Proposed ~TaskQueue() solution - GCD

Proposed ~TaskQueue() solution - GCD

TODO

BUG=None

[eladalon, 2017-08-30 09:19:35]

eladalon@webrtc.org changed reviewers:
+ nisse@webrtc.org, solenberg@webrtc.org, stefan@webrtc.org,
terelius@webrtc.org, tommi@webrtc.org

[eladalon, 2017-08-30 09:19:36]

Not for review yet. Just a basis for some discussion.

https://codereview.webrtc.org/3010733002/diff/1/webrtc/rtc_base/task_queue_gc...
File webrtc/rtc_base/task_queue_gcd.cc (right):

https://codereview.webrtc.org/3010733002/diff/1/webrtc/rtc_base/task_queue_gc...
webrtc/rtc_base/task_queue_gcd.cc:149: 
Might not hurt to add an RTC_DCHECK here to make sure nobody has posted any
additional tasks to the queue while we were waiting for VoidFunction() to be
executed; posting tasks to a TaskQueue while it's being torn down is an error.

[nisse-webrtc, 2017-08-30 09:20:36]

https://codereview.webrtc.org/3010733002/diff/1/webrtc/rtc_base/task_queue_gc...
File webrtc/rtc_base/task_queue_gcd.cc (right):

https://codereview.webrtc.org/3010733002/diff/1/webrtc/rtc_base/task_queue_gc...
webrtc/rtc_base/task_queue_gcd.cc:143: dispatch_suspend(queue_);
The interesting question is what dispatch_suspend does, if called concurrently
with a running task. If it blocks until the current task is finished, there
should be no race here.

[eladalon, 2017-08-30 09:41:27]

https://codereview.webrtc.org/3010733002/diff/1/webrtc/rtc_base/task_queue_gc...
File webrtc/rtc_base/task_queue_gcd.cc (right):

https://codereview.webrtc.org/3010733002/diff/1/webrtc/rtc_base/task_queue_gc...
webrtc/rtc_base/task_queue_gcd.cc:143: dispatch_suspend(queue_);
On 2017/08/30 09:20:35, nisse-webrtc wrote:
> The interesting question is what dispatch_suspend does, if called concurrently
> with a running task. If it blocks until the current task is finished, there
> should be no race here.

My thinking was that it would be helpful in either of these two possibilities:

* dispatch_suspend() blocks until the current task completes, and the next
pending task will not start executing until resume() is called.
* dispatch_suspend() actually pauses the currently running task.

However, some initial probing makes me suspect neither of these interpretations
is correct. Let me dig some more; I'll post an update later today.

[eladalon, 2017-08-30 10:21:43]

https://codereview.webrtc.org/3010733002/diff/1/webrtc/rtc_base/task_queue_gc...
File webrtc/rtc_base/task_queue_gcd.cc (right):

https://codereview.webrtc.org/3010733002/diff/1/webrtc/rtc_base/task_queue_gc...
webrtc/rtc_base/task_queue_gcd.cc:143: dispatch_suspend(queue_);
On 2017/08/30 09:41:27, eladalon wrote:
> On 2017/08/30 09:20:35, nisse-webrtc wrote:
> > The interesting question is what dispatch_suspend does, if called
concurrently
> > with a running task. If it blocks until the current task is finished, there
> > should be no race here.
> 
> My thinking was that it would be helpful in either of these two possibilities:
> 
> * dispatch_suspend() blocks until the current task completes, and the next
> pending task will not start executing until resume() is called.
> * dispatch_suspend() actually pauses the currently running task.
> 
> However, some initial probing makes me suspect neither of these
interpretations
> is correct. Let me dig some more; I'll post an update later today.

Okay, looks like it prevents the next task from launching, but given that the
current task might already be before/after the point where it checks the flag, I
don't see how suspend/resume helps in providing a better guarantee. We might as
well just call SetNotActive() directly.

[eladalon, 2017-08-30 10:28:09]

Patchset #2 (id:20001) has been deleted

[eladalon, 2017-08-30 10:34:52]

Please see https://codereview.webrtc.org/3004883002 instead.