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

Signed-off-by: Sean Quah <seanq@element.io>

1 files changed, 4 insertions, 1 deletions

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()