Content-Type returned by JSON is not application/json when used in special middlewares

[aharbis]

💡 The feature or bug you are proposing

Using bunrouter.JSON in handlers passed to either WithNotFoundHandler or WithMethodNotAllowedHandler results in application/text Content-Type header.

📄 The description of the bug or the logic behind your proposal

Constructing my router like this:

router := bunrouter.New(
	bunrouter.WithMiddleware(reqLogMiddleware),
	bunrouter.WithMiddleware(errorHandlerMiddleware),
	bunrouter.WithNotFoundHandler(notFoundHandler),
	bunrouter.WithMethodNotAllowedHandler(methodNotAllowedHandler),
)

Both of my notFoundHandler and methodNotAllowedHandler middlewares send a valid JSON response with:

bunrouter.JSON(w, err)

Where err is a custom HTTPError type with json marshaling tags.

The JSON marshaling does work and the response received on the client is something like:

{"code":"not_found","message":"no route found for /"}

But the Content-Type header received is text/plain; charset=utf-8.

🚀 The expected result

bunrouter.JSON should always set application/json as the Content-Type, even when used in special middlewares.

[aharbis]

Actually, this may not be scoped to just those middlewares. I am also seeing similar behavior for other routes, where neither of the NotFound nor MethodNotFound handlers were involved.

[aharbis]

I think I found the root cause, note this comment in http.ResponseWriter:

// Changing the header map after a call to WriteHeader (or
// Write) has no effect unless the HTTP status code was of the
// 1xx class or the modified headers are trailers.

However in the bunrouter documentation, and indeed my code, the following pattern is used:

w.WriteHeader(httpErr.statusCode)
_ = bunrouter.JSON(w, httpErr)

This places the call to w.Header().Set() (inside bunrouter.JSON()) after w.WriteHeader(), which prevents the Content-Type from being set as expected.

[vmihailenco]

Yes, you must be careful with WriteHeader. It is not specific to BunRouter and BunRouter does not offer a universal solution for this problem.

https://bunrouter.uptrace.dev/guide/golang-http-error-handling.html may give you some ideas. Also see uptrace for a more realistic example.