terraform: don't increment state if one is nil

terraform/state.go

@@ -214,6 +214,15 @@ func (s *State) DeepCopy() *State {
 // IncrementSerialMaybe increments the serial number of this state
 // if it different from the other state.
 func (s *State) IncrementSerialMaybe(other *State) {
+	if s == nil {
+		return
+	}
+	if other == nil {
+		return
+	}
+	if s.Serial > other.Serial {
+		return
+	}
 	if !s.Equal(other) {
 		s.Serial++
 	}

terraform/state_test.go

@@ -178,6 +178,40 @@ func TestStateEqual(t *testing.T) {
 	}
 }
 
+func TestStateIncrementSerialMaybe(t *testing.T) {
+	cases := map[string]struct {
+		S1, S2 *State
+		Serial int64
+	}{
+		"S2 is nil": {
+			&State{},
+			nil,
+			0,
+		},
+		"S2 is identical": {
+			&State{},
+			&State{},
+			0,
+		},
+		"S2 is different": {
+			&State{},
+			&State{
+				Modules: []*ModuleState{
+					&ModuleState{Path: rootModulePath},
+				},
+			},
+			1,
+		},
+	}
+
+	for name, tc := range cases {
+		tc.S1.IncrementSerialMaybe(tc.S2)
+		if tc.S1.Serial != tc.Serial {
+			t.Fatalf("Bad: %s\nGot: %d", name, tc.S1.Serial)
+		}
+	}
+}
+
 func TestResourceStateEqual(t *testing.T) {
 	cases := []struct {
 		Result   bool