bugfix: calculate the PL for non-creators correctly in v11 rooms (#18547)

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

changelog.d/18547.bugfix

@@ -0,0 +1 @@
+Fix an issue where during state resolution for v11 rooms Synapse would incorrectly calculate the power level of the creator when there was no power levels event in the room.

synapse/state/v2.py

@@ -255,7 +255,7 @@ async def _get_power_level_for_sender(
             )
             if aev and (aev.type, aev.state_key) == (EventTypes.Create, ""):
                 creator = (
-                    event.sender
+                    aev.sender
                     if event.room_version.implicit_room_creator
                     else aev.content.get("creator")
                 )

tests/state/test_v2.py

@@ -961,25 +961,47 @@ class AuthChainDifferenceTestCase(unittest.TestCase):
                     f"wrong pl for {user_id} on v{room_version.identifier}",
                 )
 
-            # the creator alone without PL is 100
-            got_creator_pl = self.successResultOf(
-                defer.ensureDeferred(
-                    _get_power_level_for_sender(
-                        ROOM_ID,
-                        member_event.event_id,
-                        {
-                            member_event.event_id: member_event,
-                            create_event.event_id: create_event,
-                        },
-                        store,
+            # the creator alone without PL is 100, everyone else is 0
+            want_pls = {
+                ALICE: 100,
+                BOB: 0,
+                CHARLIE: 0,
+            }
+            for user_id, want_pl in want_pls.items():
+                test_event = make_event_from_dict(
+                    {
+                        "room_id": ROOM_ID,
+                        "sender": user_id,
+                        "type": EventTypes.Topic,
+                        "state_key": "",
+                        "content": {"topic": "Test"},
+                        "auth_events": [
+                            create_event.event_id,
+                            member_event.event_id,
+                            pl_event.event_id,
+                        ],
+                        "prev_events": [pl_event.event_id],
+                    },
+                    room_version,
+                )
+                got_pl = self.successResultOf(
+                    defer.ensureDeferred(
+                        _get_power_level_for_sender(
+                            ROOM_ID,
+                            test_event.event_id,
+                            {
+                                test_event.event_id: test_event,
+                                create_event.event_id: create_event,
+                            },
+                            store,
+                        )
                     )
                 )
-            )
-            self.assertEqual(
-                got_creator_pl,
-                100,
-                f"wrong pl for creator with no PL event on v{room_version.identifier}",
-            )
+                self.assertEqual(
+                    got_pl,
+                    want_pl,
+                    f"wrong pl for {user_id} with no PL event on v{room_version.identifier}",
+                )
 
 
 T = TypeVar("T")