terraform: support MergeDiff for nil ResourceState

terraform/state.go

@@ -34,10 +34,16 @@ type ResourceState struct {
 func (s *ResourceState) MergeDiff(
 	d map[string]*ResourceAttrDiff,
 	computedID string) ResourceState {
-	result := *s
+	var result ResourceState
+	if s != nil {
+		result = *s
+	}
+
 	result.Attributes = make(map[string]string)
-	for k, v := range s.Attributes {
-		result.Attributes[k] = v
+	if s != nil {
+		for k, v := range s.Attributes {
+			result.Attributes[k] = v
+		}
 	}
 	for k, diff := range d {
 		if diff.NewComputed {

terraform/state_test.go

@@ -41,3 +41,24 @@ func TestResourceState_MergeDiff(t *testing.T) {
 		t.Fatalf("bad: %#v", rs2.Attributes)
 	}
 }
+
+func TestResourceState_MergeDiff_nil(t *testing.T) {
+	var rs *ResourceState = nil
+
+	diff := map[string]*ResourceAttrDiff{
+		"foo": &ResourceAttrDiff{
+			Old: "",
+			New: "baz",
+		},
+	}
+
+	rs2 := rs.MergeDiff(diff, "computed")
+
+	expected := map[string]string{
+		"foo": "baz",
+	}
+
+	if !reflect.DeepEqual(expected, rs2.Attributes) {
+		t.Fatalf("bad: %#v", rs2.Attributes)
+	}
+}