🍒[5.9][TaskGroup] Reenable test and fix memory issue #67700
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released memory. Because the waitAll() group method is called before the group returns from withTaskGroup and is used to ensure all tasks have completed before we destroy the task group, this method is then immediately followed by calling TaskGroup::destroy. If we, as previously, first resume the waiting task and then attempt to unlock the lock held by the group, we are in an unsafe situation with racing the task group destroy() and the unlock(). Therefore, we must release the lock before we resume the waiting task.
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@swift-ci please test |
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Description: This re-enables a task group test and fixes a memory issue where we realized we must release the group lock before resuming the waiting task. If we don't do this, we're racing the group's
destroy()(coming from the parent task, waiting onwaitAllwith theunlock()), risking callingunlock()on already released memory - very rarely causing crashes due to this. It is hard to reproduce this issue, due to the race appearing very rarely, but it is consistently reproducible in some environments.Risk: Low, the fix reorders an unlock() with the waitingTask resume which cannot cause any other consistency issues as there are no other actions taken after the resume. Arguably, this always should have been release + run order.
Reward: Medium, it seems this crash does not manifest frequently in real applications, however it is a real issue we need to fix.
Review by: @al45tair @erik
Testing: CI testing, confirmed the fix on specially prepared environments.
Original PR: #67392
Radar: rdar://112759360 rdar://110474311