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 &

This was originally committed as 48efb23 and backpatched down to
v16, but since then there have been reports of this happening on
v14 and v15 as well so this is a backpatch of 48efb23 down to 14.

Bug: #17695
Author: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Alexander Lakhin <exclusion@gmail.com>
Reported-by: bowenshi <zxwsbg@qq.com>
Reported-by: Alexander Pyhalov <a.pyhalov@postgrespro.ru>
Reported-by: Teja Mupparti
Discussion: https://postgr.es/m/17695-6be9277c9295985f@postgresql.org
Backpatch-through: v14

diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index 0b303f9a23552dab4fd5f3d7719fd4f09f247f8c..2ca8e6e9affdc8ad8ce6f44a3b5085b3d0b27f21 100644 (file)
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -2028,8 +2028,12 @@ 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;