Fix propagating exceptions

[jakubsvehla]

Fixes propagating exceptions when the PROPAGATE_EXCEPTIONS config setting is set to True by fixing exception handling in ASGIHTTPConnection. Currently, when PROPAGATE_EXCEPTIONS is set to True it returns the traceback as HTML instead of propagating the exception.

• fixes #383

[davidism]

Please verify that this modified truth table matches the behavior in Flask.

[jakubsvehla]

The PROPAGATE_EXCEPTIONS flag in Flask behaves differently in two ways:

1. None is not treated as False (as currently in Quart). When it’s None, it takes the value of the testing flag.
2. The PROPAGATE_EXCEPTIONS flag has precedence over the testing flag. So if PROPAGATE_EXCEPTIONS is False and testing True, then it won’t propagate exceptions (in Quart it would right now).

So Flask’s truth table (when ignoring the debug flag) is as follows:

PROPAGATE_EXCEPTIONS	testing	Raise (Flask)	Raise (Quart after the current fix)
None	True	True	True
None	False	False	False
True	True	True	True
True	False	True	True
False	True	False	True	<- This line differs
False	False	False	False

The difference is that in Flask, setting PROPAGATE_EXCEPTIONS to False overrides the testing flag.

So the question is whether we want to replicate Flask’s behavior or simply fix the bug. I think both behaviors make sense but the previous Quart’s behavior (before this fix) is in my opinion clearly wrong since it does not propagate exceptions when the PROPAGATE_EXCEPTIONS is True which I think should be the expected behavior.

You can find the relevant code in Flask here: https://github.com/pallets/flask/blob/bc098406af9537aacc436cb2ea777fbc9ff4c5aa/src/flask/app.py#L841

As far as I know, there are no tests for the interaction of the two flags. They are tested independently here: https://github.com/pallets/flask/blob/bc098406af9537aacc436cb2ea777fbc9ff4c5aa/tests/test_basic.py#L1574