Split purge API into events vs state

synapse/storage/data_stores/main/state.py

@@ -989,6 +989,29 @@ class StateGroupWorkerStore(
 
         return self.runInteraction("store_state_group", _store_state_group_txn)
 
+    @defer.inlineCallbacks
+    def get_referenced_state_groups(self, state_groups):
+        """Check if the state groups are referenced by events.
+
+        Args:
+            state_groups (Iterable[int])
+
+        Returns:
+            Deferred[set[int]]: The subset of state groups that are
+            referenced.
+        """
+
+        rows = yield self._simple_select_many_batch(
+            table="event_to_state_groups",
+            column="state_group",
+            iterable=state_groups,
+            keyvalues={},
+            retcols=("DISTINCT state_group",),
+            desc="get_referenced_state_groups",
+        )
+
+        return set(row["state_group"] for row in rows)
+
 
 class StateBackgroundUpdateStore(
     StateGroupBackgroundUpdateStore, BackgroundUpdateStore