raft: remove panic when we see a proposal with no leader.

This panic can never be reached when using raft.Node, because we only
read from propc when there is a leader. However, it is possible to see
this error when using raft the raft object directly (as in MultiNode),
and in this case it is better to simply drop the proposal (as if we had
sent it to a leader that immediately vanished).

Add an error return to MemoryStorage.Append for consistency.

raft/raft.go

@@ -479,7 +479,8 @@ func stepLeader(r *raft, m pb.Message) {
 func stepCandidate(r *raft, m pb.Message) {
 	switch m.Type {
 	case pb.MsgProp:
-		panic("no leader")
+		log.Printf("raft: no leader; dropping proposal")
+		return
 	case pb.MsgApp:
 		r.becomeFollower(r.Term, m.From)
 		r.handleAppendEntries(m)
@@ -510,7 +511,8 @@ func stepFollower(r *raft, m pb.Message) {
 	switch m.Type {
 	case pb.MsgProp:
 		if r.lead == None {
-			panic("no leader")
+			log.Printf("raft: no leader; dropping proposal")
+			return
 		}
 		m.To = r.lead
 		r.send(m)

raft/storage.go

@@ -180,18 +180,18 @@ func (ms *MemoryStorage) Compact(i uint64, cs *pb.ConfState, data []byte) error
 }
 
 // Append the new entries to storage.
-func (ms *MemoryStorage) Append(entries []pb.Entry) {
+func (ms *MemoryStorage) Append(entries []pb.Entry) error {
 	ms.Lock()
 	defer ms.Unlock()
 	if len(entries) == 0 {
-		return
+		return nil
 	}
 	first := ms.snapshot.Metadata.Index + 1
 	last := entries[0].Index + uint64(len(entries)) - 1
 
 	// shortcut if there is no new entry.
 	if last < first {
-		return
+		return nil
 	}
 	// truncate old entries
 	if first > entries[0].Index {
@@ -209,4 +209,5 @@ func (ms *MemoryStorage) Append(entries []pb.Entry) {
 		log.Panicf("missing log entry [last: %d, append at: %d]",
 			ms.snapshot.Metadata.Index+uint64(len(ms.ents)), entries[0].Index)
 	}
+	return nil
 }