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[3.9] bpo-45997: Fix asyncio.Semaphore re-acquiring order (GH-31910) (GH-32049)

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Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>.
(cherry picked from commit 32e77154dd)

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>
This commit is contained in:
Andrew Svetlov 2022-03-22 17:16:27 +02:00 • committed by GitHub
parent 2d5e9f8d62
commit f47984b560
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3 changed files with 36 additions and 6 deletions
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15
Lib/asyncio/locks.py
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@ -378,6 +378,7 @@ class Semaphore(_ContextManagerMixin):
warnings.warn("The loop argument is deprecated since Python 3.8, "
"and scheduled for removal in Python 3.10.",
DeprecationWarning, stacklevel=2)
self._wakeup_scheduled = False
def __repr__(self):
res = super().__repr__()
@ -391,6 +392,7 @@ class Semaphore(_ContextManagerMixin):
waiter = self._waiters.popleft()
if not waiter.done():
waiter.set_result(None)
self._wakeup_scheduled = True
return
def locked(self):
@ -406,16 +408,17 @@ class Semaphore(_ContextManagerMixin):
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._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():
self._wake_up_next()
# reset _wakeup_scheduled *after* waiting for a future
self._wakeup_scheduled = False
except exceptions.CancelledError:
self._wake_up_next()
raise
self._value -= 1
return True