Fix run_as_background_process not be awaited properly causing LoggingContext problems (#18938)

Basically, searching for any instance of
`run_as_background_process(...)` and making sure we wrap the deferred in
`make_deferred_yieldable(...)` if we try to `await` the result to make
it follow the [Synapse logcontext
rules](https://github.com/element-hq/synapse/blob/develop/docs/log_contexts.md).

Part of #18905

Author: MadLittleMods

changelog.d/18938.misc

@@ -0,0 +1 @@
+Fix `run_as_background_process` not be awaited properly causing `LoggingContext` problems in experimental [MSC4140](https://github.com/matrix-org/matrix-spec-proposals/pull/4140): Delayed events implementation.

synapse/handlers/delayed_events.py

@@ -21,9 +21,12 @@
 from synapse.api.errors import ShadowBanError
 from synapse.api.ratelimiting import Ratelimiter
 from synapse.config.workers import MAIN_PROCESS_INSTANCE_NAME
+from synapse.logging.context import make_deferred_yieldable
 from synapse.logging.opentracing import set_tag
 from synapse.metrics import SERVER_NAME_LABEL, event_processing_positions
-from synapse.metrics.background_process_metrics import run_as_background_process
+from synapse.metrics.background_process_metrics import (
+    run_as_background_process,
+)
 from synapse.replication.http.delayed_events import (
     ReplicationAddedDelayedEventRestServlet,
 )
@@ -328,7 +331,7 @@ async def cancel(self, requester: Requester, delay_id: str) -> None:
             requester,
             (requester.user.to_string(), requester.device_id),
         )
-        await self._initialized_from_db
+        await make_deferred_yieldable(self._initialized_from_db)
 
         next_send_ts = await self._store.cancel_delayed_event(
             delay_id=delay_id,
@@ -354,7 +357,7 @@ async def restart(self, requester: Requester, delay_id: str) -> None:
             requester,
             (requester.user.to_string(), requester.device_id),
         )
-        await self._initialized_from_db
+        await make_deferred_yieldable(self._initialized_from_db)
 
         next_send_ts = await self._store.restart_delayed_event(
             delay_id=delay_id,
@@ -380,7 +383,7 @@ async def send(self, requester: Requester, delay_id: str) -> None:
         # Use standard request limiter for sending delayed events on-demand,
         # as an on-demand send is similar to sending a regular event.
         await self._request_ratelimiter.ratelimit(requester)
-        await self._initialized_from_db
+        await make_deferred_yieldable(self._initialized_from_db)
 
         event, next_send_ts = await self._store.process_target_delayed_event(
             delay_id=delay_id,