etcdserver: don't try to apply empty message list

If all messages have been applied, don't apply an empty messages list;
otherwise appliedi will update to 0 and etcd will panic.

Fixes #4278

etcdserver/server.go

@@ -667,6 +667,9 @@ func (s *EtcdServer) applyEntries(ep *etcdProgress, apply *apply) {
 	if ep.appliedi+1-firsti < uint64(len(apply.entries)) {
 		ents = apply.entries[ep.appliedi+1-firsti:]
 	}
+	if len(ents) == 0 {
+		return
+	}
 	var shouldstop bool
 	if ep.appliedi, shouldstop = s.apply(ents, &ep.confState); shouldstop {
 		go s.stopWithDelay(10*100*time.Millisecond, fmt.Errorf("the member has been permanently removed from the cluster"))

etcdserver/server_test.go

@@ -154,6 +154,56 @@ func TestDoBadLocalAction(t *testing.T) {
 	}
 }
 
+// TestApplyRepeat tests that server handles repeat raft messages gracefully
+func TestApplyRepeat(t *testing.T) {
+	n := newNodeConfChangeCommitterRecorder()
+	n.readyc <- raft.Ready{
+		SoftState: &raft.SoftState{RaftState: raft.StateLeader},
+	}
+	cl := newTestCluster(nil)
+	st := store.New()
+	cl.SetStore(store.New())
+	cl.AddMember(&Member{ID: 1234})
+	s := &EtcdServer{
+		r: raftNode{
+			Node:        n,
+			raftStorage: raft.NewMemoryStorage(),
+			storage:     &storageRecorder{},
+			transport:   rafthttp.NewNopTransporter(),
+		},
+		cfg:      &ServerConfig{},
+		store:    st,
+		cluster:  cl,
+		reqIDGen: idutil.NewGenerator(0, time.Time{}),
+	}
+	s.start()
+	req := &pb.Request{Method: "QGET", ID: uint64(1)}
+	ents := []raftpb.Entry{{Index: 1, Data: pbutil.MustMarshal(req)}}
+	n.readyc <- raft.Ready{CommittedEntries: ents}
+	// dup msg
+	n.readyc <- raft.Ready{CommittedEntries: ents}
+
+	// use a conf change to block until dup msgs are all processed
+	cc := &raftpb.ConfChange{Type: raftpb.ConfChangeRemoveNode, NodeID: 2}
+	ents = []raftpb.Entry{{
+		Index: 2,
+		Type:  raftpb.EntryConfChange,
+		Data:  pbutil.MustMarshal(cc),
+	}}
+	n.readyc <- raft.Ready{CommittedEntries: ents}
+	act, err := n.Wait(1)
+	s.Stop()
+
+	// only want to confirm etcdserver won't panic; no data to check
+
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(act) == 0 {
+		t.Fatalf("expected len(act)=0, got %d", len(act))
+	}
+}
+
 func TestApplyRequest(t *testing.T) {
 	tests := []struct {
 		req pb.Request