Invalid `SERVER_NAME` + `url_for` leads to `AttributeError` about flask

[davidism]

I'm having a hard time evaluating this. None of the suggested fixes are really useful.

1 is not possible. 3 is not possible, except to "handle" it by raising an error, which is what happens when the app context (instead of request context) can't create an adapter. 2 is possible, at least to raise a clearer error message closer to where the problem occurs, as explained for 3, but it shouldn't be something the dev should consider handling, since ultimately it's an issue with the server environment.

There is a little known feature called app.url_build_error_handlers, which is a list of handlers to call when adapter.build() raises a BuildError. Right now, it's only called when adapter.build() fails with a BuildError, not for other errors or errors raised by Flask when wrapping that call. This could be extended so that the RuntimeError and ValueError that url_for raises in certain situations would be a BuildError instead and pass through the same handlers. By default, no handlers means that the error is re-raised.

[kasium]

Thanks for checking this. I know it's a weird one, with not a clear path forward.

I like the idea to register a url build error handler 👍

[kasium]

Hi, I now found a reproducible example by just providing an invalid Host header value. I also reported it to cheroot: cherrypy/cheroot#646