Ensure fed-sender catchup does not block for full state (#15248)

* Reproduce bad scenario in test
* Avoid catchup optimisation for partial state rooms

tests/federation/test_federation_catch_up.py

@@ -1,4 +1,5 @@
-from typing import Callable, List, Optional, Tuple
+from typing import Callable, Collection, List, Optional, Tuple
+from unittest import mock
 from unittest.mock import Mock
 
 from twisted.test.proto_helpers import MemoryReactor
@@ -500,3 +501,87 @@ class FederationCatchUpTestCases(FederatingHomeserverTestCase):
         self.assertEqual(len(sent_pdus), 1)
         self.assertEqual(sent_pdus[0].event_id, event_2.event_id)
         self.assertFalse(per_dest_queue._catching_up)
+
+    def test_catch_up_is_not_blocked_by_remote_event_in_partial_state_room(
+        self,
+    ) -> None:
+        """Detects (part of?) https://github.com/matrix-org/synapse/issues/15220."""
+        # ARRANGE:
+        # - a local user (u1)
+        # - a room which contains u1 and two remote users, @u2:host2 and @u3:other
+        # - events in that room such that
+        #   - history visibility is restricted
+        #   - u1 sent message events e1 and e2
+        #   - afterwards, u3 sent a remote event e3
+        # - catchup to begin for host2; last successfully sent event was e1
+        per_dest_queue, sent_pdus = self.make_fake_destination_queue()
+
+        self.register_user("u1", "you the one")
+        u1_token = self.login("u1", "you the one")
+        room = self.helper.create_room_as("u1", tok=u1_token)
+        self.helper.send_state(
+            room_id=room,
+            event_type="m.room.history_visibility",
+            body={"history_visibility": "joined"},
+            tok=u1_token,
+        )
+        self.get_success(
+            event_injection.inject_member_event(self.hs, room, "@u2:host2", "join")
+        )
+        self.get_success(
+            event_injection.inject_member_event(self.hs, room, "@u3:other", "join")
+        )
+
+        # create some events
+        event_id_1 = self.helper.send(room, "hello", tok=u1_token)["event_id"]
+        event_id_2 = self.helper.send(room, "world", tok=u1_token)["event_id"]
+        # pretend that u3 changes their displayname
+        event_id_3 = self.get_success(
+            event_injection.inject_member_event(self.hs, room, "@u3:other", "join")
+        ).event_id
+
+        # destination_rooms should already be populated, but let us pretend that we already
+        # sent (successfully) up to and including event id 1
+        event_1 = self.get_success(self.hs.get_datastores().main.get_event(event_id_1))
+        assert event_1.internal_metadata.stream_ordering is not None
+        self.get_success(
+            self.hs.get_datastores().main.set_destination_last_successful_stream_ordering(
+                "host2", event_1.internal_metadata.stream_ordering
+            )
+        )
+
+        # also fetch event 2 so we can compare its stream ordering to the sender's
+        # last_successful_stream_ordering later
+        event_2 = self.get_success(self.hs.get_datastores().main.get_event(event_id_2))
+
+        # Mock event 3 as having partial state
+        self.get_success(
+            event_injection.mark_event_as_partial_state(self.hs, event_id_3, room)
+        )
+
+        # Fail the test if we block on full state for event 3.
+        async def mock_await_full_state(event_ids: Collection[str]) -> None:
+            if event_id_3 in event_ids:
+                raise AssertionError("Tried to await full state for event_id_3")
+
+        # ACT
+        with mock.patch.object(
+            self.hs.get_storage_controllers().state._partial_state_events_tracker,
+            "await_full_state",
+            mock_await_full_state,
+        ):
+            self.get_success(per_dest_queue._catch_up_transmission_loop())
+
+        # ASSERT
+        # We should have:
+        # - not sent event 3: it's not ours, and the room is partial stated
+        # - fallen back to sending event 2: it's the most recent event in the room
+        #   we tried to send to host2
+        # - completed catch-up
+        self.assertEqual(len(sent_pdus), 1)
+        self.assertEqual(sent_pdus[0].event_id, event_id_2)
+        self.assertFalse(per_dest_queue._catching_up)
+        self.assertEqual(
+            per_dest_queue._last_successful_stream_ordering,
+            event_2.internal_metadata.stream_ordering,
+        )