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Commit6c6d6ba

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author
Amit Kapila
committed
Fix the Drop Database hang.
The drop database command waits for the logical replication sync worker toaccept ProcSignalBarrier and the worker's slot creation waits for the dropdatabase to finish which leads to a deadlock. This happens because thetablesync worker holds interrupts while creating a slot.We prevent cancel/die interrupts while creating a slot in the table syncworker because it is possible that before the server finishes thiscommand, a concurrent drop subscription happens which would completewithout removing this slot and that leads to the slot existing until theend of walsender. However, the slot will eventually get dropped at thewalsender exit time, so there is no danger of the dangling slot.This patch reallows cancel/die interrupts while creating a slot andmodifies the test to wait for slots to become zero to prevent finding anephemeral slot.The reported hang doesn't happen in PG14 as the drop database starts towait for ProcSignalBarrier with PG15 (commits4eb2176 ande2f65f4)but it is good to backpatch this till PG14 as it is not a good idea toprevent interrupts during a network call that could block indefinitely.Reported-by: Lakshmi Narayanan SreetharDiagnosed-by: Andres FreundAuthor: Hou ZhijieReviewed-by: Vignesh C, Amit KapilaBackpatch-through: 14, where it was introduced in commit6b67d72Discussion:https://postgr.es/m/CA+kvmZELXQ4ZD3U=XCXuG3KvFgkuPoN1QrEj8c-rMRodrLOnsg@mail.gmail.com
1 parent728f86f commit6c6d6ba

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2 files changed

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2 files changed

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‎src/backend/replication/logical/tablesync.c

Lines changed: 0 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -1396,17 +1396,10 @@ LogicalRepSyncTableStart(XLogRecPtr *origin_startpos)
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* Create a new permanent logical decoding slot. This slot will be used
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* for the catchup phase after COPY is done, so tell it to use the
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* snapshot to make the final data consistent.
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*
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* Prevent cancel/die interrupts while creating slot here because it is
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* possible that before the server finishes this command, a concurrent
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* drop subscription happens which would complete without removing this
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* slot leading to a dangling slot on the server.
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*/
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HOLD_INTERRUPTS();
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walrcv_create_slot(LogRepWorkerWalRcvConn,
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slotname, false/* permanent */ , false/* two_phase */ ,
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CRS_USE_SNAPSHOT,origin_startpos);
1409-
RESUME_INTERRUPTS();
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/*
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* Setup replication origin tracking. The purpose of doing this before the

‎src/test/subscription/t/004_sync.pl

Lines changed: 7 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -163,9 +163,13 @@
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# subscriber is stuck on data copy for constraint violation.
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$node_subscriber->safe_psql('postgres',"DROP SUBSCRIPTION tap_sub");
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$result =$node_publisher->safe_psql('postgres',
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"SELECT count(*) FROM pg_replication_slots");
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is($result,qq(0),
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# When DROP SUBSCRIPTION tries to drop the tablesync slot, the slot may not
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# have been created, which causes the slot to be created after the DROP
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# SUSCRIPTION finishes. Such slots eventually get dropped at walsender exit
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# time. So, to prevent being affected by such ephemeral tablesync slots, we
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# wait until all the slots have been cleaned.
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ok($node_publisher->poll_query_until(
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'postgres','SELECT count(*) = 0 FROM pg_replication_slots'),
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'DROP SUBSCRIPTION during error can clean up the slots on the publisher');
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$node_subscriber->stop('fast');

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