Add storage method have_events

synapse/storage/__init__.py

@@ -422,6 +422,35 @@ class DataStore(RoomMemberStore, RoomStore,
             ],
         )
 
+    def have_events(self, event_ids):
+        """Given a list of event ids, check if we have already processed them.
+
+        Returns:
+            dict: Has an entry for each event id we already have seen. Maps to
+            the rejected reason string if we rejected the event, else maps to
+            None.
+        """
+        def f(txn):
+            sql = (
+                "SELECT e.event_id, reason FROM events as e "
+                "LEFT JOIN rejections as r ON e.event_id = r.event_id "
+                "WHERE event_id = ?"
+            )
+
+            res = {}
+            for event_id in event_ids:
+                txn.execute(sql, (event_id,))
+                row = txn.fetchone()
+                if row:
+                    _, rejected = row
+                    res[event_id] = rejected
+
+            return res
+
+        return self.runInteraction(
+            "have_events", f,
+        )
+
 
 def schema_path(schema):
     """ Get a filesystem path for the named database schema