Stop `get_joined_users` corruption from custom statuses (#7376)

Fix a bug where the `get_joined_users` cache could be corrupted by custom
status events (or other state events with a state_key matching the user ID).

The bug was introduced by #2229, but has largely gone unnoticed since then.

Fixes #7099, #7373.

changelog.d/7376.bugfix

@@ -0,0 +1 @@
+Fix a bug which could cause messages not to be sent over federation, when state events with state keys matching user IDs (such as custom user statuses) were received.

synapse/storage/data_stores/main/roommember.py

@@ -576,7 +576,8 @@ class RoomMemberWorkerStore(EventsWorkerStore):
                         if key[0] == EventTypes.Member
                     ]
                     for etype, state_key in context.delta_ids:
-                        users_in_room.pop(state_key, None)
+                        if etype == EventTypes.Member:
+                            users_in_room.pop(state_key, None)
 
         # We check if we have any of the member event ids in the event cache
         # before we ask the DB

tests/storage/test_roommember.py

@@ -22,6 +22,8 @@ from synapse.rest.client.v1 import login, room
 from synapse.types import Requester, UserID
 
 from tests import unittest
+from tests.test_utils import event_injection
+from tests.utils import TestHomeServer
 
 
 class RoomMemberStoreTestCase(unittest.HomeserverTestCase):
@@ -38,7 +40,7 @@ class RoomMemberStoreTestCase(unittest.HomeserverTestCase):
         )
         return hs
 
-    def prepare(self, reactor, clock, hs):
+    def prepare(self, reactor, clock, hs: TestHomeServer):
 
         # We can't test the RoomMemberStore on its own without the other event
         # storage logic
@@ -114,6 +116,52 @@ class RoomMemberStoreTestCase(unittest.HomeserverTestCase):
         # It now knows about Charlie's server.
         self.assertEqual(self.store._known_servers_count, 2)
 
+    def test_get_joined_users_from_context(self):
+        room = self.helper.create_room_as(self.u_alice, tok=self.t_alice)
+        bob_event = event_injection.inject_member_event(
+            self.hs, room, self.u_bob, Membership.JOIN
+        )
+
+        # first, create a regular event
+        event, context = event_injection.create_event(
+            self.hs,
+            room_id=room,
+            sender=self.u_alice,
+            prev_event_ids=[bob_event.event_id],
+            type="m.test.1",
+            content={},
+        )
+
+        users = self.get_success(
+            self.store.get_joined_users_from_context(event, context)
+        )
+        self.assertEqual(users.keys(), {self.u_alice, self.u_bob})
+
+        # Regression test for #7376: create a state event whose key matches bob's
+        # user_id, but which is *not* a membership event, and persist that; then check
+        # that `get_joined_users_from_context` returns the correct users for the next event.
+        non_member_event = event_injection.inject_event(
+            self.hs,
+            room_id=room,
+            sender=self.u_bob,
+            prev_event_ids=[bob_event.event_id],
+            type="m.test.2",
+            state_key=self.u_bob,
+            content={},
+        )
+        event, context = event_injection.create_event(
+            self.hs,
+            room_id=room,
+            sender=self.u_alice,
+            prev_event_ids=[non_member_event.event_id],
+            type="m.test.3",
+            content={},
+        )
+        users = self.get_success(
+            self.store.get_joined_users_from_context(event, context)
+        )
+        self.assertEqual(users.keys(), {self.u_alice, self.u_bob})
+
 
 class CurrentStateMembershipUpdateTestCase(unittest.HomeserverTestCase):
     def prepare(self, reactor, clock, homeserver):