handler500 called multiple times.

[David Szotten]

i recently came across a situation where a (poorly designed) handler500 view ended up itself raising exceptions. i was slightly surprised to realise that the error handler ends up getting called once for each middleware. is this expected/desired (or e.g. just a side-effect of the current implementation)?

[Mariusz Felisiak]

handler500 should return HttpResponseServerError, when it raises an exception then it's propagated to the next middleware and a custom handler is called again. This behavior is a side effect of an incorrect implementation of a custom handler500. It's called only once when returns HttpResponseServerError (as documented).

[David Szotten]

apologies if i was being unclear. i acknowledge that the root cause was a bug in my 500 handler "(poorly designed)". however, my question was about how django should behave in the presence of such bugs. bugs are a fact of life, and for when they happen in regular view code, django provides the handler500 hook for custom error pages. it doesn't seem unreasonable that from time to time we also have bugs in our error handlers, and my question is how we want django to behave then. is the current behaviour of re-calling the handler500 once for each middleware desired in this case, or would we be interested in exploring other options? (or, perhaps it's not ideal but too rare to bother improving)

[Mariusz Felisiak]

This is made by design (see ​process_exception_by_middleware()), exceptions are processed by middlewares (see ​#process-exception), and a default exception handler (handler500) is used when process_exception() is not implemented. That's why your custom (buggy) handler is used by all middlewares. Your implementation doesn't follow documentation and I don't think there is anything to change in Django.