From: Daniel Gustafsson <dgustafsson@postgresql.org>
Date: Tue, 4 Jul 2023 15:36:13 +0000 (+0200)
Subject: Fix assertion failure in snapshot building
X-Git-Tag: REL_16_BETA3~61
X-Git-Url: http://git.postgresql.org/gitweb/?a=commitdiff_plain;h=eb27d3dc88879cb148ae0365394aae0ab84fa1a7;p=postgresql.git

Fix assertion failure in snapshot building

Clear any potential stale next_phase_at value from the snapshot
builder which otherwise may trip an assertion check ensuring
that there is no next_phase_at value.

This can be reproduced by running 80 concurrent sessions like
the below where $c is a loop counter (assumes there has been
1..$c databases created) :

  echo "
    CREATE TABLE replication_example(id SERIAL PRIMARY KEY,
                                     somedata int,
                                     text varchar(120));
    SELECT 'init' FROM
      pg_create_logical_replication_slot('regression_slot_$c',
                                         'test_decoding');
    SELECT data FROM
      pg_logical_slot_get_changes('regression_slot_$c', NULL,
                                  NULL, 'include-xids', '0',
                                  'skip-empty-xacts', '1');
  " | psql -d regress_$c >>psql.log &

Backpatch down to v16.

Bug: #17695
Author: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Alexander Lakhin <exclusion@gmail.com>
Reported-by: bowenshi <zxwsbg@qq.com>
Discussion: https://postgr.es/m/17695-6be9277c9295985f@postgresql.org
Backpatch-through: v16
---

diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index 0786bb0ab71..e9b672ead28 100644
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -1950,8 +1950,12 @@ SnapBuildRestore(SnapBuild *builder, XLogRecPtr lsn)
 	if (TransactionIdPrecedes(ondisk.builder.xmin, builder->initial_xmin_horizon))
 		goto snapshot_not_interesting;
 
-	/* consistent snapshots have no next phase */
+	/*
+	 * Consistent snapshots have no next phase. Reset next_phase_at as it is
+	 * possible that an old value may remain.
+	 */
 	Assert(ondisk.builder.next_phase_at == InvalidTransactionId);
+	builder->next_phase_at = InvalidTransactionId;
 
 	/* ok, we think the snapshot is sensible, copy over everything important */
 	builder->xmin = ondisk.builder.xmin;