dynamic/msgregistry: Add typed errors for lookups

Adds typed errors used when looking up a
message and finding an enum or vice versa.

dynamic/msgregistry/message_registry.go

@@ -22,6 +22,42 @@ import (
 
 const googleApisDomain = "type.googleapis.com"
 
+// ErrUnexpectedEnum is returned if the URL that was requested
+// resolved to an enum instead of a message.
+type ErrUnexpectedEnum struct {
+	URL                string
+	FullyQualifiedName string
+}
+
+func (e *ErrUnexpectedEnum) Error() string {
+	return fmt.Sprintf("type for URL %q is the wrong type: wanted message, got enum", e.URL)
+}
+
+// Is determines how ErrUnexpectedEnum is compared
+// against other errors when using errors.Is.
+func (e *ErrUnexpectedEnum) Is(err error) bool {
+	_, ok := err.(*ErrUnexpectedEnum)
+	return ok
+}
+
+// ErrUnexpectedMessage is returned if the URL that was requested
+// resolved to a message instead of an enum.
+type ErrUnexpectedMessage struct {
+	URL                string
+	FullyQualifiedName string
+}
+
+func (e *ErrUnexpectedMessage) Error() string {
+	return fmt.Sprintf("type for URL %q is the wrong type: wanted enum, got message", e.URL)
+}
+
+// Is determines how ErrUnexpectedMessage is compared
+// against other errors when using errors.Is.
+func (e *ErrUnexpectedMessage) Is(err error) bool {
+	_, ok := err.(*ErrUnexpectedMessage)
+	return ok
+}
+
 // MessageRegistry is a registry that maps URLs to message types. It allows for marshalling
 // and unmarshalling Any types to and from dynamic messages.
 type MessageRegistry struct {
@@ -188,8 +224,10 @@ func (r *MessageRegistry) getRegisteredMessageTypeByUrl(url string) (*desc.Messa
 		if m != nil {
 			if md, ok := m.(*desc.MessageDescriptor); ok {
 				return md, nil
-			} else {
-				return nil, fmt.Errorf("type for URL %v is the wrong type: wanted message, got enum", url)
+			}
+			return nil, &ErrUnexpectedEnum{
+				URL:                url,
+				FullyQualifiedName: m.GetFullyQualifiedName(),
 			}
 		}
 	}
@@ -236,8 +274,10 @@ func (r *MessageRegistry) getRegisteredEnumTypeByUrl(url string) (*desc.EnumDesc
 	if m != nil {
 		if ed, ok := m.(*desc.EnumDescriptor); ok {
 			return ed, nil
-		} else {
-			return nil, fmt.Errorf("type for URL %v is the wrong type: wanted enum, got message", url)
+		}
+		return nil, &ErrUnexpectedMessage{
+			URL:                url,
+			FullyQualifiedName: m.GetFullyQualifiedName(),
 		}
 	}
 	return nil, nil

dynamic/msgregistry/message_registry_test.go

@@ -1,6 +1,7 @@
 package msgregistry
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -59,9 +60,9 @@ func TestMessageRegistry_LookupTypes(t *testing.T) {
 
 	// wrong type
 	_, err = mr.FindMessageTypeByUrl("foo.bar/google.protobuf.FieldDescriptorProto.Type")
-	testutil.Require(t, err != nil && strings.Contains(err.Error(), "wanted message, got enum"))
+	testutil.Require(t, errors.Is(err, &ErrUnexpectedEnum{}))
 	_, err = mr.FindEnumTypeByUrl("foo.bar/google.protobuf.DescriptorProto")
-	testutil.Require(t, err != nil && strings.Contains(err.Error(), "wanted enum, got message"))
+	testutil.Require(t, errors.Is(err, &ErrUnexpectedMessage{}))
 
 	// unmarshal any successfully finds the registered type
 	b, err := proto.Marshal(md.AsProto())