Cannot extract extension from FieldOptions message

[tiziano88]

even after defining an extension, calling proto.HasExtension(fd.Options, pb.E_MyExtension) always returns false.

On my machine, FieldOptions has the following methods:

func (m *FieldOptions) Reset()                    { *m = FieldOptions{} }
func (m *FieldOptions) String() string            { return proto.CompactTextString(m) }
func (*FieldOptions) ProtoMessage()               {}
func (*FieldOptions) Descriptor() ([]byte, []int) { return fileDescriptor0, []int{11} }

var extRange_FieldOptions = []proto.ExtensionRange{
	{1000, 536870911},
}

func (*FieldOptions) ExtensionRangeArray() []proto.ExtensionRange {
	return extRange_FieldOptions
}

but HasExtension checks whether the message is extendable:

// extendableProto is an interface implemented by any protocol buffer generated by the current
// proto compiler that may be extended.
type extendableProto interface {
	Message
	ExtensionRangeArray() []ExtensionRange
	extensionsWrite() map[int32]Extension
	extensionsRead() (map[int32]Extension, sync.Locker)
}

// extendableProtoV1 is an interface implemented by a protocol buffer generated by the previous
// version of the proto compiler that may be extended.
type extendableProtoV1 interface {
	Message
	ExtensionRangeArray() []ExtensionRange
	ExtensionMap() map[int32]Extension
}

...

// extendable returns the extendableProto interface for the given generated proto message.
// If the proto message has the old extension format, it returns a wrapper that implements
// the extendableProto interface.
func extendable(p interface{}) (extendableProto, bool) {
	if ep, ok := p.(extendableProto); ok {
		return ep, ok
	}
	if ep, ok := p.(extendableProtoV1); ok {
		return extensionAdapter{ep}, ok
	}
	return nil, false
}

but it looks like FieldOptions does not have the extensionsWrite and extensionsRead methods, so this fails immediately.

[tiziano88]

OK, I noticed that FieldOptions actually gains those methods by embedding the XXX_InternalExtensions struct:

// XXX_InternalExtensions is an internal representation of proto extensions.
//
// Each generated message struct type embeds an anonymous XXX_InternalExtensions field,
// thus gaining the unexported 'extensions' method, which can be called only from the proto package.
//
// The methods of XXX_InternalExtensions are not concurrency safe in general,
// but calls to logically read-only methods such as has and get may be executed concurrently.
type XXX_InternalExtensions struct {
...
}

// extensionsWrite returns the extension map, creating it on first use.
func (e *XXX_InternalExtensions) extensionsWrite() map[int32]Extension {
...
}

// extensionsRead returns the extensions map for read-only use.  It may be nil.
// The caller must hold the returned mutex's lock when accessing Elements within the map.
func (e *XXX_InternalExtensions) extensionsRead() (map[int32]Extension, sync.Locker) {
...
}

Still can't explain why extendable does not pick them up.

[tiziano88]

Trying to statically pass the message to a method accepting a extendableProto type, getting the following error:

./proto.go:55: cannot use fd.Options (type *"github.com/golang/protobuf/protoc-gen-go/descriptor".FieldOptions) as type "xxx.com/vendor/github.com/golang/protobuf/proto".extendableProto in argument to "xxx.com/vendor/github.com/golang/protobuf/proto".Extendable:
	*"github.com/golang/protobuf/protoc-gen-go/descriptor".FieldOptions does not implement "xxx.com/vendor/github.com/golang/protobuf/proto".extendableProto (wrong type for ExtensionRangeArray method)
		have ExtensionRangeArray() []"github.com/golang/protobuf/proto".ExtensionRange
		want ExtensionRangeArray() []"xxx.com/vendor/github.com/golang/protobuf/proto".ExtensionRange

so this seems caused by the interplay between vendored and standard dependencies.

[shiywang]

hi @tiziano88 I met the same problem using official website's example, and it seems like a vendor problem
when using old vendor and run those testing code

func TestABCMessage(t *testing.T) {
	var msg VersionFlags
	_, md := ForMessage(&msg)
	opts := md.Field[0].GetOptions()
	foo, err := proto.GetExtension(opts, E_Info)
	if err != nil {
		spew.Dump(err)
		spew.Dump(foo) // 123
	}
	spew.Dump(foo)
}

it throws

/tmp/TestABCMessage_in_version_abc_test_gogo -test.v -test.run ^TestABCMessage$
(*errors.errorString)(0xc420214af0)(proto: not an extendable proto)
(interface {}) <nil>

but when I update the vendor it works

/tmp/TestABCMessage_in_version_abc_test_gogo -test.v -test.run ^TestABCMessage$
(*cmd.FlagDetail)(0xc420790240)(name:"client" value:"false" shorthand:"c" usage:"Client version only (no server required)." )