proposal: net/http: customizable Not Found and Method Not Allowed handlers

[hossein1376]

Proposal Details

With the HTTP router changes in go1.22, many developers are considering standard library for daily use or production. Although, as of now, NotFound and MethodNotAllowed handlers are hard coded, with no way of change or overriding them.

If we look at thefindHandler function, we see:

	//...
	allowedMethods := mux.matchingMethods(host, path)
	if len(allowedMethods) > 0 {
		return HandlerFunc(func(w ResponseWriter, r *Request) {
			w.Header().Set("Allow", strings.Join(allowedMethods, ", "))
			Error(w, StatusText(StatusMethodNotAllowed), StatusMethodNotAllowed)
		}), "", nil, nil
	}
	return NotFoundHandler(), "", nil, nil
	//...

I propose adding the option to set these two handlers.

Implementation
Referring to my PR (#67970), a new MuxOption type is introduced and two functions called WithNotFoundHandler and WithNotAllowedHandler are added.
Also, function NewServeMux is updated to accept variadic parameter of type MuxOption. This ensures backward compatibility, while allowing for customization.

To make sure the default value of ServeMux is valid as well, findHandler function is updated to use custom handlers if provided, or to use the default handlers otherwise.

[gabyhelp]

Similar Issues

• net/http: customize ServeMux with not found and not allowed handlers #67970
• net/http: a little more flexibility on NotFound #10707
• proposal: net/http: Custom handlers for 404 and 405 HTTP response codes #65648
• http.ServeMux could allow a user specified default handler #263

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

Duplicate of #65648

[hossein1376]

Duplicate of #65648

That issue does not suggest a solution. This one offers a clear approach, plus opening the door for more customizations of the ServeMux type in the future.

[seankhliao]

we haven't agreed in #65648 that it is a concrete issue to address first, let's keep the discussion in one place