grpc-gateway: do no require manual grpc gateway registration

[julienrbrt]

Follow-up of #22701, #20798

Our goal is to kill that explicit gRPC gateway registration (RegisterGRPCGatewayRoutes and generated code), and do some proto options parsing directly in the grpcgateway server. This will allow us to let user have modules even smaller and no SDK dependency and gRPC dependencies at all (roughly this: #20798 (comment))

ref: #22701 (review)

While gRPC gateway has been wired in #22701, as said in the message above, the goal is to removal totally the grpc gateway manual registration from the modules. This still needs to be implemented.

[julienrbrt]

In addition to modules, we should allow the registration of the following routes in the server (extensible like the gRPC server): https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.2/runtime/app.go#L224-L231

[technicallyty]

I am interested in taking a stab at this!

[julienrbrt]

I am interested in taking a stab at this!

Cool, thanks!

[technicallyty]

kill that explicit gRPC gateway registration (RegisterGRPCGatewayRoutes and generated code)

i'm not sure that this is possible with removing gateway code. the custom endpoint from google.api.http.get get generated into the gateway (*.pb.gw.go) code.

if we keep the gw generated code, im still not sure how to proceed as the custom endpoint is not exported. it's used internally in the mux registration code.

any idea on how to proceed here? is there an approach/element to this im missing?

[julienrbrt]

The idea was mainly to ditch that whole generated code, but keep the google.api.http.get annotations.
This simplify the module.go of modules, and their dependencies.

The gRPC gateway server (server/v2/api/grpc-gateway), would have a catch all handler to answer to any path, and that handler would dynamically try to figure out which message was aimed the request.

For example:

• GET /cosmos/bank/v1beta1/params -> split (3 first?) / -> cosmos.bank.v1beta1 -> Find service -> Find Params
  Then you can do a POST request to the internal rest endpoint (or just copy the logic in the grpc-gateway server) server/v2/api/rest)

That should work for most of them, this is when there are parameters that things get hacky:

• GET /cosmos/bank/v1beta1/balances/cosmos123 -> split (3 first?) / -> cosmos.bank.v1beta1 -> Find service -> Loop over the whole service to find matching rpc call
  Then using the rest v2 stuff again

Again, that server won't be pretty, and probably slower than with the generated code, but it is more important to have killed the generated code + sdk import in modules. gRPC gateway server is there for backward compat (as handlers do not work with it). The idea is to have everyone slowly migrate to rest v2.