net/http/httputil: add a custom-error-page ResponseWriter wrapper type

[bep]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?

Not relevant.

1. What operating system and processor architecture are you using (go env)?

Not relevant.

1. What did you do?

Try to present a nice 404.html to the end user on Page Not Found when using the Go builtin file server.

1. What did you expect to see?

This should be a common enough use case that I shouldn't have to drag big external libraries in to solve this.

1. What did you see instead?

That I had to drag in big external libraries to solve this.

See gohugoio/hugo#874

[cespare]

Probably a dup of #10123 or one of the previous issues discussing this.

A big external library is not required. One way you can handle this is by providing your own ResponseWriter which spits out your custom page after it sees WriteHeader(404).

[bep]

A big external library is not required.

I was just quoting several people who have tried to solve this issue in a clean way in the built-in dev file server in Hugo. I haven't looked into this my self, but if the "common Go developer" are having a hard time cracking this nut, you should consider to help him/her.

[hansrodtang]

A quick and dirty proof-of-concept.
It should be pretty simple to implement your own handler in a similar way.

[bep]

It should be pretty simple to implement your own handler in a similar way.

Sure, but this is still 50ish lines of code that every person (I know, not every ...) who wants to use the FileServer must implement. You added the FileServer abstraction at one point, so you might as well finish it. Where to stop, you might ask, and that is a good question. A unified way of handling the most common error situation, a file that doesn't exist, is well within the borders.

[gkop]

It would be nice if we could handle things other than status code, for example timeouts. In the meantime we're using https://github.com/vulcand/oxy , which provides error handling in its reverse proxy.

[rsc]

Dup of #10123.