Fix rare error in ReadWriteLock when writers complete immediately (#12105)

Signed-off-by: Sean Quah <seanq@element.io>
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Sean Quah 2022-03-01 15:27:15 +00:00 committed by GitHub
parent 300ed0b8a6
commit 4d6b6c17c8
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3 changed files with 35 additions and 1 deletions

1
changelog.d/12105.bugfix Normal file
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@ -0,0 +1 @@
Fix an extremely rare, long-standing bug in `ReadWriteLock` that would cause an error when a newly unblocked writer completes instantly.

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@ -555,7 +555,10 @@ class ReadWriteLock:
finally:
with PreserveLoggingContext():
new_defer.callback(None)
if self.key_to_current_writer[key] == new_defer:
# `self.key_to_current_writer[key]` may be missing if there was another
# writer waiting for us and it completed entirely within the
# `new_defer.callback()` call above.
if self.key_to_current_writer.get(key) == new_defer:
self.key_to_current_writer.pop(key)
return _ctx_manager()

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@ -13,6 +13,7 @@
# limitations under the License.
from twisted.internet import defer
from twisted.internet.defer import Deferred
from synapse.util.async_helpers import ReadWriteLock
@ -83,3 +84,32 @@ class ReadWriteLockTestCase(unittest.TestCase):
self.assertTrue(d.called)
with d.result:
pass
def test_lock_handoff_to_nonblocking_writer(self):
"""Test a writer handing the lock to another writer that completes instantly."""
rwlock = ReadWriteLock()
key = "key"
unblock: "Deferred[None]" = Deferred()
async def blocking_write():
with await rwlock.write(key):
await unblock
async def nonblocking_write():
with await rwlock.write(key):
pass
d1 = defer.ensureDeferred(blocking_write())
d2 = defer.ensureDeferred(nonblocking_write())
self.assertFalse(d1.called)
self.assertFalse(d2.called)
# Unblock the first writer. The second writer will complete without blocking.
unblock.callback(None)
self.assertTrue(d1.called)
self.assertTrue(d2.called)
# The `ReadWriteLock` should operate as normal.
d3 = defer.ensureDeferred(nonblocking_write())
self.assertTrue(d3.called)