Please document how to handle "If-Match" or return 412 on PRECONDITION_FAILED

[gnu-lorien]

I'm trying to implement "If-Match" headers using gRPC-Gateway.

My first attempt was to return PRECONDITION_FAILED from my gRPC service after doing the ETag checks. At some point this worked in gRPC-Gateway, but this issue changed the behavior to no longer do that: #972

Looking in the existing documentation this seems to imply that this is the correct place to set up custom http status codes https://grpc-ecosystem.github.io/grpc-gateway/docs/mapping/customizing_your_gateway/#controlling-http-response-status-codes. Unfortunately this does not seem to be called in the situation where I've returned PRECONDITION_FAILED.

The following issue implies that what I should actually do is override the default http error codes in this situation: #1064. Unfortunately when I try the solution proffered in that issue I receive the following errors:

../../gateway/gateway.go:76:20: cannot assign to runtime.HTTPError
../../gateway/gateway.go:83:3: undefined: runtime.DefaultHTTPError

I would greatly appreciate a complete example that shows how to customize the http return code or that otherwise gives the best guidance on how to handle something like "If-Match."

[gnu-lorien]

Noting that I am very bad at Go, this is what I came up with.

func myHTTPErrorHandler(ctx context.Context, mux *runtime.ServeMux, marshaler runtime.Marshaler, w http.ResponseWriter, r *http.Request, err error) {
	var customStatus *runtime.HTTPStatusError
	ifMatchHeader := r.Header.Get("if-match")
	if ifMatchHeader == "" || errors.As(err, &customStatus) {
		runtime.DefaultHTTPErrorHandler(ctx, mux, marshaler, w, r, err)
	} else {
		s := status.Convert(err)
		if codes.FailedPrecondition == s.Code() {
			myError := &runtime.HTTPStatusError{
				HTTPStatus: 412,
				Err:        status.Error(codes.FailedPrecondition, http.StatusText(412)),
			}
			runtime.DefaultHTTPErrorHandler(ctx, mux, marshaler, w, r, myError)
		} else {
			runtime.DefaultHTTPErrorHandler(ctx, mux, marshaler, w, r, err)
		}
	}
}

	var opts []runtime.ServeMuxOption
	opts = append(opts, runtime.WithErrorHandler(myHTTPErrorHandler))
	gwmux := runtime.NewServeMux(opts...)

[johanbrandhorst]

This looks about right to me, does this not work for you? I'd make some style suggestions but the logic looks alright. Were you able to find https://grpc-ecosystem.github.io/grpc-gateway/docs/mapping/customizing_your_gateway/#error-handler in the docs? I don't know if we need a new docs section specifically for this error, do you?

[stale]

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