[Python-ideas] Asynchronous exception handling around with/try statement borders

[Erik Bray]

On Wed, Jun 28, 2017 at 3:09 PM, Erik Bray <erik.m.bray at gmail.com> wrote:
> On Wed, Jun 28, 2017 at 2:26 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> On 28 June 2017 at 21:40, Erik Bray <erik.m.bray at gmail.com> wrote:
>>> My colleague's contention is that given
>>>
>>> lock = threading.Lock()
>>>
>>> this is simply *wrong*:
>>>
>>> lock.acquire()
>>> try:
>>>     do_something()
>>> finally:
>>>     lock.release()
>>>
>>> whereas this is okay:
>>>
>>> with lock:
>>>     do_something()
>>
>> Technically both are slightly racy with respect to async signals (e.g.
>> KeyboardInterrupt), but the with statement form is less exposed to the
>> problem (since it does more of its work in single opcodes).
>>
>> Nathaniel Smith posted a good write-up of the technical details to the
>> issue tracker based on his work with trio:
>> https://bugs.python.org/issue29988
>
> Interesting; thanks for pointing this out.  Part of me felt like this
> has to have come up before but my searching didn't bring this up
> somehow (and even then it's only a couple months old itself).
>
> I didn't think about the possible race condition before
> WITH_CLEANUP_START, but obviously that's a possibility as well.
> Anyways since this is already acknowledged as a real bug I guess any
> further followup can happen on the issue tracker.

On second thought, maybe there is a case to made w.r.t. making a
documentation change about the semantics of the `with` statement:

The old-style syntax cannot make any guarantees about atomicity w.r.t.
async events.  That is, there's no way syntactically in Python to
declare that no exception will be raised between "lock.acquire()" and
the setup of the "try/finally" blocks.

However, if issue-29988 were *fixed* somehow (and I'm not convinced it
can't be fixed in the limited case of `with` statements) then there
really would be a major semantic difference of the `with` statement in
that it does support this invariant.  Then the question is whether
that difference be made a requirement of the language (probably too
onerous a requirement?), or just a feature of CPython (which should
still be documented one way or the other IMO).

Erik