Skip processing stats for broken rooms. (#14873)

* Skip processing stats for broken rooms.

* Newsfragment

* Use a custom exception.

4 files changed, 72 insertions, 36 deletions

diff --git a/changelog.d/14873.bugfix b/changelog.d/14873.bugfix
new file mode 100644
index 0000000000..9b058576cd
--- /dev/null
+++ b/changelog.d/14873.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where the `populate_room_stats` background job could fail on broken rooms.
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index 24127d0364..f42af34a2f 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -110,6 +110,10 @@ event_fetch_ongoing_gauge = Gauge(
 )
 
 
+class InvalidEventError(Exception):
+    """The event retrieved from the database is invalid and cannot be used."""
+
+
 @attr.s(slots=True, auto_attribs=True)
 class EventCacheEntry:
     event: EventBase
@@ -1310,7 +1314,7 @@ class EventsWorkerStore(SQLBaseStore):
                 # invites, so just accept it for all membership events.
                 #
                 if d["type"] != EventTypes.Member:
-                    raise Exception(
+                    raise InvalidEventError(
                         "Room %s for event %s is unknown" % (d["room_id"], event_id)
                     )
 
diff --git a/synapse/storage/databases/main/stats.py b/synapse/storage/databases/main/stats.py
index 356d4ca788..0c1cbd540d 100644
--- a/synapse/storage/databases/main/stats.py
+++ b/synapse/storage/databases/main/stats.py
@@ -29,6 +29,7 @@ from synapse.storage.database import (
     LoggingDatabaseConnection,
     LoggingTransaction,
 )
+from synapse.storage.databases.main.events_worker import InvalidEventError
 from synapse.storage.databases.main.state_deltas import StateDeltasStore
 from synapse.types import JsonDict
 from synapse.util.caches.descriptors import cached
@@ -554,7 +555,17 @@ class StatsStore(StateDeltasStore):
             "get_initial_state_for_room", _fetch_current_state_stats
         )
 
-        state_event_map = await self.get_events(event_ids, get_prev_content=False)  # type: ignore[attr-defined]
+        try:
+            state_event_map = await self.get_events(event_ids, get_prev_content=False)  # type: ignore[attr-defined]
+        except InvalidEventError as e:
+            # If an exception occurs fetching events then the room is broken;
+            # skip process it to avoid being stuck on a room.
+            logger.warning(
+                "Failed to fetch events for room %s, skipping stats calculation: %r.",
+                room_id,
+                e,
+            )
+            return
 
         room_state: Dict[str, Union[None, bool, str]] = {
             "join_rules": None,
diff --git a/tests/storage/databases/main/test_room.py b/tests/storage/databases/main/test_room.py
index 7d961fac64..3108ca3444 100644
--- a/tests/storage/databases/main/test_room.py
+++ b/tests/storage/databases/main/test_room.py
@@ -40,9 +40,23 @@ class RoomBackgroundUpdateStoreTestCase(HomeserverTestCase):
         self.token = self.login("foo", "pass")
 
     def _generate_room(self) -> str:
-        room_id = self.helper.create_room_as(self.user_id, tok=self.token)
+        """Create a room and return the room ID."""
+        return self.helper.create_room_as(self.user_id, tok=self.token)
 
-        return room_id
+    def run_background_updates(self, update_name: str) -> None:
+        """Insert and run the background update."""
+        self.get_success(
+            self.store.db_pool.simple_insert(
+                "background_updates",
+                {"update_name": update_name, "progress_json": "{}"},
+            )
+        )
+
+        # ... and tell the DataStore that it hasn't finished all updates yet
+        self.store.db_pool.updates._all_done = False
+
+        # Now let's actually drive the updates to completion
+        self.wait_for_background_updates()
 
     def test_background_populate_rooms_creator_column(self) -> None:
         """Test that the background update to populate the rooms creator column
@@ -71,22 +85,7 @@ class RoomBackgroundUpdateStoreTestCase(HomeserverTestCase):
         )
         self.assertEqual(room_creator_before, None)
 
-        # Insert and run the background update.
-        self.get_success(
-            self.store.db_pool.simple_insert(
-                "background_updates",
-                {
-                    "update_name": _BackgroundUpdates.POPULATE_ROOMS_CREATOR_COLUMN,
-                    "progress_json": "{}",
-                },
-            )
-        )
-
-        # ... and tell the DataStore that it hasn't finished all updates yet
-        self.store.db_pool.updates._all_done = False
-
-        # Now let's actually drive the updates to completion
-        self.wait_for_background_updates()
+        self.run_background_updates(_BackgroundUpdates.POPULATE_ROOMS_CREATOR_COLUMN)
 
         # Make sure the background update filled in the room creator
         room_creator_after = self.get_success(
@@ -137,22 +136,7 @@ class RoomBackgroundUpdateStoreTestCase(HomeserverTestCase):
             )
         )
 
-        # Insert and run the background update
-        self.get_success(
-            self.store.db_pool.simple_insert(
-                "background_updates",
-                {
-                    "update_name": _BackgroundUpdates.ADD_ROOM_TYPE_COLUMN,
-                    "progress_json": "{}",
-                },
-            )
-        )
-
-        # ... and tell the DataStore that it hasn't finished all updates yet
-        self.store.db_pool.updates._all_done = False
-
-        # Now let's actually drive the updates to completion
-        self.wait_for_background_updates()
+        self.run_background_updates(_BackgroundUpdates.ADD_ROOM_TYPE_COLUMN)
 
         # Make sure the background update filled in the room type
         room_type_after = self.get_success(
@@ -164,3 +148,39 @@ class RoomBackgroundUpdateStoreTestCase(HomeserverTestCase):
             )
         )
         self.assertEqual(room_type_after, RoomTypes.SPACE)
+
+    def test_populate_stats_broken_rooms(self) -> None:
+        """Ensure that re-populating room stats skips broken rooms."""
+
+        # Create a good room.
+        good_room_id = self._generate_room()
+
+        # Create a room and then break it by having no room version.
+        room_id = self._generate_room()
+        self.get_success(
+            self.store.db_pool.simple_update(
+                table="rooms",
+                keyvalues={"room_id": room_id},
+                updatevalues={"room_version": None},
+                desc="test",
+            )
+        )
+
+        # Nuke any current stats in the database.
+        self.get_success(
+            self.store.db_pool.simple_delete(
+                table="room_stats_state", keyvalues={"1": 1}, desc="test"
+            )
+        )
+
+        self.run_background_updates("populate_stats_process_rooms")
+
+        # Only the good room appears in the stats tables.
+        results = self.get_success(
+            self.store.db_pool.simple_select_onecol(
+                table="room_stats_state",
+                keyvalues={},
+                retcol="room_id",
+            )
+        )
+        self.assertEqual(results, [good_room_id])