Optional specification of stream content type

[huin]

We have grpc-gateway based services that include both unary and streamed responses, using JSONPb as the marshaler. There is a niggle where the response type for the streamed response is application/json, but we'd like it to be application/x-ndjson in the streamed response case.

References to other Issues or PRs

This looks like it would have provided a solution for at least one comment on #581 (#581 (comment) in particular).

Have you read the Contributing Guidelines?

Yes. This does not appear to need file regeneration.

Brief description of what is fixed or changed

Support specifying streamed response content types, without changing the existing behaviour of grpc-gateway unless consumers opt in by providing their own Marshaler to set the stream content type.

Other comments

[johanbrandhorst]

Hi, thanks for your PR. I don't think we should need any new functionality for this. Marshalers in the grpc-gateway are registered by incoming Accept: header contents. Could you do something like

type myJSONPbWrapper struct {
    *runtime.JSONPb
}

func (myJSONPbWrapper) ContentType() string {
    return "application/x-ndjson"
}

and

mux := runtime.NewServeMux(
    runtime.WithMarshalerOption("application/json", &runtime.JSONPb{}),
    runtime.WithMarshalerOption("application/x-ndjson", myJSONPbWrapper{&runtime.JSONPb{}}),
)

?

[huin]

I don't think that quite captures the solution I was seeking.

Using the Accept header requires that the client provides the content type that it will receive in the response Content-Type header, and the server will blindly repeat back whichever of the two options it is given in code, regardless of the actual content type that it sends back.

I was aiming for something where the server would authoritatively state the content type on endpoints that have a streaming result.

For more context: in our situation, the client is not under our control, as we provide the serving grpc-gateway service, but not the client that is consuming a grpc-gateway service.

[johanbrandhorst]

OK, fair enough, I do think there's a pretty good use case for this. It sorta sucks that we can introduce this retroactively though since it might break users. What do you think about adding an example of an ndjson stream to our docs page? It could wrap the JSONPb marshaler to implement the streaming content type and to set the delimiter. Maybe a new doc in https://github.com/grpc-ecosystem/grpc-gateway/tree/main/docs/docs/mapping?

[huin]

OK, fair enough, I do think there's a pretty good use case for this. It sorta sucks that we can introduce this retroactively though since it might break users. What do you think about adding an example of an ndjson stream to our docs page? It could wrap the JSONPb marshaler to implement the streaming content type and to set the delimiter. Maybe a new doc in https://github.com/grpc-ecosystem/grpc-gateway/tree/main/docs/docs/mapping?

Willdo, I'll take a crack at that now.

[huin]

I've added another commit to add another page of documentation, and refer to it from an existing page.

[johanbrandhorst]

Excellent, I love the new docs page!

[johanbrandhorst]

Thanks for your contribution!

[huin]

No problem, thanks for reviewing!