Re: [PATCH v3 10/15] qemu_iotests: extent QMP socket timeout when using valgrind

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

>> So the current plan I have for _qmp_timer is:
>>
>> - As Max suggested, move it in __init__ and check there for the 
>> wrapper contents. If we need to block forever (gdb, valgrind), we set 
>> it to None. Otherwise to 15 seconds. I think setting it always to None 
>> is not ideal, because if you are testing something that deadlocks (see 
>> my attempts to remove/add locks in QEMU multiqueue) and the socket is 
>> set to block forever, you don't know if the test is super slow or it 
>> just deadlocked.
>>
> 
> I agree with your concern on rational defaults, let's focus on that 
> briefly:
> 
> Let's have QEMUMachine default to *no timeouts* moving forward, and have 
> the timeouts be *opt-in*. This keeps the Machine class somewhat pure and 
> free of opinions. The separation of mechanism and policy.
> 
> Next, instead of modifying hundreds of tests to opt-in to the timeout, 
> let's modify the VM class in iotests.py to opt-in to that timeout, 
> restoring the current "safe" behavior of iotests.
> 
> The above items can happen in a single commit, preserving behavior in 
> the bisect.
> 
> Finally, we can add a non-private property that individual tests can 
> re-override to opt BACK out of the default.
> 
> Something as simple as:
> 
> vm.qmp_timeout = None
> 
> would be just fine.
>

I applied these suggested changes, will send v4 and we'll see what comes 
out of it.

>> Well, one can argue that in both cases this is not the expected 
>> behavior, but I think having an upper bound on each QMP command 
>> execution would be good.
>>
>> - pass _qmp_timer directly to self._qmp.accept() in _post launch, 
>> leaving _launch() intact. I think this makes sense because as you also 
>> mentioned, changing _post_launch() into taking a parameter requires 
>> changing also all subclasses and pass values around.
>>
> 
> Sounds OK. If we do change the defaults back to "No Timeout" in a way 
> that allows an override by an opinionated class, we'll already have the 
> public property, though, so a parameter might not be needed.
> 
> (Yes, this is the THIRD time I've changed my mind in 48 hours.)
> 
>> Any opinion on this is very welcome.
>>
> 
> Brave words!
> 
> My last thought here is that I still don't like the idea of QEMUMachine 
> class changing its timeout behavior based on the introspection of 
> wrapper args.
> 
> It feels much more like the case that a caller who is knowingly wrapping 
> it with a program that delays its execution should change its parameters 
> accordingly based on what the caller knows about what they're trying to 
> accomplish.
> 
> Does that make the code too messy? I understand you probably want to 
> ensure that adding a GDB wrapper is painless and simple, so it might not 
> be great to always ask a caller to remember to set some timeout value to 
> use it.
> 
I am not sure I follow you here, where do you want to move this logic? 
Can you please elaborate more, I did not understand what you mean.

I understand that tweaking the timers in iotests.py with checks like

if not (qemu_gdb or qemu_valgrind):
	normal timer

may not be the most beautiful piece of code, but as you said it keeps 
things as simple as they can.

> I figure that the right place to do this, though, is wherever the 
> boilerplate code gets written that knows how to set up the right gdb 
> args and so on, and not in machine.py. It sounds like iotests.py code to 
> me, maybe in the VM class.

After the changes suggested on qmp_timeout, iotests.py already contains 
the only code to perform the setup right for gdb and valgrind, and 
machine.py is not touched (except for qmp_timeout). iotests.py will 
essentially contain a couple of ifs like the one above, changing the 
timer when gdb and valgring are *not* needed.

Emanuele