.TrimSpace(string(data))
 		if s == "" {
 			return fmt.Errorf("cannot find revision")
 		}
-		if len(s) != 40 {
+		if len(s) != 40 { // correct for both hg and git
 			return fmt.Errorf("%s returned invalid hash: %s", r.Master.VCS, s)
 		}
 		hash = s