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

[Kevin Wolf <kwolf@redhat.com>]

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
then just assert in the C code that both flags aren't set at the same
time?

> >> > @@ -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.

> >> Some extra churn due to type hints here.  Distracting.  Suggest not to
> >> mix adding type hints to existing code with feature work.
> >
> > If you would be open for a compromise, I could leave options
> > unannotated, but keep the typed parameter list.
> 
> Keeping just the function annotation is much less distracting.  I can't
> reject that with a "separate patches for separate things" argument.
> 
> I'd still prefer not to, because:
> 
> * If we do add systematic type hints in the near future, then delaying
>   this one until then shouldn't hurt your productivity.
> 
> * If we don't, this lone one won't help your productivity much, but
>   it'll look out of place.
> 
> I really don't want us to add type hints as we go, because such
> open-ended "while we touch it anyway" conversions take forever and a
> day.  Maximizes the confusion integral over time.

I think it's a first time that I'm asked not to document things, but
I'll remove them.

Kevin