Feature request: allow transformations between descriptor.proto objects and reflection objects

[murgatroid99]

protobuf.js version: 6

It would be useful to be able to take a message of a type defined in google/protobuf/descriptor.proto, and convert it into the corresponding reflection object, and also be able to convert the other way. Of course, some of them don't really make sense, but I would like, for example, to be able to load a DescriptorProto message instance as a Message object, and conversely to be able to extract a DescriptorProto instance from a Message object.

[dcodeIO]

That's basically self-describing messages-support, right?

[murgatroid99]

Not really. The types in descriptor.proto are structurally equivalent to the information you can load from a .proto file. For example, and instance of the DescriptorProto message type contains the same information as a message declaration in a .proto file. What I'm asking for is a transformation between instances of these messages and the ProtoBuf.js reflection types.

[dcodeIO]

To my understanding, DescriptorProtos reference enums and messages of other DescriptorProtos, including only nested enums and messages. Hence, resolving any such type referencing a non-nested enum or message would throw.

Only a FileDescriptorSet, which is used in self-describing messages, would contain all relevant DescriptorProtos making it possibly to successfully work with (like encode and decode) the types imported this way.

I might just be misunderstanding your use-case, though, so feel free to point me into the right direction.

[murgatroid99]

Right, DescriptorProto may have been a bad example. It is likely that I would only need to transform between instances of FileDescriptorSet and Root objects. The goal here is to support the gRPC server reflection protocol.

[dcodeIO]

I quickly put something together as a starting point, see commit above. The reason why I'd prefer an extension here is that it's usually not necessary to include the entire descriptor (functionality) as part of the standard library.

Idea is that doing require("protobufjs/ext/descriptor") loads the extension along with the descriptor definitions and then adds fromDescriptor, #toDescriptor methods to all reflection types. See protobuf.Type.fromDescriptor for how these methods could look like.

[murgatroid99]

That looks like what I was asking for.

[dcodeIO]

Sigh, this will take a while. Added a few comments to document what's (still) necessary.

[dcodeIO]

Still pretty rough around the edges, but it's now able to convert a root with loaded descriptor.proto in it to a FileDescriptorSet, encode it, decode it back and have it return something similar (still needs a real test suite) to the descriptor we initially put in. Wheee...