return the actually committed index if it's greater than the official…

… committed index for ReadIndex

Signed-off-by: Benjamin Wang <[email protected]>

raft.go

@@ -1303,7 +1303,10 @@ func stepLeader(r *raft, m pb.Message) error {
 	case pb.MsgReadIndex:
 		// only one voting member (the leader) in the cluster
 		if r.prs.IsSingleton() {
-			if resp := r.responseToReadIndexReq(m, r.raftLog.committed); resp.To != None {
+			// It's possible that the "officially committed index" < "actually committed index",
+			// raft may return an out of date readindex in such case.
+			readIndex := max(r.raftLog.committed, r.prs.Committed())
+			if resp := r.responseToReadIndexReq(m, readIndex); resp.To != None {
 				r.send(resp)
 			}
 			return nil