Re: [PATCH v3 1/4] qapi: Add a 'coroutine' flag for commands

[Markus Armbruster <armbru@redhat.com>]

Kevin Wolf <kwolf@redhat.com> writes:

> Am 17.01.2020 um 08:57 hat Markus Armbruster geschrieben:
>> Kevin Wolf <kwolf@redhat.com> writes:
>> 
>> > Am 16.01.2020 um 14:00 hat Markus Armbruster geschrieben:
>> >> Kevin Wolf <kwolf@redhat.com> writes:
>> >> > I have no idea if we will eventually get a case where the command wants
>> >> > to behave different between the two modes and actually has use for a
>> >> > coroutine. I hope not.
>> >> >
>> >> > But using two bools rather than a single enum keeps the code simple and
>> >> > leaves us all options open if it turns out that we do have a use case.
>> >> 
>> >> I can buy the argument "the two are conceptually orthogonal, although we
>> >> don't haven't found a use for one of the four cases".
>> >> 
>> >> Let's review the four combinations of the two flags once more:
>> >> 
>> >> * allow-oob: false, coroutine: false
>> >> 
>> >>   Handler runs in main loop, outside coroutine context.  Okay.
>> >> 
>> >> * allow-oob: false, coroutine: true
>> >> 
>> >>   Handler runs in main loop, in coroutine context.  Okay.
>> >> 
>> >> * allow-oob: true, coroutine: false
>> >> 
>> >>   Handler may run in main loop or in iothread, outside coroutine
>> >>   context.  Okay.
>> >> 
>> >> * allow-oob: true, coroutine: true
>> >> 
>> >>   Handler may run (in main loop, in coroutine context) or (in iothread,
>> >>   outside coroutine context).  This "in coroutine context only with
>> >>   execute, not with exec-oob" behavior is a bit surprising.
>> >> 
>> >>   We could document it, noting that it may change to always run in
>> >>   coroutine context.  Or we simply reject this case as "not
>> >>   implemented".  Since we have no uses, I'm leaning towards reject.  One
>> >>   fewer case to test then.
>> >
>> > What would be the right mode of rejecting it?
>> >
>> > I assume we should catch it somewhere in the QAPI generator (where?) and
>> 
>> check_flags() in expr.py?
>
> Looks like the right place, thanks.
>
>> > then just assert in the C code that both flags aren't set at the same
>> > time?
>> 
>> I think you already do, in do_qmp_dispatch():
>> 
>>     assert(!(oob && qemu_in_coroutine()));
>> 
>> Not sure that's the best spot.  Let's see when I review PATCH 3.
>
> This asserts that exec-oob handlers aren't executed in coroutine
> context. It doesn't assert that the handler doesn't have QCO_COROUTINE
> and QCO_ALLOW_OOB set at the same time.

Asserting this explicitly can't hurt.  qmp_register_command()?

>> >> >> > @@ -194,8 +195,9 @@ out:
>> >> >> >      return ret
>> >> >> >  
>> >> >> >  
>> >> >> > -def gen_register_command(name, success_response, allow_oob, allow_preconfig):
>> >> >> > -    options = []
>> >> >> > +def gen_register_command(name: str, success_response: bool, allow_oob: bool,
>> >> >> > +                         allow_preconfig: bool, coroutine: bool) -> str:
>> >> >> > +    options = [] # type: List[str]
>> >> 
>> >> One more: this is a PEP 484 type hint.  With Python 3, we can use PEP
>> >> 526 instead:
>> >> 
>> >>           options: List[str] = []
>> >> 
>> >> I think we should.
>> >
>> > This requires Python 3.6, unfortunately. The minimum requirement for
>> > building QEMU is 3.5.
>> 
>> *Sigh*
>
> One of the reasons why I would have preferred 3.6 as the minimum, but
> our policy says that Debian oldstabe is still relevant for another two
> years. *shrug*

3.5 EOL is scheduled for 2020-09-13.
https://devguide.python.org/#status-of-python-branches

Whether Debian can support it beyond that date seems doubtful.

For another reason to want 3.6, see
[PATCH] qapi: Fix code generation with Python 3.5
Message-Id: <20200116202558.31473-1-armbru@redhat.com>