no way to access data for unrecognized extensions

[jhump]

Access to extensions is provided by top-level functions in the proto package (in extensions.go). But they omit any way to query for the values of unrecognized extensions (as raw unparsed bytes).

My immediate use case is to be able to re-parse custom method options on a descriptor proto using a dynamic extension registry (not limited to just linked-in extensions). This allows a "server browser" tool to show more useful information about a server's exposed services instead of just "unknown fields/extensions". Using the server reflection API, the client can easily ask the server for all known extensions and then re-parse the descriptors (for example, using a dynamic message).

For symmetry with how C++ and Java libraries support extension and unknown fields, it would be preferred for unrecognized extensions to end up in XXX_unrecognized instead of hidden inside of a map of unparsed extension values that is inaccessible. (It used to be accessible via the ExtensionMap() method, but that is no longer generated by protoc-gen-go.)

I also brought this up on the protobuf mailing list, where I think @xfxyjwf expressed a similar sentiment:
https://groups.google.com/d/topic/protobuf/HTNFwKLxtRg/discussion

I believe this could be addressed by merely changing the unmarshalling code to append the data to XXX_unrecognized instead of storing in the map when the looked up *ExtensionDesc is nil.

[jhump]

@dsnet any updates on how/when this issue might be resolved?

[jhump]

@dsnet, what do you think about #420 as a solution for this? Unlike #386, it has no compatibility issues. Instead, it increases API surface area by one method.

[dsnet]

I'll take a look. With Go1.9 out the door, I'm swinging my attention back over to protos.

[dsnet]

Closing as fixed with #583. Track #479 for the merging of dev into master.