More documentation needed on exceptions in middlewares

[pakal <chambon.pascal@…>]

I have not found any documentation on the behaviour of django when an exception is raised, not from a view, but from a middleware method.

Eg. if a process_exception() raises an unexpected exception, how will django deal with that ? And if a process_response() fails when treating a normal http response ?

I can have some information by testing these tests, but as long as it's undocumented I can't really rely on it.

[pakal <chambon.pascal@…>]

My tests show that if process_request, process_view or process_exception raise an error, normal handling of this error occurs, depending on the "DEBUG" setting : printing full stack information, or sending an email to admins.

But if process_response raises an error, there is visibly no fallback solution : the server simply outputs a raw traceback, and admins are not notified of the error.
Shouldn't the exception management system protect "process_response" too, so that admin notification and the display of a simple "error 500" page occurs ?

[Gabriel Hurley]

Looks like this could use a little DDN... is this a documentation issue or a code issue? What *is* the desired behavior here?

[pascal chambon]

I guess the whole middleware processing should be protected by lowest-level exception handlers.

It seems that neither email sending, nor debug page processing, require the middlewares to function, anyway.
So process_response should be handled like other middleware functions - if they raise smth, catch it and trigger the critical eror system.

[Pakal]

I've browsed django's core handlers modules, and there is a very weird design there. Look at that code, which is the same in modpython and wsgi handlers :

                response = self.get_response(request)

                # Apply response middleware
                for middleware_method in self._response_middleware:
                    response = middleware_method(request, response)
                response = self.apply_response_fixes(request, response)

get_response() is the part dealing with request/view/exception middleware, and its result is protected by a try...except leading to handle_uncaught_exception() and its email-sending/traceback-display systems.

But why are response middlewares left out of this method, and thus insecurely left without exception catcher ? Shouldn't we simply apply these response middlewares at the same level as others ??

[Russell Keith-Magee]

@Pakal -- This discrepancy was addressed in r14393.

[Pakal]

Excellent, thanks B-)