tree 2bbc1eb72d6f098da719b3e3fffd16eb3c613532
parent c9f7a622db5026e9c690878b58c1e54af8fc130c
author Daniel Dunbar <daniel_dunbar@apple.com> 1509924082 -0800
committer Daniel Dunbar <daniel_dunbar@apple.com> 1509924146 -0800

[Core] Fix a race condition in task cancellation.

 - This also renames the cancellation method to be more obvious about
   when it is used.

 - When task cancellation was done as part of an error handling path,
   there was a potential race with any outstanding work being completed.
   We solve this by forcing (engine-level) cancellation to wait for any
   outstanding work before completing (to avoid needing to introduce
   any additional synchronization logic).

 - This is currently very improbable, since errors aren’t ever expected,
   but that will change once we expose engine cancellation to clients.