terraform: Diff.Prune

terraform/diff.go

@@ -32,6 +32,30 @@ type Diff struct {
 	Modules []*ModuleDiff
 }
 
+// Prune cleans out unused structures in the diff without affecting
+// the behavior of the diff at all.
+//
+// This is not safe to call concurrently. This is safe to call on a
+// nil Diff.
+func (d *Diff) Prune() {
+	if d == nil {
+		return
+	}
+
+	// Prune all empty modules
+	newModules := make([]*ModuleDiff, 0, len(d.Modules))
+	for _, m := range d.Modules {
+		// If the module isn't empty, we keep it
+		if !m.Empty() {
+			newModules = append(newModules, m)
+		}
+	}
+	if len(newModules) == 0 {
+		newModules = nil
+	}
+	d.Modules = newModules
+}
+
 // AddModule adds the module with the given path to the diff.
 //
 // This should be the preferred method to add module diffs since it
@@ -212,6 +236,10 @@ func (d *ModuleDiff) ChangeType() DiffChangeType {
 
 // Empty returns true if the diff has no changes within this module.
 func (d *ModuleDiff) Empty() bool {
+	if d.Destroy {
+		return false
+	}
+
 	if len(d.Resources) == 0 {
 		return true
 	}

terraform/diff_test.go

@@ -103,6 +103,53 @@ func TestDiffEqual(t *testing.T) {
 	}
 }
 
+func TestDiffPrune(t *testing.T) {
+	cases := map[string]struct {
+		D1, D2 *Diff
+	}{
+		"nil": {
+			nil,
+			nil,
+		},
+
+		"empty": {
+			new(Diff),
+			new(Diff),
+		},
+
+		"empty module": {
+			&Diff{
+				Modules: []*ModuleDiff{
+					&ModuleDiff{Path: []string{"root", "foo"}},
+				},
+			},
+			&Diff{},
+		},
+
+		"destroy module": {
+			&Diff{
+				Modules: []*ModuleDiff{
+					&ModuleDiff{Path: []string{"root", "foo"}, Destroy: true},
+				},
+			},
+			&Diff{
+				Modules: []*ModuleDiff{
+					&ModuleDiff{Path: []string{"root", "foo"}, Destroy: true},
+				},
+			},
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			tc.D1.Prune()
+			if !tc.D1.Equal(tc.D2) {
+				t.Fatalf("bad:\n\n%#v\n\n%#v", tc.D1, tc.D2)
+			}
+		})
+	}
+}
+
 func TestModuleDiff_ChangeType(t *testing.T) {
 	cases := []struct {
 		Diff   *ModuleDiff