reflect/prototype: fix name resolution when file has no package name

A .proto source file with no 'package' statement may still contain
references to descriptors within the file.

Change-Id: I86e942c9c06e5a2915e9722162e0455ffa9ba2ab
Reviewed-on: https://go-review.googlesource.com/c/140899
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

cmd/protoc-gen-go/testdata/nopackage/nopackage.pb.go

@@ -59,7 +59,7 @@ func (Enum) EnumDescriptor() ([]byte, []int) {
 
 type Message struct {
 	StringField          *string  `protobuf:"bytes,1,opt,name=string_field,json=stringField" json:"string_field,omitempty"`
-	EnumField            *Enum    `protobuf:"varint,2,opt,name=enum_field,json=enumField,enum=Enum" json:"enum_field,omitempty"`
+	EnumField            *Enum    `protobuf:"varint,2,opt,name=enum_field,json=enumField,enum=Enum,def=0" json:"enum_field,omitempty"`
 	XXX_NoUnkeyedLiteral struct{} `json:"-"`
 	XXX_unrecognized     []byte   `json:"-"`
 	XXX_sizecache        int32    `json:"-"`
@@ -90,6 +90,8 @@ func (m *Message) XXX_DiscardUnknown() {
 
 var xxx_messageInfo_Message proto.InternalMessageInfo
 
+const Default_Message_EnumField Enum = Enum_ZERO
+
 func (m *Message) GetStringField() string {
 	if m != nil && m.StringField != nil {
 		return *m.StringField
@@ -101,7 +103,7 @@ func (m *Message) GetEnumField() Enum {
 	if m != nil && m.EnumField != nil {
 		return *m.EnumField
 	}
-	return Enum_ZERO
+	return Default_Message_EnumField
 }
 
 func init() {
@@ -112,13 +114,14 @@ func init() {
 func init() { proto.RegisterFile("nopackage/nopackage.proto", fileDescriptor_f33a1d5d178c43c9) }
 
 var fileDescriptor_f33a1d5d178c43c9 = []byte{
-	// 127 bytes of a gzipped FileDescriptorProto
+	// 130 bytes of a gzipped FileDescriptorProto
 	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0xff, 0xe2, 0x92, 0xcc, 0xcb, 0x2f, 0x48,
-	0x4c, 0xce, 0x4e, 0x4c, 0x4f, 0xd5, 0x87, 0xb3, 0xf4, 0x0a, 0x8a, 0xf2, 0x4b, 0xf2, 0x95, 0x82,
+	0x4c, 0xce, 0x4e, 0x4c, 0x4f, 0xd5, 0x87, 0xb3, 0xf4, 0x0a, 0x8a, 0xf2, 0x4b, 0xf2, 0x95, 0x22,
 	0xb8, 0xd8, 0x7d, 0x53, 0x8b, 0x8b, 0x13, 0xd3, 0x53, 0x85, 0x14, 0xb9, 0x78, 0x8a, 0x4b, 0x8a,
 	0x32, 0xf3, 0xd2, 0xe3, 0xd3, 0x32, 0x53, 0x73, 0x52, 0x24, 0x18, 0x15, 0x18, 0x35, 0x38, 0x83,
-	0xb8, 0x21, 0x62, 0x6e, 0x20, 0x21, 0x21, 0x15, 0x2e, 0xae, 0xd4, 0xbc, 0xd2, 0x5c, 0xa8, 0x02,
-	0x26, 0x05, 0x46, 0x0d, 0x3e, 0x23, 0x56, 0x3d, 0xd7, 0xbc, 0xd2, 0xdc, 0x20, 0x4e, 0x90, 0x04,
-	0x58, 0x95, 0x96, 0x00, 0x17, 0x0b, 0x48, 0x48, 0x88, 0x83, 0x8b, 0x25, 0xca, 0x35, 0xc8, 0x5f,
-	0x80, 0x01, 0x10, 0x00, 0x00, 0xff, 0xff, 0xb2, 0xbc, 0x78, 0x1f, 0x81, 0x00, 0x00, 0x00,
+	0xb8, 0x21, 0x62, 0x6e, 0x20, 0x21, 0x21, 0x2d, 0x2e, 0xae, 0xd4, 0xbc, 0xd2, 0x5c, 0xa8, 0x02,
+	0x26, 0x05, 0x46, 0x0d, 0x3e, 0x23, 0x56, 0x3d, 0xd7, 0xbc, 0xd2, 0x5c, 0x2b, 0x96, 0x28, 0xd7,
+	0x20, 0xff, 0x20, 0x4e, 0x90, 0x34, 0x58, 0xad, 0x96, 0x00, 0x17, 0x0b, 0x48, 0x42, 0x88, 0x83,
+	0x0b, 0x2c, 0x25, 0xc0, 0x00, 0x08, 0x00, 0x00, 0xff, 0xff, 0x31, 0x29, 0xe4, 0xb2, 0x87, 0x00,
+	0x00, 0x00,
 }

cmd/protoc-gen-go/testdata/nopackage/nopackage.proto

@@ -12,5 +12,5 @@ enum Enum {
 
 message Message {
   optional string string_field = 1;
-  optional Enum   enum_field   = 2;
+  optional Enum   enum_field   = 2 [default=ZERO];
 }

reflect/prototype/protofile_type.go

@@ -544,6 +544,8 @@ func resolveReference(parent pref.Descriptor, refName pref.FullName) pref.Descri
 			} else if refName[len(curName)] == '.' {
 				refName = refName[len(curName)+len("."):]
 				break // e.g., refName: foo.firetruck.driver, curName: foo.firetruck
+			} else if len(curName) == 0 {
+				break // FileDescriptor has no package name
 			}
 			// No match. (e.g., refName: foo.firetruck, curName: foo.fire)
 		}