raft: change raftLog.maybeAppend to return the last new index

As per @unihorn's comment on #1366, we change raftLog.maybeAppend to
return the last new index of entries in maybeAppend.

raft/log.go

@@ -53,8 +53,10 @@ func (l *raftLog) String() string {
 	return fmt.Sprintf("offset=%d committed=%d applied=%d len(ents)=%d", l.offset, l.committed, l.applied, len(l.ents))
 }
 
-func (l *raftLog) maybeAppend(index, logTerm, committed uint64, ents ...pb.Entry) bool {
-	lastnewi := index + uint64(len(ents))
+// maybeAppend returns (0, false) if the entries cannot be appended. Otherwise,
+// it returns (last index of entries, true).
+func (l *raftLog) maybeAppend(index, logTerm, committed uint64, ents ...pb.Entry) (lastnewi uint64, ok bool) {
+	lastnewi = index + uint64(len(ents))
 	if l.matchTerm(index, logTerm) {
 		from := index + 1
 		ci := l.findConflict(from, ents)
@@ -70,9 +72,9 @@ func (l *raftLog) maybeAppend(index, logTerm, committed uint64, ents ...pb.Entry
 		if l.committed < tocommit {
 			l.committed = tocommit
 		}
-		return true
+		return lastnewi, true
 	}
-	return false
+	return 0, false
 }
 
 func (l *raftLog) append(after uint64, ents ...pb.Entry) uint64 {

raft/raft.go

@@ -379,11 +379,7 @@ func (r *raft) Step(m pb.Message) error {
 }
 
 func (r *raft) handleAppendEntries(m pb.Message) {
-	if r.raftLog.maybeAppend(m.Index, m.LogTerm, m.Commit, m.Entries...) {
-		mlastIndex := m.Index
-		if len(m.Entries) != 0 {
-			mlastIndex = m.Entries[len(m.Entries)-1].Index
-		}
+	if mlastIndex, ok := r.raftLog.maybeAppend(m.Index, m.LogTerm, m.Commit, m.Entries...); ok {
 		r.send(pb.Message{To: m.From, Type: pb.MsgAppResp, Index: mlastIndex})
 	} else {
 		r.send(pb.Message{To: m.From, Type: pb.MsgAppResp, Index: m.Index, Reject: true})