Very first cut of calculating actions for events as they come in. Doesn't store them yet. Not very efficient.

synapse/storage/registration.py

@@ -291,6 +291,18 @@ class RegistrationStore(SQLBaseStore):
             defer.returnValue(ret['user_id'])
         defer.returnValue(None)
 
+    @defer.inlineCallbacks
+    def get_all_user_ids(self):
+        """Returns all user ids registered on this homeserver"""
+        return self.runInteraction(
+            "get_all_user_ids",
+            self._get_all_user_ids_txn
+        )
+
+    def _get_all_user_ids_txn(self, txn):
+        txn.execute("SELECT name from users")
+        return [r[0] for r in txn.fetchall()]
+
     @defer.inlineCallbacks
     def count_all_users(self):
         """Counts all users registered on the homeserver."""