receive older committed index should not be error

log.go

@@ -237,7 +237,7 @@ func (l *Log) GetEntriesAfter(index uint64) ([]*LogEntry, uint64) {
 
 	// Return an error if the index doesn't exist.
 	if index > (uint64(len(l.entries)) + l.startIndex) {
-		panic(fmt.Sprintf("raft: Index is beyond end of log: % v%v", len(l.entries), index))
+		panic(fmt.Sprintf("raft: Index is beyond end of log: %v %v", len(l.entries), index))
 	}
 
 	// If we're going from the beginning of the log then return the whole log.
@@ -320,14 +320,30 @@ func (l *Log) SetCommitIndex(index uint64) error {
 	l.mutex.Lock()
 	defer l.mutex.Unlock()
 
-	// Do not allow previous indices to be committed again.
-	if index < l.commitIndex {
-		return fmt.Errorf("raft.Log: Commit index (%d) ahead of requested commit index (%d)", l.commitIndex, index)
-	}
 	if index > l.startIndex+uint64(len(l.entries)) {
 		return fmt.Errorf("raft.Log: Commit index (%d) out of range (%d)", index, len(l.entries))
 	}
 
+	// Do not allow previous indices to be committed again.
+
+	// This could happens, since the guarantee is that the new leader has up-to-dated 
+	// log entires rather than has most up-to-dated committed index
+
+	// For example, Leader 1 send log 80 to follower 2 and follower 3
+	// follower 2 and follow 3 all got the new entries and reply
+	// leader 1 committed entry 80 and send reply to follower 2 and follower3
+	// follower 2 receive the new committed index and update committed index to 80
+	// leader 1 fail to send the committed index to follower 3
+	// follower 3 promote to leader (server 1 and server 2 will vote, since leader 3
+	// has up-to-dated the entries) 
+	// when new leader 3 send heartbeat with committed index = 0 to follower 2,
+	// follower 2 should reply success and let leader 3 update the committed index to 80
+
+
+	if index < l.commitIndex {
+		return nil
+	}
+
 	// Find all entries whose index is between the previous index and the current index.
 	for i := l.commitIndex + 1; i <= index; i++ {
 		entryIndex := i - 1 - l.startIndex
@@ -335,6 +351,7 @@ func (l *Log) SetCommitIndex(index uint64) error {
 
 		// Write to storage.
 		if err := entry.Encode(l.file); err != nil {
+			panic(err)
 			return err
 		}