Race condition when waiting on Promise resolved in other thread

I’m using with LibUsb to achieve asynchronous transfers. Without going into too much details I’m using two threads:

1. Worker: creates promise (P) and starts async USB transfer with supplied callback © that resolves promise. After starting transfer, thread wait for promise result.
2. LibUSB event loop: triggers event processing in LibUSB, which will call callback C which will resolve promise and resume Worker thread.

All calls to LibUSB are native calls (ctypes).

Occasionally (one in hundreds) I’m getting AssertionFailed exception on this line: https://github.com/syrusakbary/promise/blob/master/promise/promise.py#L414 with traceback:

Traceback (most recent call last):
  File "C:\Python38\lib\threading.py", line 932, in _bootstrap_inner
    self.run()
  File "C:\Python38\lib\threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "<snip>", line 66, in _run
    packet = self._segmentation.read_packet()
  File "<snip>", line 60, in read_packet
    payload = self._low.low_read(part_size)
  File "<snip>.py", line 56, in low_read
    return bytes(ret.get())
  File "C:\Python38\lib\site-packages\promise\promise.py", line 511, in get
    self._wait(timeout or DEFAULT_TIMEOUT)
  File "C:\Python38\lib\site-packages\promise\promise.py", line 506, in _wait
    self.wait(self, timeout)
  File "C:\Python38\lib\site-packages\promise\promise.py", line 502, in wait
    async_instance.wait(promise, timeout)
  File "C:\Python38\lib\site-packages\promise\async_.py", line 117, in wait
    target.scheduler.wait(target, timeout)
  File "C:\Python38\lib\site-packages\promise\schedulers\immediate.py", line 24, in wait
    promise._then(on_resolve_or_reject, on_resolve_or_reject)
  File "C:\Python38\lib\site-packages\promise\promise.py", line 577, in _then
    target._add_callbacks(did_fulfill, did_reject, promise)
  File "C:\Python38\lib\site-packages\promise\promise.py", line 414, in _add_callbacks
    assert not self._rejection_handler0"
AssertionError

Traceback indicates that assertion failed in context on Worker thread.

I was able to create (sort of…) reproduction of this issue: https://github.com/Novakov/promise/tree/race-condition Unfortunately it’s not great. Running test https://github.com/Novakov/promise/blob/race-condition/tests/test_thread_safety.py#L122 shows the issue in most cases.

Investigation of Promise state suggests that in line https://github.com/syrusakbary/promise/blob/master/promise/promise.py#L577 promise is still pending, than context switch happens, promise gets resolved in loop thread, context switch happends again and _add_callbacks continues on now resolved promise.

Machine details OS: Windows 10 x64 CPU: 4 cores, 8 logical threads Python: 3.8.1 (tags/v3.8.1:1b293b6, Dec 18 2019, 23:11:46) [MSC v.1916 64 bit (AMD64)]

I was not able to reproduce the issue on Linux machine, however it maybe matter of probability

[alex-verve]

I think I fixed the problem. Review is welcome https://github.com/syrusakbary/promise/pull/89

[alex-verve]

Also getting these errors (14 times per 2.5 million requests)

  File "/var/task/graphql/execution/executor.py", line 218, in complete_value_catching_error
    completed = complete_value(exe_context, return_type, field_asts, info, result)
, 
  File "/var/task/graphql/execution/executor.py", line 262, in complete_value
    lambda error: Promise.rejected(GraphQLLocatedError(field_asts, original_error=error))
, 
  File "/var/task/promise/promise.py", line 630, in then
    return self._then(did_fulfill, did_reject)
, 
  File "/var/task/promise/promise.py", line 577, in _then
    target._add_callbacks(did_fulfill, did_reject, promise)
, 
  File "/var/task/promise/promise.py", line 414, in _add_callbacks
    assert not self._rejection_handler0

We’re executing Threads with callbacks which resolve promise (similar to @Novakov case)