Add method for custom formatting of stream errors.

[johanbrandhorst]

Alternative to discussion in #538 (comment). This would allow a user to get errors as marshalled status.Proto() types with the use of both WithProtoStreamErrorFormatter and WithProtoErrorHandler.

This is pretty experimental so I'm happy to make any changes.

[abursavich]

I would much prefer the handler's signature to be more in the style of (Default)HTTPError, so that a client can write the stream error however they wish. In my case, I would like to construct my own error status proto directly and avoid my marshalers having to handle maps.

[johanbrandhorst]

Could you come up with a practical alternative? The stream is already established at this point, so a handler function may require rewriting of much of the logic.

[abursavich]

Here's my first pass: https://github.com/abursavich/grpc-gateway/commit/900ec66e401eb8dd606ddf41cd1a492d260354a8

[johanbrandhorst]

@abursavich Thanks for that! I think one problem with your approach is that we expose a new function type, used by the user, which has an empty interface in the signature. That's pretty poor interface design. It also rewrites quite a bit of the logic, and it seems to add an error case if the response is nil, which may break backwards compatibility? I think your suggestion is a step in the right direction, as I would also like more control over the writing of mesages, but maybe the configuration should be more fine grained? One option for formatting the error and one for formatting a successful reply? I'm not sure.

[abursavich]

FWIW, the error on nil response was already there, I just lifted it out of streamChunk (which I deleted):

grpc-gateway/runtime/handler.go

Lines 185 to 187 in cde2f8f

	if result == nil {
	return streamChunk(nil, fmt.Errorf("empty response"))
	}

I don't like the empty interface much either, but I was matching the existing interface for Marshaler:

grpc-gateway/runtime/marshaler.go

Lines 7 to 20 in e4b8a93

	// Marshaler defines a conversion between byte sequence and gRPC payloads / fields.
	type Marshaler interface {
	// Marshal marshals "v" into byte sequence.
	Marshal(v interface{}) ([]byte, error)
	// Unmarshal unmarshals "data" into "v".
	// "v" must be a pointer value.
	Unmarshal(data []byte, v interface{}) error
	// NewDecoder returns a Decoder which reads byte sequence from "r".
	NewDecoder(r io.Reader) Decoder
	// NewEncoder returns an Encoder which writes bytes sequence into "w".
	NewEncoder(w io.Writer) Encoder
	// ContentType returns the Content-Type which this marshaler is responsible for.
	ContentType() string
	}

[johanbrandhorst]

My heart is not really in this change after merging #561, so I will close this to let someone else provide a better solution to the specific problem of customizing stream error returns.