http.EncodeJSONResponse body writing is not compatible with net/http

[kerma]

What did you do?

1. Implemented http.StatusCoder() which returns 304 (Not Modified)
2. Observed an error when http.EncodeJSONResponse is called: http: request method or response status code does not allow body

It seems that body is not written only on 204: https://github.com/go-kit/kit/blob/master/transport/http/server.go#L177

However, stdlib defines "no body allowed" as 100-199, 204, 304: https://cs.opensource.google/go/go/+/refs/tags/go1.22.3:src/net/http/transfer.go;l=460

What did you expect?

No error.

What happened instead?

Got an error.

[peterbourgon]

Oops. Seems like a simple PR to fix.

[kerma]

Using this fix myself:

func EncodeJSONResponse(_ context.Context, w http.ResponseWriter, response any) error {
	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	if headerer, ok := response.(http.Headerer); ok {
		for k, values := range headerer.Headers() {
			for _, v := range values {
				w.Header().Add(k, v)
			}
		}
	}
	code := http.StatusOK
	if sc, ok := response.(http.StatusCoder); ok {
		code = sc.StatusCode()
	}
	w.WriteHeader(code)
	if !bodyAllowedForStatus(code) {
		return nil
	}
	return json.NewEncoder(w).Encode(response)
}

// bodyAllowedForStatus reports whether a given response status code
// permits a body. See RFC 7230, section 3.3.
// forked from net/http/transfer.go
func bodyAllowedForStatus(status int) bool {
	switch {
	case status >= 100 && status <= 199:
		return false
	case status == 204:
		return false
	case status == 304:
		return false
	}
	return true
}