Host cache_joined_hosts_for_event to caller (#10986)

`_check_event_auth` is only called in two places, and only one of those sets
`send_on_behalf_of`. Warming the cache isn't really part of auth anyway, so
moving it out makes a lot more sense.

changelog.d/10986.misc

@@ -0,0 +1 @@
+Clean up some of the federation event authentication code for clarity.

synapse/handlers/federation_event.py

@@ -356,6 +356,11 @@ async def on_send_membership_event(
                 )
 
         # all looks good, we can persist the event.
+
+        # First, precalculate the joined hosts so that the federation sender doesn't
+        # need to.
+        await self._event_creation_handler.cache_joined_hosts_for_event(event, context)
+
         await self._run_push_actions_and_persist_event(event, context)
         return event, context
 
@@ -1299,17 +1304,10 @@ async def _check_event_auth(
         except AuthError as e:
             logger.warning("Failed auth resolution for %r because %s", event, e)
             context.rejected = RejectedReason.AUTH_ERROR
+            return context
 
-        if not context.rejected:
-            await self._check_for_soft_fail(event, state, backfilled, origin=origin)
-            await self._maybe_kick_guest_users(event)
-
-        # If we are going to send this event over federation we precaclculate
-        # the joined hosts.
-        if event.internal_metadata.get_send_on_behalf_of():
-            await self._event_creation_handler.cache_joined_hosts_for_event(
-                event, context
-            )
+        await self._check_for_soft_fail(event, state, backfilled, origin=origin)
+        await self._maybe_kick_guest_users(event)
 
         return context