config: Parsing of data.TYPE.NAME.FIELD variables

This allows ${data.TYPE.NAME.FIELD} interpolation syntax at the
configuration level, though since there is no special handling of them
in the core package this currently just acts as an alias for
${TYPE.NAME.FIELD}.

config/config.go

@@ -568,7 +568,7 @@ func (c *Config) Validate() error {
 				continue
 			}
 
-			id := fmt.Sprintf("%s.%s", rv.Type, rv.Name)
+			id := rv.ResourceId()
 			if _, ok := resources[id]; !ok {
 				errs = append(errs, fmt.Errorf(
 					"%s: unknown resource '%s' referenced in variable %s",

config/interpolate.go

@@ -58,6 +58,7 @@ const (
 // A ResourceVariable is a variable that is referencing the field
 // of a resource, such as "${aws_instance.foo.ami}"
 type ResourceVariable struct {
+	Mode  ResourceMode
 	Type  string // Resource type, i.e. "aws_instance"
 	Name  string // Resource name
 	Field string // Resource field
@@ -171,11 +172,28 @@ func (v *PathVariable) FullKey() string {
 }
 
 func NewResourceVariable(key string) (*ResourceVariable, error) {
-	parts := strings.SplitN(key, ".", 3)
-	if len(parts) < 3 {
-		return nil, fmt.Errorf(
-			"%s: resource variables must be three parts: type.name.attr",
-			key)
+	var mode ResourceMode
+	var parts []string
+	if strings.HasPrefix(key, "data.") {
+		mode = DataResourceMode
+		parts = strings.SplitN(key, ".", 4)
+		if len(parts) < 4 {
+			return nil, fmt.Errorf(
+				"%s: data variables must be four parts: data.TYPE.NAME.ATTR",
+				key)
+		}
+
+		// Don't actually need the "data." prefix for parsing, since it's
+		// always constant.
+		parts = parts[1:]
+	} else {
+		mode = ManagedResourceMode
+		parts = strings.SplitN(key, ".", 3)
+		if len(parts) < 3 {
+			return nil, fmt.Errorf(
+				"%s: resource variables must be three parts: TYPE.NAME.ATTR",
+				key)
+		}
 	}
 
 	field := parts[2]
@@ -201,6 +219,7 @@ func NewResourceVariable(key string) (*ResourceVariable, error) {
 	}
 
 	return &ResourceVariable{
+		Mode:  mode,
 		Type:  parts[0],
 		Name:  parts[1],
 		Field: field,
@@ -211,7 +230,14 @@ func NewResourceVariable(key string) (*ResourceVariable, error) {
 }
 
 func (v *ResourceVariable) ResourceId() string {
-	return fmt.Sprintf("%s.%s", v.Type, v.Name)
+	switch v.Mode {
+	case ManagedResourceMode:
+		return fmt.Sprintf("%s.%s", v.Type, v.Name)
+	case DataResourceMode:
+		return fmt.Sprintf("data.%s.%s", v.Type, v.Name)
+	default:
+		panic(fmt.Errorf("unknown resource mode %s", v.Mode))
+	}
 }
 
 func (v *ResourceVariable) FullKey() string {

config/interpolate_test.go

@@ -81,6 +81,9 @@ func TestNewResourceVariable(t *testing.T) {
 		t.Fatalf("err: %s", err)
 	}
 
+	if v.Mode != ManagedResourceMode {
+		t.Fatalf("bad: %#v", v)
+	}
 	if v.Type != "foo" {
 		t.Fatalf("bad: %#v", v)
 	}
@@ -99,6 +102,33 @@ func TestNewResourceVariable(t *testing.T) {
 	}
 }
 
+func TestNewResourceVariableData(t *testing.T) {
+	v, err := NewResourceVariable("data.foo.bar.baz")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if v.Mode != DataResourceMode {
+		t.Fatalf("bad: %#v", v)
+	}
+	if v.Type != "foo" {
+		t.Fatalf("bad: %#v", v)
+	}
+	if v.Name != "bar" {
+		t.Fatalf("bad: %#v", v)
+	}
+	if v.Field != "baz" {
+		t.Fatalf("bad: %#v", v)
+	}
+	if v.Multi {
+		t.Fatal("should not be multi")
+	}
+
+	if v.FullKey() != "data.foo.bar.baz" {
+		t.Fatalf("bad: %#v", v)
+	}
+}
+
 func TestNewUserVariable(t *testing.T) {
 	v, err := NewUserVariable("var.bar")
 	if err != nil {