[FIXED] A peer-remove of an R1 could brick the stream. (#4420)

We should not remove a peer from a stream when we can not find a
replacement unless R>1.

Signed-off-by: Derek Collison <derek@nats.io>

Resolves #4396

server/jetstream_cluster.go

@@ -5184,7 +5184,12 @@ func (cc *jetStreamCluster) remapStreamAssignment(sa *streamAssignment, removePe
 		return true
 	}
 
-	// If we are here let's remove the peer at least.
+	// If R1 just return to avoid bricking the stream.
+	if sa.Group.node == nil || len(sa.Group.Peers) == 1 {
+		return false
+	}
+
+	// If we are here let's remove the peer at least, as long as we are R>1
 	for i, peer := range sa.Group.Peers {
 		if peer == removePeer {
 			sa.Group.Peers[i] = sa.Group.Peers[len(sa.Group.Peers)-1]

server/jetstream_super_cluster_test.go

@@ -4016,3 +4016,34 @@ func TestJetStreamSuperClusterMovingR1Stream(t *testing.T) {
 		return nil
 	})
 }
+
+// https://github.com/nats-io/nats-server/issues/4396
+func TestJetStreamSuperClusterR1StreamPeerRemove(t *testing.T) {
+	sc := createJetStreamSuperCluster(t, 1, 3)
+	defer sc.shutdown()
+
+	nc, js := jsClientConnect(t, sc.serverByName("C1-S1"))
+	defer nc.Close()
+
+	_, err := js.AddStream(&nats.StreamConfig{
+		Name:     "TEST",
+		Subjects: []string{"foo"},
+		Replicas: 1,
+	})
+	require_NoError(t, err)
+
+	si, err := js.StreamInfo("TEST")
+	require_NoError(t, err)
+
+	// Call peer remove on the only peer the leader.
+	resp, err := nc.Request(fmt.Sprintf(JSApiStreamRemovePeerT, "TEST"), []byte(`{"peer":"`+si.Cluster.Leader+`"}`), time.Second)
+	require_NoError(t, err)
+	var rpr JSApiStreamRemovePeerResponse
+	require_NoError(t, json.Unmarshal(resp.Data, &rpr))
+	require_False(t, rpr.Success)
+	require_True(t, rpr.Error.ErrCode == 10075)
+
+	// Stream should still be in place and useable.
+	_, err = js.StreamInfo("TEST")
+	require_NoError(t, err)
+}