Well, CREATE TRIGGER's lock seems to be already enough.

And Postgres has LOCK TABLE cmd anyway.

Author: arssher

pg_shardman--0.0.1.sql

@@ -407,8 +407,6 @@ $$ LANGUAGE plpgsql;
 CREATE FUNCTION readonly_table_on(relation regclass)
 	RETURNS void AS $$
 BEGIN
-	-- Grab exclusive lock on table to finish existing transactions
-	PERFORM shardman.ae_lock_table(relation);
 	-- Create go away trigger to prevent any new ones
 	PERFORM shardman.readonly_table_off(relation);
 	EXECUTE format(

src/udf.c

@@ -269,15 +269,3 @@ pq_conninfo_parse(PG_FUNCTION_ARGS)
 	PQconninfoFree(opts);
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
-
-/*
- * Obtain AccessExclusiveLock on table
- */
-PG_FUNCTION_INFO_V1(ae_lock_table);
-Datum
-ae_lock_table(PG_FUNCTION_ARGS)
-{
-	Oid relid = PG_GETARG_OID(0);
-	LockRelationOid(relid, AccessExclusiveLock);
-	PG_RETURN_VOID();
-}