diff options
| -rw-r--r-- | changelog.d/12105.bugfix | 1 | ||||
| -rw-r--r-- | synapse/util/async_helpers.py | 5 | ||||
| -rw-r--r-- | tests/util/test_rwlock.py | 30 |
3 files changed, 35 insertions, 1 deletions
diff --git a/changelog.d/12105.bugfix b/changelog.d/12105.bugfix new file mode 100644 index 0000000000..f42e63e01f --- /dev/null +++ b/changelog.d/12105.bugfix @@ -0,0 +1 @@ +Fix an extremely rare, long-standing bug in `ReadWriteLock` that would cause an error when a newly unblocked writer completes instantly. diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py index 81320b8972..60c03a66fd 100644 --- a/synapse/util/async_helpers.py +++ b/synapse/util/async_helpers.py @@ -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() diff --git a/tests/util/test_rwlock.py b/tests/util/test_rwlock.py index a10071c70f..0774625b85 100644 --- a/tests/util/test_rwlock.py +++ b/tests/util/test_rwlock.py @@ -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) |