helper/diff: ComputedAttrsUpdate

helper/diff/resource_builder.go

@@ -45,6 +45,10 @@ type ResourceBuilder struct {
 	// resource creation time.
 	ComputedAttrs []string
 
+	// ComputedAttrsUpdate are the attributes that are computed
+	// at resource update time (this includes creation).
+	ComputedAttrsUpdate []string
+
 	// PreProcess is a mapping of exact keys that are sent through
 	// a pre-processor before comparing values. The original value will
 	// be put in the "NewExtra" field of the diff.
@@ -185,6 +189,23 @@ func (b *ResourceBuilder) Diff(
 		}
 	}
 
+	// If we're changing anything, then mark the updated
+	// attributes.
+	if len(attrs) > 0 {
+		for _, k := range b.ComputedAttrsUpdate {
+			if _, ok := attrs[k]; ok {
+				continue
+			}
+
+			old := s.Attributes[k]
+			attrs[k] = &terraform.ResourceAttrDiff{
+				Old:         old,
+				NewComputed: true,
+				Type:        terraform.DiffAttrOutput,
+			}
+		}
+	}
+
 	// Build our resulting diff if we had attributes change
 	var result *terraform.ResourceDiff
 	if len(attrs) > 0 {

helper/diff/resource_builder_test.go

@@ -170,6 +170,38 @@ func TestResourceBuilder_complexReplace(t *testing.T) {
 	}
 }
 
+func TestResourceBuilder_computedAttrsUpdate(t *testing.T) {
+	rb := &ResourceBuilder{
+		Attrs: map[string]AttrType{
+			"foo": AttrTypeUpdate,
+		},
+		ComputedAttrsUpdate: []string{
+			"bar",
+		},
+	}
+
+	state := &terraform.ResourceState{
+		Attributes: map[string]string{"foo": "foo"},
+	}
+	c := testConfig(t, map[string]interface{}{
+		"foo": "bar",
+	}, nil)
+
+	diff, err := rb.Diff(state, c)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if diff == nil {
+		t.Fatal("diff shold not be nil")
+	}
+
+	actual := testResourceDiffStr(diff)
+	expected := testRBComputedAttrUpdate
+	if actual != expected {
+		t.Fatalf("bad: %s", actual)
+	}
+}
+
 func TestResourceBuilder_new(t *testing.T) {
 	rb := &ResourceBuilder{
 		Attrs: map[string]AttrType{
@@ -407,6 +439,11 @@ const testRBComplexReplaceDiff = `UPDATE
   IN  listener.0.value: "" => "50"
 `
 
+const testRBComputedAttrUpdate = `UPDATE
+  OUT bar: "" => "<computed>"
+  IN  foo: "foo" => "bar"
+`
+
 const testRBNewDiff = `UPDATE
   IN  foo:        "" => "bar"
   OUT private_ip: "" => "<computed>"