Fix rare deadlock failure in create_am regression test.

tglsfdc · tglsfdc · commit 44f4986dd96b · 2020-09-04T12:40:28.000-04:00
The "DROP ACCESS METHOD gist2" test will require locking the index to be dropped and then its table; while most ordinary operations lock a table first then its index. While no concurrent test scripts should be touching fast_emp4000, autovacuum might chance to be processing that table when the DROP runs, resulting in a deadlock failure. This is pretty rare but we see it in the buildfarm from time to time. To fix, acquire a lock on fast_emp4000 before issuing the DROP. Since the point of the exercise is mostly to prevent buildfarm failures, back-patch to 9.6 where this test was introduced. Discussion: https://postgr.es/m/839004.1599185607@sss.pgh.pa.us
diff --git a/src/test/regress/expected/create_am.out b/src/test/regress/expected/create_am.out
@@ -99,5 +99,10 @@ ERROR:  cannot drop access method gist2 because other objects depend on it
 DETAIL:  index grect2ind2 depends on operator class box_ops for access method gist2
 HINT:  Use DROP ... CASCADE to drop the dependent objects too.
 -- Drop access method cascade
+-- To prevent a (rare) deadlock against autovacuum,
+-- we must lock the table that owns the index that will be dropped
+BEGIN;
+LOCK TABLE fast_emp4000;
 DROP ACCESS METHOD gist2 CASCADE;
 NOTICE:  drop cascades to index grect2ind2
+COMMIT;
diff --git a/src/test/regress/sql/create_am.sql b/src/test/regress/sql/create_am.sql
@@ -67,4 +67,9 @@ ROLLBACK;
 DROP ACCESS METHOD gist2;
 
 -- Drop access method cascade
+-- To prevent a (rare) deadlock against autovacuum,
+-- we must lock the table that owns the index that will be dropped
+BEGIN;
+LOCK TABLE fast_emp4000;
 DROP ACCESS METHOD gist2 CASCADE;
+COMMIT;
