raft: do not attach term to MsgReadIndex

Fix #6744.

MsgReadIndex, as MsgProp, is to be forwarded to leader.
So we should treat it as local message.

raft/node_test.go

@@ -190,6 +190,44 @@ func TestNodeReadIndex(t *testing.T) {
 	}
 }
 
+// TestNodeReadIndexToOldLeader ensures that raftpb.MsgReadIndex to old leader
+// gets forwarded to the new leader and 'send' method does not attach its term.
+func TestNodeReadIndexToOldLeader(t *testing.T) {
+	r1 := newTestRaft(1, []uint64{1, 2, 3}, 10, 1, NewMemoryStorage())
+	r2 := newTestRaft(2, []uint64{1, 2, 3}, 10, 1, NewMemoryStorage())
+	r3 := newTestRaft(3, []uint64{1, 2, 3}, 10, 1, NewMemoryStorage())
+	nt := newNetwork(r1, r2, r3)
+
+	// elect r1 as leader
+	nt.send(raftpb.Message{From: 1, To: 1, Type: raftpb.MsgHup})
+
+	// elect r3 as leader, r1 is now follower
+	nt.send(raftpb.Message{From: 3, To: 3, Type: raftpb.MsgHup})
+
+	// to old leader r1
+	r2.Step(raftpb.Message{
+		From:    2,
+		To:      1,
+		Type:    raftpb.MsgReadIndex,
+		Entries: []raftpb.Entry{{Data: []byte("testdata")}},
+	})
+
+	// term should not be attached
+	m := r2.msgs[0]
+	if m.Term != 0 {
+		t.Fatalf("m.Term expected 0, got %d", m.Term)
+	}
+
+	// old leader r1 forwards to new leader r3
+	m.To = 3
+	r1.Step(m)
+
+	m = r1.msgs[0]
+	if m.Term != 0 {
+		t.Fatalf("m.Term expected 0, got %d", m.Term)
+	}
+}
+
 // TestNodeProposeConfig ensures that node.ProposeConfChange sends the given configuration proposal
 // to the underlying raft.
 func TestNodeProposeConfig(t *testing.T) {

raft/raft.go

@@ -356,7 +356,10 @@ func (r *raft) send(m pb.Message) {
 		// do not attach term to MsgProp
 		// proposals are a way to forward to the leader and
 		// should be treated as local message.
-		if m.Type != pb.MsgProp {
+		//
+		// do not attach term to MsgReadIndex
+		// for the same reason
+		if m.Type != pb.MsgProp && m.Type != pb.MsgReadIndex {
 			m.Term = r.Term
 		}
 	}