config: eval HEL as long as the result changes [GH-2909]

config/interpolate_walk.go

@@ -118,9 +118,15 @@ func (w *interpolationWalker) Primitive(v reflect.Value) error {
 		return err
 	}
 
-	// If the AST we got is just a literal string value, then we ignore it
-	if _, ok := astRoot.(*ast.LiteralNode); ok {
-		return nil
+	// If the AST we got is just a literal string value with the same
+	// value then we ignore it. We have to check if its the same value
+	// because it is possible to input a string, get out a string, and
+	// have it be different. For example: "foo-$${bar}" turns into
+	// "foo-${bar}"
+	if n, ok := astRoot.(*ast.LiteralNode); ok {
+		if s, ok := n.Value.(string); ok && s == v.String() {
+			return nil
+		}
 	}
 
 	if w.ContextF != nil {

config/lang/eval_test.go

@@ -24,6 +24,14 @@ func TestEval(t *testing.T) {
 			ast.TypeString,
 		},
 
+		{
+			"foo $${bar}",
+			nil,
+			false,
+			"foo ${bar}",
+			ast.TypeString,
+		},
+
 		{
 			"foo ${bar}",
 			&ast.BasicScope{

config/raw_config_test.go

@@ -114,6 +114,38 @@ func TestRawConfig_double(t *testing.T) {
 	}
 }
 
+func TestRawConfigInterpolate_escaped(t *testing.T) {
+	raw := map[string]interface{}{
+		"foo": "bar-$${baz}",
+	}
+
+	rc, err := NewRawConfig(raw)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	// Before interpolate, Config() should be the raw
+	if !reflect.DeepEqual(rc.Config(), raw) {
+		t.Fatalf("bad: %#v", rc.Config())
+	}
+
+	if err := rc.Interpolate(nil); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := rc.Config()
+	expected := map[string]interface{}{
+		"foo": "bar-${baz}",
+	}
+
+	if !reflect.DeepEqual(actual, expected) {
+		t.Fatalf("bad: %#v", actual)
+	}
+	if len(rc.UnknownKeys()) != 0 {
+		t.Fatalf("bad: %#v", rc.UnknownKeys())
+	}
+}
+
 func TestRawConfig_merge(t *testing.T) {
 	raw1 := map[string]interface{}{
 		"foo": "${var.foo}",