Allow MSC4155 to work if m.invite_permission_config was previously set (#20476)

Allow MSC4155 to work if m.invite_permission_config was previously set

Fixes #20474

changelog.d/20476.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.149.0 where an empty `m.invite_permission_config` account data event would take precedence over [MSC4155](https://github.com/matrix-org/matrix-spec-proposals/pull/4155) invite filtering rules.

synapse/storage/databases/main/account_data.py

@@ -575,9 +575,9 @@ class AccountDataWorkerStore(PushRulesWorkerStore, CacheInvalidationWorkerStore)
         data = await self.get_global_account_data_by_type_for_user(
             user_id, AccountDataTypes.INVITE_PERMISSION_CONFIG
         )
-        # If the user has an MSC4380-style config setting, prioritise that
-        # above an MSC4155 one
-        if data is not None:
+        # If the user has an MSC4380-style config setting AND has indicated
+        # a preference within it (no empty dict), prioritise that above an MSC4155 one.
+        if data:
             return MSC4380InviteRulesConfig.from_account_data(data)
 
         if self._msc4155_enabled:

tests/handlers/test_room_member.py

@@ -680,6 +680,39 @@ class TestMSC4380InviteBlocking(FederatingHomeserverTestCase):
             )
         )
 
+    @override_config({"experimental_features": {"msc4155_enabled": True}})
+    def test_msc4380_empty_config_falls_back_to_msc4155(self) -> None:
+        """Test that an empty MSC4380 config does not take precedence over an
+        MSC4155 config."""
+        room_id = self.helper.create_room_as(self.alice, tok=self.alice_token)
+
+        self.get_success(
+            self.store.add_account_data_for_user(
+                self.bob,
+                AccountDataTypes.INVITE_PERMISSION_CONFIG,
+                {},
+            )
+        )
+        self.get_success(
+            self.store.add_account_data_for_user(
+                self.bob,
+                AccountDataTypes.MSC4155_INVITE_PERMISSION_CONFIG,
+                {"blocked_users": [self.alice]},
+            )
+        )
+
+        f = self.get_failure(
+            self.handler.update_membership(
+                requester=create_requester(self.alice),
+                target=UserID.from_string(self.bob),
+                room_id=room_id,
+                action=Membership.INVITE,
+            ),
+            SynapseError,
+        ).value
+        self.assertEqual(f.code, 403)
+        self.assertEqual(f.errcode, "M_INVITE_BLOCKED")
+
     def test_msc4380_block_invite_remote(self) -> None:
         """Test that MSC4380 will block a user from being invited to a room by a remote user."""
         # A remote user who sends the invite