state/remote: increment serial properly

state/remote/state.go

@@ -13,7 +13,7 @@ import (
 type State struct {
 	Client Client
 
-	state *terraform.State
+	state, readState *terraform.State
 }
 
 // StateReader impl.
@@ -43,11 +43,14 @@ func (s *State) RefreshState() error {
 	}
 
 	s.state = state
+	s.readState = state
 	return nil
 }
 
 // StatePersister impl.
 func (s *State) PersistState() error {
+	s.state.IncrementSerialMaybe(s.readState)
+
 	var buf bytes.Buffer
 	if err := terraform.WriteState(s.state, &buf); err != nil {
 		return err

state/remote/state_test.go

@@ -7,8 +7,11 @@ import (
 )
 
 func TestState(t *testing.T) {
-	s := &State{Client: new(InmemClient)}
-	s.WriteState(state.TestStateInitial())
+	s := &State{
+		Client:    new(InmemClient),
+		state:     state.TestStateInitial(),
+		readState: state.TestStateInitial(),
+	}
 	if err := s.PersistState(); err != nil {
 		t.Fatalf("err: %s", err)
 	}