logging HTTP 404/405 errors with Golang 1.22+ net/http ServeMux

Question

I’m playing around with net/http after reading about the routing upgrades in 1.22, the goal here is to only use stdlib, so I'm intentionally avoiding frameworks and routers that are not part of the stdlib such as gin or gorilla/chi.

...

func main() {
    mux := http.NewServeMux()

    mux.Handle("GET /hello/{$}", handler1)
    mux.Handle("GET /headers/{$}", handler2)


    http.ListenAndServe(port, mux)
}

does what I expect it to do:

GET requests to /hello/ or /hello route to handler 1

GET requests to /headers/ or /headers route to handler 2

POST requests to handled paths return a HTTP 405

unhandled paths return a HTTP 404

However, in the case of a 404 or 405 these are simply responses and are not recorded in stdout, is there a simple way to also add log these bad requests without adding extra handlers/middleware?

Writing a handler as a catch all such as mux.Handle("/", badHandler) would require extra complexity to deal with 404 vs 405, if there is an alternative I'd like to avoid it.

building middleware to spy on a responseWriter, checking the statusCode, and logging on 404/405 seems like it would also potentially work depending on what is made public by the http package (though I haven't looked into this option yet), but is more complex of a solution I would otherwise hope is available.

No reason preventing me from middleware, I'm just looking for an alternative solution, as I believed there was a simpler method I was overlooking. Wrapping the ResponseWriter in a middleware does present some issues however which are well documented in this GitHub Issue I found recently. Seems like it is a short coming of the stdlib — Andrew Sirolly
– Andrew Sirolly, Commented Jul 13, 2024 at 22:17

Lorretta McCabe · Accepted Answer · 2024-07-14 11:41:32Z

8

Use middleware. It's not very complex and is the only option as of Go 1.22.

type logWriter struct {
    http.ResponseWriter
    code int
}

func (lw *logWriter) WriteHeader(code int) {
    lw.code = code
    lw.ResponseWriter.WriteHeader(code)
}

// Unwrap supports http.ResponseController.
func (lw *logWriter) Unwrap() http.ResponseWriter { return lw.ResponseWriter }

func wrap(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        lw := &logWriter{w, http.StatusOK}
        next.ServeHTTP(lw, r)
        fmt.Printf("%d: %s\n", lw.code, r.URL.Path)
    })
}

Use the middleware like this:

http.ListenAndServe(port, wrap(mux))

Run the middleware on the PlayGround

Wrapping the response writer using middleware was a problem in the past because the wrapper can block access to optional interfaces such as http.Flusher and http.Hijacker. The http.ResponseController type was added to Go 1.20 to address this shortcoming. If the response writer wrapper implements the Unwrap() http.ResponseWriter method (as done in this answer), then the application can access the optional interfaces through the response controller.

edited Jul 14, 2024 at 11:41

answered Jul 13, 2024 at 20:28

Lorretta McCabe

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3 Comments

Andrew Sirolly Over a year ago

There's some well discussed shortcomings when using middleware for this in this GitHub discussion which I found after asking the question.

Lorretta McCabe Over a year ago

@AndrewSirolly The response controller addresses the shortcoming. My answer supports the response controller.

Markus W Mahlberg Over a year ago

This is the most elegant solution I have seen thus far.

Andrew Sirolly · Accepted Answer · 2024-07-13 22:30:25Z

1

As thoroughly discussed in this Github Issue of the official Go repository, there is no built in capability to add logging without middleware to intercept the WriteHeader() function call as of go 1.22

answered Jul 13, 2024 at 22:30

Andrew Sirolly

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