bpo-45997: Fix asyncio.Semaphore re-acquiring order (GH-31910)

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>
This commit is contained in:
Andrew Svetlov 2022-03-22 16:02:51 +02:00 committed by GitHub
parent 673755bfba
commit 32e77154dd
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3 changed files with 36 additions and 6 deletions

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@ -6,6 +6,7 @@ import collections
from . import exceptions
from . import mixins
from . import tasks
class _ContextManagerMixin:
@ -346,6 +347,7 @@ class Semaphore(_ContextManagerMixin, mixins._LoopBoundMixin):
raise ValueError("Semaphore initial value must be >= 0")
self._value = value
self._waiters = collections.deque()
self._wakeup_scheduled = False
def __repr__(self):
res = super().__repr__()
@ -359,6 +361,7 @@ class Semaphore(_ContextManagerMixin, mixins._LoopBoundMixin):
waiter = self._waiters.popleft()
if not waiter.done():
waiter.set_result(None)
self._wakeup_scheduled = True
return
def locked(self):
@ -374,15 +377,16 @@ class Semaphore(_ContextManagerMixin, mixins._LoopBoundMixin):
called release() to make it larger than 0, and then return
True.
"""
while self._value <= 0:
# _wakeup_scheduled is set if *another* task is scheduled to wakeup
# but its acquire() is not resumed yet
while self._wakeup_scheduled or self._value <= 0:
fut = self._get_loop().create_future()
self._waiters.append(fut)
try:
await fut
except:
# See the similar code in Queue.get.
fut.cancel()
if self._value > 0 and not fut.cancelled():
# reset _wakeup_scheduled *after* waiting for a future
self._wakeup_scheduled = False
except exceptions.CancelledError:
self._wake_up_next()
raise
self._value -= 1

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@ -917,6 +917,31 @@ class SemaphoreTests(unittest.IsolatedAsyncioTestCase):
sem.release()
self.assertFalse(sem.locked())
async def test_acquire_fifo_order(self):
sem = asyncio.Semaphore(1)
result = []
async def coro(tag):
await sem.acquire()
result.append(f'{tag}_1')
await asyncio.sleep(0.01)
sem.release()
await sem.acquire()
result.append(f'{tag}_2')
await asyncio.sleep(0.01)
sem.release()
async with asyncio.TaskGroup() as tg:
tg.create_task(coro('c1'))
tg.create_task(coro('c2'))
tg.create_task(coro('c3'))
self.assertEqual(
['c1_1', 'c2_1', 'c3_1', 'c1_2', 'c2_2', 'c3_2'],
result
)
if __name__ == '__main__':
unittest.main()

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@ -0,0 +1 @@
Fix :class:`asyncio.Semaphore` re-aquiring FIFO order.