Keep track when we try and fail to process a pulled event (#13589)

We can follow-up this PR with:

 1. Only try to backfill from an event if we haven't tried recently -> https://github.com/matrix-org/synapse/issues/13622
 1. When we decide to backfill that event again, process it in the background so it doesn't block and make `/messages` slow when we know it will probably fail again -> https://github.com/matrix-org/synapse/issues/13623
 1. Generally track failures everywhere we try and fail to pull an event over federation -> https://github.com/matrix-org/synapse/issues/13700

Fix https://github.com/matrix-org/synapse/issues/13621

Part of https://github.com/matrix-org/synapse/issues/13356

Mentioned in [internal doc](https://docs.google.com/document/d/1lvUoVfYUiy6UaHB6Rb4HicjaJAU40-APue9Q4vzuW3c/edit#bookmark=id.qv7cj51sv9i5)

1 files changed, 222 insertions, 0 deletions

diff --git a/tests/handlers/test_federation_event.py b/tests/handlers/test_federation_event.py
index 51c8dd6498..b5b89405a4 100644
--- a/tests/handlers/test_federation_event.py
+++ b/tests/handlers/test_federation_event.py
@@ -227,3 +227,225 @@ class FederationEventHandlerTests(unittest.FederatingHomeserverTestCase):
 
         if prev_exists_as_outlier:
             self.mock_federation_transport_client.get_event.assert_not_called()
+
+    def test_process_pulled_event_records_failed_backfill_attempts(
+        self,
+    ) -> None:
+        """
+        Test to make sure that failed backfill attempts for an event are
+        recorded in the `event_failed_pull_attempts` table.
+
+        In this test, we pretend we are processing a "pulled" event via
+        backfill. The pulled event has a fake `prev_event` which our server has
+        obviously never seen before so it attempts to request the state at that
+        `prev_event` which expectedly fails because it's a fake event. Because
+        the server can't fetch the state at the missing `prev_event`, the
+        "pulled" event fails the history check and is fails to process.
+
+        We check that we correctly record the number of failed pull attempts
+        of the pulled event and as a sanity check, that the "pulled" event isn't
+        persisted.
+        """
+        OTHER_USER = f"@user:{self.OTHER_SERVER_NAME}"
+        main_store = self.hs.get_datastores().main
+
+        # Create the room
+        user_id = self.register_user("kermit", "test")
+        tok = self.login("kermit", "test")
+        room_id = self.helper.create_room_as(room_creator=user_id, tok=tok)
+        room_version = self.get_success(main_store.get_room_version(room_id))
+
+        # We expect an outbound request to /state_ids, so stub that out
+        self.mock_federation_transport_client.get_room_state_ids.return_value = make_awaitable(
+            {
+                # Mimic the other server not knowing about the state at all.
+                # We want to cause Synapse to throw an error (`Unable to get
+                # missing prev_event $fake_prev_event`) and fail to backfill
+                # the pulled event.
+                "pdu_ids": [],
+                "auth_chain_ids": [],
+            }
+        )
+        # We also expect an outbound request to /state
+        self.mock_federation_transport_client.get_room_state.return_value = make_awaitable(
+            StateRequestResponse(
+                # Mimic the other server not knowing about the state at all.
+                # We want to cause Synapse to throw an error (`Unable to get
+                # missing prev_event $fake_prev_event`) and fail to backfill
+                # the pulled event.
+                auth_events=[],
+                state=[],
+            )
+        )
+
+        pulled_event = make_event_from_dict(
+            self.add_hashes_and_signatures_from_other_server(
+                {
+                    "type": "test_regular_type",
+                    "room_id": room_id,
+                    "sender": OTHER_USER,
+                    "prev_events": [
+                        # The fake prev event will make the pulled event fail
+                        # the history check (`Unable to get missing prev_event
+                        # $fake_prev_event`)
+                        "$fake_prev_event"
+                    ],
+                    "auth_events": [],
+                    "origin_server_ts": 1,
+                    "depth": 12,
+                    "content": {"body": "pulled"},
+                }
+            ),
+            room_version,
+        )
+
+        # The function under test: try to process the pulled event
+        with LoggingContext("test"):
+            self.get_success(
+                self.hs.get_federation_event_handler()._process_pulled_event(
+                    self.OTHER_SERVER_NAME, pulled_event, backfilled=True
+                )
+            )
+
+        # Make sure our failed pull attempt was recorded
+        backfill_num_attempts = self.get_success(
+            main_store.db_pool.simple_select_one_onecol(
+                table="event_failed_pull_attempts",
+                keyvalues={"event_id": pulled_event.event_id},
+                retcol="num_attempts",
+            )
+        )
+        self.assertEqual(backfill_num_attempts, 1)
+
+        # The function under test: try to process the pulled event again
+        with LoggingContext("test"):
+            self.get_success(
+                self.hs.get_federation_event_handler()._process_pulled_event(
+                    self.OTHER_SERVER_NAME, pulled_event, backfilled=True
+                )
+            )
+
+        # Make sure our second failed pull attempt was recorded (`num_attempts` was incremented)
+        backfill_num_attempts = self.get_success(
+            main_store.db_pool.simple_select_one_onecol(
+                table="event_failed_pull_attempts",
+                keyvalues={"event_id": pulled_event.event_id},
+                retcol="num_attempts",
+            )
+        )
+        self.assertEqual(backfill_num_attempts, 2)
+
+        # And as a sanity check, make sure the event was not persisted through all of this.
+        persisted = self.get_success(
+            main_store.get_event(pulled_event.event_id, allow_none=True)
+        )
+        self.assertIsNone(
+            persisted,
+            "pulled event that fails the history check should not be persisted at all",
+        )
+
+    def test_process_pulled_event_clears_backfill_attempts_after_being_successfully_persisted(
+        self,
+    ) -> None:
+        """
+        Test to make sure that failed pull attempts
+        (`event_failed_pull_attempts` table) for an event are cleared after the
+        event is successfully persisted.
+
+        In this test, we pretend we are processing a "pulled" event via
+        backfill. The pulled event succesfully processes and the backward
+        extremeties are updated along with clearing out any failed pull attempts
+        for those old extremities.
+
+        We check that we correctly cleared failed pull attempts of the
+        pulled event.
+        """
+        OTHER_USER = f"@user:{self.OTHER_SERVER_NAME}"
+        main_store = self.hs.get_datastores().main
+
+        # Create the room
+        user_id = self.register_user("kermit", "test")
+        tok = self.login("kermit", "test")
+        room_id = self.helper.create_room_as(room_creator=user_id, tok=tok)
+        room_version = self.get_success(main_store.get_room_version(room_id))
+
+        # allow the remote user to send state events
+        self.helper.send_state(
+            room_id,
+            "m.room.power_levels",
+            {"events_default": 0, "state_default": 0},
+            tok=tok,
+        )
+
+        # add the remote user to the room
+        member_event = self.get_success(
+            event_injection.inject_member_event(self.hs, room_id, OTHER_USER, "join")
+        )
+
+        initial_state_map = self.get_success(
+            main_store.get_partial_current_state_ids(room_id)
+        )
+
+        auth_event_ids = [
+            initial_state_map[("m.room.create", "")],
+            initial_state_map[("m.room.power_levels", "")],
+            member_event.event_id,
+        ]
+
+        pulled_event = make_event_from_dict(
+            self.add_hashes_and_signatures_from_other_server(
+                {
+                    "type": "test_regular_type",
+                    "room_id": room_id,
+                    "sender": OTHER_USER,
+                    "prev_events": [member_event.event_id],
+                    "auth_events": auth_event_ids,
+                    "origin_server_ts": 1,
+                    "depth": 12,
+                    "content": {"body": "pulled"},
+                }
+            ),
+            room_version,
+        )
+
+        # Fake the "pulled" event failing to backfill once so we can test
+        # if it's cleared out later on.
+        self.get_success(
+            main_store.record_event_failed_pull_attempt(
+                pulled_event.room_id, pulled_event.event_id, "fake cause"
+            )
+        )
+        # Make sure we have a failed pull attempt recorded for the pulled event
+        backfill_num_attempts = self.get_success(
+            main_store.db_pool.simple_select_one_onecol(
+                table="event_failed_pull_attempts",
+                keyvalues={"event_id": pulled_event.event_id},
+                retcol="num_attempts",
+            )
+        )
+        self.assertEqual(backfill_num_attempts, 1)
+
+        # The function under test: try to process the pulled event
+        with LoggingContext("test"):
+            self.get_success(
+                self.hs.get_federation_event_handler()._process_pulled_event(
+                    self.OTHER_SERVER_NAME, pulled_event, backfilled=True
+                )
+            )
+
+        # Make sure the failed pull attempts for the pulled event are cleared
+        backfill_num_attempts = self.get_success(
+            main_store.db_pool.simple_select_one_onecol(
+                table="event_failed_pull_attempts",
+                keyvalues={"event_id": pulled_event.event_id},
+                retcol="num_attempts",
+                allow_none=True,
+            )
+        )
+        self.assertIsNone(backfill_num_attempts)
+
+        # And as a sanity check, make sure the "pulled" event was persisted.
+        persisted = self.get_success(
+            main_store.get_event(pulled_event.event_id, allow_none=True)
+        )
+        self.assertIsNotNone(persisted, "pulled event was not persisted at all")