Don't cancel Deferreds that readers or writers are waiting on

Author: Sean Quah

synapse/util/async_helpers.py

@@ -513,9 +513,10 @@ async def _ctx_manager() -> AsyncIterator[None]:
             try:
                 # Wait for the latest writer to finish.
                 # May raise a `CancelledError` if the `Deferred` wrapping us is
-                # cancelled.
+                # cancelled. The `Deferred` we are waiting on must not be cancelled,
+                # since we do not own it.
                 if curr_writer:
-                    await make_deferred_yieldable(curr_writer)
+                    await make_deferred_yieldable(stop_cancellation(curr_writer))
                 yield
             finally:
                 with PreserveLoggingContext():
@@ -546,8 +547,9 @@ async def _ctx_manager() -> AsyncIterator[None]:
             try:
                 # Wait for all current readers and the latest writer to finish.
                 # May raise a `CancelledError` if the `Deferred` wrapping us is
-                # cancelled.
-                await make_deferred_yieldable(to_wait_on_defer)
+                # cancelled. The `Deferred`s we are waiting on must not be cancelled,
+                # since we do not own them.
+                await make_deferred_yieldable(stop_cancellation(to_wait_on_defer))
                 yield
             finally: