raft: don't assume MsgSnap comes from leader

We can't consider MsgSnap to be from the leader of Message.Term until we
address cockroachdb#127348 and cockroachdb#127349.

Epic: None
Release note: None

pkg/raft/raft.go

@@ -2075,6 +2075,11 @@ func stepCandidate(r *raft, m pb.Message) error {
 	case pb.MsgProp:
 		r.logger.Infof("%x no leader at term %d; dropping proposal", r.id, r.Term)
 		return ErrProposalDropped
+	case pb.MsgSnap:
+		// TODO(nvanbenschoten): we can't consider MsgSnap to be from the leader of
+		// Message.Term until we address #127348 and #127349.
+		r.becomeFollower(m.Term, None)
+		r.handleSnapshot(m)
 	case myVoteRespType:
 		gr, rj, res := r.poll(m.From, m.Type, !m.Reject)
 		r.logger.Infof("%x has received %d %s votes and %d vote rejections", r.id, gr, m.Type, rj)

pkg/raft/raft_test.go

@@ -2615,7 +2615,7 @@ func TestRestoreFromSnapMsg(t *testing.T) {
 	sm := newTestRaft(2, 10, 1, newTestMemoryStorage(withPeers(1, 2)))
 	sm.Step(m)
 
-	assert.Equal(t, pb.PeerID(1), sm.lead)
+	assert.Equal(t, None, sm.lead)
 	// TODO(bdarnell): what should this test?
 }
 

pkg/raft/util.go

@@ -50,8 +50,10 @@ var isResponseMsg = [...]bool{
 }
 
 var isMsgFromLeader = [...]bool{
-	pb.MsgApp:             true,
-	pb.MsgSnap:            true,
+	pb.MsgApp: true,
+	// TODO(nvanbenschoten): we can't consider MsgSnap to be from the leader of
+	// Message.Term until we address #127348 and #127349.
+	// pb.MsgSnap:            true,
 	pb.MsgHeartbeat:       true,
 	pb.MsgTimeoutNow:      true,
 	pb.MsgFortifyLeader:   true,

pkg/raft/util_test.go

@@ -165,7 +165,7 @@ func TestMsgFromLeader(t *testing.T) {
 		{pb.MsgAppResp, false},
 		{pb.MsgVote, false},
 		{pb.MsgVoteResp, false},
-		{pb.MsgSnap, true},
+		{pb.MsgSnap, false},
 		{pb.MsgHeartbeat, true},
 		{pb.MsgHeartbeatResp, false},
 		{pb.MsgTimeoutNow, true},