Migrate work for finished CurrentThreadExecutor to previous executor
#494
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A CurrentThreadExecutor may terminate with work still remaining in its queue, or new work may be submitted later. We previously discarded remaining work, leading to deadlocks, and raised an error on submitting late work. But if there’s another CurrentThreadExecutor for the same thread below us on the stack, we should instead migrate the extra work there to allow it to eventually run. Doing this in a thread-safe way requires replacing the queue.Queue abstraction with collections.deque and threading.ConditionVariable (the same primitives used to implement queue.Queue). Fixes django#491; fixes django#492. Signed-off-by: Anders Kaseorg <[email protected]>
Signed-off-by: Anders Kaseorg <[email protected]>
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| # Resubmit remaining work to the closest running CurrentThreadExecutor | ||
| # on the stack, if any. | ||
| if self._work_items: | ||
| executor = self._old_executor | ||
| while executor is not None: | ||
| with executor._work_ready: | ||
| if not executor._broken: | ||
| executor._work_items.extend(self._work_items) | ||
| executor._work_ready.notify() | ||
| break | ||
| executor = executor._old_executor |
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Alternatively, we could execute the remaining work immediately. This would block the async_to_sync caller for longer, but would avoid the possibility of silently discarding the work if we don’t find a running CurrentThreadExecutor:
while self._work_items:
work_item = self._work_items.popleft()
work_item.run()Which strategy makes more sense?
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A
CurrentThreadExecutormay terminate with work still remaining in its queue, or new work may be submitted later. We previously discarded remaining work, leading to deadlocks, and raised an error on submitting late work. But if there’s anotherCurrentThreadExecutorfor the same thread below us on the stack, we should instead migrate the extra work there to allow it to eventually run.Doing this in a thread-safe way requires replacing the
queue.Queueabstraction withcollections.dequeandthreading.ConditionVariable(the same primitives used to implementqueue.Queue).async_to_sync→sync_to_async→async_to_sync→create_task→sync_to_async#492CurrentThreadExecutorreentrant and reusing it #493