Golang http handler - time taken for request

I am trying to set a timer to count how much time is needed for my server to finish a request and I want the timer to stop after the last byte of the response is sent.

I found that the http server will only send the response after the handler function returns.

Is there any way to add a callback after the response is sent ?

Or is there a better way to count the time taken from the first byte of the request coming in till the last byte byte of the response is sent ?

The easier but not as accurate way to do it would be using a middleware to wrap your handler function.

func timer(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		startTime := time.Now()
		h.ServeHTTP(w, r)
		duration := time.Now().Sub(startTime)
	})
}

Then

http.Handle("/route",timer(yourHandler))

This is more accurately the time taken to process the request and form the response and not the time between writes.

If you absolutely need a more accurate duration then the parts of code you're looking to change reside in the net/http package.

It would be around here.

The highlighted line go c.serve(ctx) is where the the go routine for serving the request is spawned.

for {
	rw, e := l.Accept()
	if e != nil {
		if ne, ok := e.(net.Error); ok && ne.Temporary() {
			if tempDelay == 0 {
				tempDelay = 5 * time.Millisecond
			} else {
				tempDelay *= 2
			}
			if max := 1 * time.Second; tempDelay > max {
				tempDelay = max
			}
			srv.logf("http: Accept error: %v; retrying in %v", e, tempDelay)
			time.Sleep(tempDelay)
			continue
		}
		return e
	}
	tempDelay = 0
    
	c := srv.newConn(rw)
	c.setState(c.rwc, StateNew) // before Serve can return

> go func(){
> startTime := time.Now()
> c.serve(ctx)
> duration := time.Now().Sub(startTime)
> }()

}

Note : The request actually gets written in the net.Conn somewhere inside l.Accept() but the highlighted point is the only place where we can have the approximate start time and end time within the same scope in the code.