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

diff --git a/src/test/regress/expected/create_am.out b/src/test/regress/expected/create_am.out
index 84da403afc5db8832012056dafc8a7d68b0f37e0..b9dc82dd3c3a324cf10fc401fa1a5451c630fd87 100644 (file)
--- a/src/test/regress/expected/create_am.out
+++ b/src/test/regress/expected/create_am.out
@@ -102,8 +102,13 @@ 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;
 --
 -- Test table access methods
 --

diff --git a/src/test/regress/sql/create_am.sql b/src/test/regress/sql/create_am.sql
index a7f6de7e9be42a2cdff1471a4e7dac93989f9e3a..97df244d172a0902be6a6f2bf4b482ac0f820667 100644 (file)
--- a/src/test/regress/sql/create_am.sql
+++ b/src/test/regress/sql/create_am.sql
@@ -70,7 +70,12 @@ 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;
 
 
 --