Fix rare deadlock failure in create_am regression test.

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

2 files changed, 10 insertions, 0 deletions

diff --git a/src/test/regress/expected/create_am.out b/src/test/regress/expected/create_am.out
index 1b464aae2dc..bbd69187ac7 100644
--- 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
index 2f116d98c77..d4463c92559 100644
--- 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;