raft: leader response to learner MsgReadIndex

Leader should check message sender after receiving MsgReadIndex, even
when raft quorum is 1. The message could be sent from learner node, and
leader should respond.

raft/raft.go

@@ -1044,8 +1044,12 @@ func stepLeader(r *raft, m pb.Message) error {
 					r.send(pb.Message{To: m.From, Type: pb.MsgReadIndexResp, Index: ri, Entries: m.Entries})
 				}
 			}
-		} else {
-			r.readStates = append(r.readStates, ReadState{Index: r.raftLog.committed, RequestCtx: m.Entries[0].Data})
+		} else { // there is only one voting member (the leader) in the cluster
+			if m.From == None || m.From == r.id { // from leader itself
+				r.readStates = append(r.readStates, ReadState{Index: r.raftLog.committed, RequestCtx: m.Entries[0].Data})
+			} else { // from learner member
+				r.send(pb.Message{To: m.From, Type: pb.MsgReadIndexResp, Index: r.raftLog.committed, Entries: m.Entries})
+			}
 		}
 
 		return nil

raft/raft_test.go

@@ -2416,6 +2416,57 @@ func TestReadOnlyOptionSafe(t *testing.T) {
 	}
 }
 
+func TestReadOnlyWithLearner(t *testing.T) {
+	a := newTestLearnerRaft(1, []uint64{1}, []uint64{2}, 10, 1, NewMemoryStorage())
+	b := newTestLearnerRaft(2, []uint64{1}, []uint64{2}, 10, 1, NewMemoryStorage())
+
+	nt := newNetwork(a, b)
+	setRandomizedElectionTimeout(b, b.electionTimeout+1)
+
+	for i := 0; i < b.electionTimeout; i++ {
+		b.tick()
+	}
+	nt.send(pb.Message{From: 1, To: 1, Type: pb.MsgHup})
+
+	if a.state != StateLeader {
+		t.Fatalf("state = %s, want %s", a.state, StateLeader)
+	}
+
+	tests := []struct {
+		sm        *raft
+		proposals int
+		wri       uint64
+		wctx      []byte
+	}{
+		{a, 10, 11, []byte("ctx1")},
+		{b, 10, 21, []byte("ctx2")},
+		{a, 10, 31, []byte("ctx3")},
+		{b, 10, 41, []byte("ctx4")},
+	}
+
+	for i, tt := range tests {
+		for j := 0; j < tt.proposals; j++ {
+			nt.send(pb.Message{From: 1, To: 1, Type: pb.MsgProp, Entries: []pb.Entry{{}}})
+		}
+
+		nt.send(pb.Message{From: tt.sm.id, To: tt.sm.id, Type: pb.MsgReadIndex, Entries: []pb.Entry{{Data: tt.wctx}}})
+
+		r := tt.sm
+		if len(r.readStates) == 0 {
+			t.Fatalf("#%d: len(readStates) = 0, want non-zero", i)
+		}
+		rs := r.readStates[0]
+		if rs.Index != tt.wri {
+			t.Errorf("#%d: readIndex = %d, want %d", i, rs.Index, tt.wri)
+		}
+
+		if !bytes.Equal(rs.RequestCtx, tt.wctx) {
+			t.Errorf("#%d: requestCtx = %v, want %v", i, rs.RequestCtx, tt.wctx)
+		}
+		r.readStates = nil
+	}
+}
+
 func TestReadOnlyOptionLease(t *testing.T) {
 	a := newTestRaft(1, []uint64{1, 2, 3}, 10, 1, NewMemoryStorage())
 	b := newTestRaft(2, []uint64{1, 2, 3}, 10, 1, NewMemoryStorage())