Pass through SynapseErrors that are raised from experimental check_event_allowed callback of the module API

changelog.d/11042.bugfix

@@ -0,0 +1 @@
+Work around a regression, introduced in Synapse 1.39.0, that caused `SynapseError`s raised by the experimental Module API callback `check_event_allowed` to be ignored.

synapse/events/third_party_rules.py

@@ -217,6 +217,15 @@ async def check_event_allowed(
  for callback in self._check_event_allowed_callbacks:
  try:
  res, replacement_data = await callback(event, state_events)
+ except SynapseError as e:
+ # HACK. Being able to throw SynapseErrors is relied upon by
+ # some modules. PR #10386 accidentally broke this ability.
+ # That said, we aren't keen on exposing this implementation detail
+ # to modules and we should one day have a proper way to do what
+ # is wanted.
+ # This module API callback needs a rework so that hacks such as
+ # this one are not necessary.
+ raise e
  except Exception as e:
  logger.warning("Failed to run module API callback %s: %s", callback, e)
  continue