diff --git a/tests/storage/test_purge.py b/tests/storage/test_purge.py
index efd8d25bd1..5d6a8518c0 100644
--- a/tests/storage/test_purge.py
+++ b/tests/storage/test_purge.py
@@ -195,3 +195,78 @@ class PurgeTests(HomeserverTestCase):
self.assertEqual(second_state, {})
self.assertEqual(third_state, {})
self.assertNotEqual(last_state, {})
+
+ def test_purge_unreferenced_state_group(self) -> None:
+ """Test that purging a room also gets rid of unreferenced state groups
+ it encounters during the purge.
+
+ This is important, as otherwise these unreferenced state groups get
+ "de-deltaed" during the purge process, consuming lots of disk space.
+ """
+
+ self.helper.send(self.room_id, body="test1")
+ state1 = self.helper.send_state(
+ self.room_id, "org.matrix.test", body={"number": 2}
+ )
+ state2 = self.helper.send_state(
+ self.room_id, "org.matrix.test", body={"number": 3}
+ )
+ self.helper.send(self.room_id, body="test4")
+ last = self.helper.send(self.room_id, body="test5")
+
+ # Create an unreferenced state group that has a prev group of one of the
+ # to-be-purged events.
+ prev_group = self.get_success(
+ self.store._get_state_group_for_event(state1["event_id"])
+ )
+ unreferenced_state_group = self.get_success(
+ self.state_store.store_state_group(
+ event_id=last["event_id"],
+ room_id=self.room_id,
+ prev_group=prev_group,
+ delta_ids={("org.matrix.test", ""): state2["event_id"]},
+ current_state_ids=None,
+ )
+ )
+
+ # Get the topological token
+ token = self.get_success(
+ self.store.get_topological_token_for_event(last["event_id"])
+ )
+ token_str = self.get_success(token.to_string(self.hs.get_datastores().main))
+
+ # Purge everything before this topological token
+ self.get_success(
+ self._storage_controllers.purge_events.purge_history(
+ self.room_id, token_str, True
+ )
+ )
+
+ # Advance so that the background jobs to delete the state groups runs
+ self.reactor.advance(
+ 1 + self.state_deletion_store.DELAY_BEFORE_DELETION_MS / 1000
+ )
+
+ # We expect that the unreferenced state group has been deleted.
+ row = self.get_success(
+ self.state_store.db_pool.simple_select_one_onecol(
+ table="state_groups",
+ keyvalues={"id": unreferenced_state_group},
+ retcol="id",
+ allow_none=True,
+ desc="test_purge_unreferenced_state_group",
+ )
+ )
+ self.assertIsNone(row)
+
+ # We expect there to now only be one state group for the room, which is
+ # the state group of the last event (as the only outlier).
+ state_groups = self.get_success(
+ self.state_store.db_pool.simple_select_onecol(
+ table="state_groups",
+ keyvalues={"room_id": self.room_id},
+ retcol="id",
+ desc="test_purge_unreferenced_state_group",
+ )
+ )
+ self.assertEqual(len(state_groups), 1)
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