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Commit44f4986

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Fix rare deadlock failure in create_am regression test.
The "DROP ACCESS METHOD gist2" test will require locking the indexto be dropped and then its table; while most ordinary operationslock a table first then its index. While no concurrent test scriptsshould be touching fast_emp4000, autovacuum might chance to beprocessing that table when the DROP runs, resulting in a deadlockfailure. This is pretty rare but we see it in the buildfarm fromtime to time.To fix, acquire a lock on fast_emp4000 before issuing the DROP.Since the point of the exercise is mostly to prevent buildfarmfailures, back-patch to 9.6 where this test was introduced.Discussion:https://postgr.es/m/839004.1599185607@sss.pgh.pa.us
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‎src/test/regress/expected/create_am.out

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@@ -99,5 +99,10 @@ ERROR: cannot drop access method gist2 because other objects depend on it
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DETAIL: index grect2ind2 depends on operator class box_ops for access method gist2
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HINT: Use DROP ... CASCADE to drop the dependent objects too.
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-- Drop access method cascade
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-- To prevent a (rare) deadlock against autovacuum,
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-- we must lock the table that owns the index that will be dropped
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BEGIN;
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LOCK TABLE fast_emp4000;
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DROP ACCESS METHOD gist2 CASCADE;
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NOTICE: drop cascades to index grect2ind2
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COMMIT;

‎src/test/regress/sql/create_am.sql

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@@ -67,4 +67,9 @@ ROLLBACK;
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DROP ACCESS METHOD gist2;
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-- Drop access method cascade
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-- To prevent a (rare) deadlock against autovacuum,
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-- we must lock the table that owns the index that will be dropped
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BEGIN;
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LOCK TABLE fast_emp4000;
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DROP ACCESS METHOD gist2 CASCADE;
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COMMIT;

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