maintner: track hashtag edits on GerritMeta, make HashtagEdits method faster

Updates golang/go#24836

Change-Id: I75cae7de574af7525964bdf420328d3e553a044c
Reviewed-on: https://go-review.googlesource.com/107305
Reviewed-by: Andrew Bonventre <[email protected]>

Author: bradfitz

maintner/gerrit.go

@@ -776,10 +776,7 @@ func (gp *GerritProject) processMutation(gm *maintpb.GerritMutation) {
 
 		if clv.Version == 0 { // is a meta commit
 			gp.noteDirtyCL(cl) // needs processing at end of sync
-			cl.Meta = &GerritMeta{
-				Commit: gc,
-				CL:     cl,
-			}
+			cl.Meta = newGerritMeta(gc, cl)
 		} else {
 			cl.Commit = gc
 			cl.Version = clv.Version
@@ -837,10 +834,7 @@ func (gp *GerritProject) finishProcessingCL(cl *GerritCL) {
 			cl.Private = true
 		}
 		if gc.GerritMeta == nil {
-			gc.GerritMeta = &GerritMeta{
-				Commit: gc,
-				CL:     cl,
-			}
+			gc.GerritMeta = newGerritMeta(gc, cl)
 		}
 		if foundStatus == "" {
 			foundStatus = getGerritStatus(gc)
@@ -1280,6 +1274,24 @@ type GerritMeta struct {
 
 	// CL is the Gerrit CL this metadata is for.
 	CL *GerritCL
+
+	flags gerritMetaFlags
+}
+
+type gerritMetaFlags uint8
+
+const (
+	// metaFlagHashtagEdit indicates that the meta commit edits the hashtags on the commit.
+	metaFlagHashtagEdit gerritMetaFlags = 1 << iota
+)
+
+func newGerritMeta(gc *GitCommit, cl *GerritCL) *GerritMeta {
+	m := &GerritMeta{Commit: gc, CL: cl}
+
+	if msg := m.Commit.Msg; strings.Contains(msg, "autogenerated:gerrit:setHashtag") && m.ActionTag() == "autogenerated:gerrit:setHashtag" {
+		m.flags |= metaFlagHashtagEdit
+	}
+	return m
 }
 
 // Footer returns the "key: value" lines at the base of the commit.
@@ -1307,16 +1319,13 @@ func (m *GerritMeta) ActionTag() string {
 // HashtagEdits returns the hashtags added and removed by this meta commit,
 // and whether this meta commit actually modified hashtags.
 func (m *GerritMeta) HashtagEdits() (added, removed GerritHashtags, ok bool) {
-	msg := m.Commit.Msg
-	// Fast path to return early on the majority of non-hashtag-mutating commits:
-	if !strings.Contains(msg, "autogenerated:gerrit:setHashtag") {
-		return
-	}
-	// Parse it more properly.
-	if m.ActionTag() != "autogenerated:gerrit:setHashtag" {
+	// Return early for the majority of meta commits that don't edit hashtags.
+	if m.flags&metaFlagHashtagEdit == 0 {
 		return
 	}
 
+	msg := m.Commit.Msg
+
 	// Parse lines of form:
 	//
 	// Hashtag removed: bar

maintner/gerrit_test.go

@@ -388,7 +388,7 @@ Tag: autogenerated:gerrit:otherTag
 
 func TestParseHashtags(t *testing.T) {
 	for i, tt := range hashtagTests {
-		meta := &GerritMeta{Commit: &GitCommit{Msg: tt.commit}}
+		meta := newGerritMeta(&GitCommit{Msg: tt.commit}, nil)
 		added, removed, ok := meta.HashtagEdits()
 		if ok != (added != "" || removed != "") {
 			t.Errorf("%d. inconsistent return values: %q, %q, %v", i, added, removed, ok)