Fix waiter cancellation in asyncio.Lock (#1031) (#2037)
Avoid a deadlock when the waiter who is about to take the lock is
cancelled
Issue #27585
diff --git a/Lib/asyncio/locks.py b/Lib/asyncio/locks.py
index deefc93..9266183 100644
--- a/Lib/asyncio/locks.py
+++ b/Lib/asyncio/locks.py
@@ -176,6 +176,10 @@
yield from fut
self._locked = True
return True
+ except futures.CancelledError:
+ if not self._locked:
+ self._wake_up_first()
+ raise
finally:
self._waiters.remove(fut)
@@ -192,14 +196,17 @@
"""
if self._locked:
self._locked = False
- # Wake up the first waiter who isn't cancelled.
- for fut in self._waiters:
- if not fut.done():
- fut.set_result(True)
- break
+ self._wake_up_first()
else:
raise RuntimeError('Lock is not acquired.')
+ def _wake_up_first(self):
+ """Wake up the first waiter who isn't cancelled."""
+ for fut in self._waiters:
+ if not fut.done():
+ fut.set_result(True)
+ break
+
class Event:
"""Asynchronous equivalent to threading.Event.
diff --git a/Lib/test/test_asyncio/test_locks.py b/Lib/test/test_asyncio/test_locks.py
index 152948c..c85e8b1 100644
--- a/Lib/test/test_asyncio/test_locks.py
+++ b/Lib/test/test_asyncio/test_locks.py
@@ -176,6 +176,28 @@
self.assertTrue(tb.cancelled())
self.assertTrue(tc.done())
+ def test_finished_waiter_cancelled(self):
+ lock = asyncio.Lock(loop=self.loop)
+
+ ta = asyncio.Task(lock.acquire(), loop=self.loop)
+ test_utils.run_briefly(self.loop)
+ self.assertTrue(lock.locked())
+
+ tb = asyncio.Task(lock.acquire(), loop=self.loop)
+ test_utils.run_briefly(self.loop)
+ self.assertEqual(len(lock._waiters), 1)
+
+ # Create a second waiter, wake up the first, and cancel it.
+ # Without the fix, the second was not woken up.
+ tc = asyncio.Task(lock.acquire(), loop=self.loop)
+ lock.release()
+ tb.cancel()
+ test_utils.run_briefly(self.loop)
+
+ self.assertTrue(lock.locked())
+ self.assertTrue(ta.done())
+ self.assertTrue(tb.cancelled())
+
def test_release_not_acquired(self):
lock = asyncio.Lock(loop=self.loop)