Limit concurrent AS joins (#11996)

Initially introduced in matrix-org-hotfixes by e5537cf (and tweaked by later commits).

Fixes #11995

See also #4826

changelog.d/11996.misc

@@ -0,0 +1 @@
+Limit concurrent joins from applications services.

synapse/handlers/room_member.py

@@ -82,6 +82,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self.event_auth_handler = hs.get_event_auth_handler()
 
         self.member_linearizer: Linearizer = Linearizer(name="member")
+        self.member_as_limiter = Linearizer(max_count=10, name="member_as_limiter")
 
         self.clock = hs.get_clock()
         self.spam_checker = hs.get_spam_checker()
@@ -500,6 +501,13 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
 
         key = (room_id,)
 
+        as_id = object()
+        if requester.app_service:
+            as_id = requester.app_service.id
+
+        # We first linearise by the application service (to try to limit concurrent joins
+        # by application services), and then by room ID.
+        with (await self.member_as_limiter.queue(as_id)):
             with (await self.member_linearizer.queue(key)):
                 result = await self.update_membership_locked(
                     requester,