BUG #19556: Segmentation fault in test_decoding

The following bug has been logged on the website:

Bug reference: 19556
Logged by: Alexander Kozhemyakin
Email address: a(dot)kozhemyakin(at)postgrespro(dot)ru
PostgreSQL version: 19beta2
Operating system: ubuntu 26.04
Description:

Hi,

The following script causes the server to crash with a Segmentation fault.

initdb -D data
echo "
max_prepared_transactions = '15'
wal_level = logical " >> data/postgresql.auto.conf
pg_ctl -D data -l logfile start

psql <<EOF
SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot',
'test_decoding', false, true);
CREATE TABLE test (id int PRIMARY KEY, data text);
INSERT INTO test VALUES (1, 'test data');
BEGIN;
SELECT * FROM test WHERE id = 1 FOR SHARE;
PREPARE TRANSACTION 'p1';
SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL,
'include-xids', '0', 'skip-empty-xacts', '1');
EOF

backtrace
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x0000749f6ee23f54 in pg_decode_prepare_txn (ctx=0x6344c3d7f3f0,
txn=0x6344c3d7d320, prepare_lsn=24628632) at
/pgpro/postgres/contrib/test_decoding/test_decoding.c:378
378 if (data->skip_empty_xacts && !txndata->xact_wrote_changes)
(gdb) bt
#0 0x0000749f6ee23f54 in pg_decode_prepare_txn (ctx=0x6344c3d7f3f0,
txn=0x6344c3d7d320, prepare_lsn=24628632) at
/pgpro/postgres/contrib/test_decoding/test_decoding.c:378
#1 0x00006344a8e8293c in prepare_cb_wrapper (cache=<optimized out>,
txn=<optimized out>, prepare_lsn=<optimized out>) at
/pgpro/postgres/src/backend/replication/logical/logical.c:985
#2 0x00006344a8e90c24 in ReorderBufferPrepare (rb=0x6344c3cd9e10,
xid=xid(at)entry=745, gid=gid(at)entry=0x7ffcb3620ef4 "p1") at
/pgpro/postgres/src/backend/replication/logical/reorderbuffer.c:2934
#3 0x00006344a8e7fa26 in DecodePrepare (ctx=0x6344c3d7f3f0,
buf=0x7ffcb3621030, parsed=0x7ffcb3620ea0) at
/pgpro/postgres/src/backend/replication/logical/decode.c:815
#4 xact_decode (ctx=0x6344c3d7f3f0, buf=0x7ffcb3621030) at
/pgpro/postgres/src/backend/replication/logical/decode.c:347
#5 0x00006344a8e7f223 in LogicalDecodingProcessRecord
(ctx=ctx(at)entry=0x6344c3d7f3f0, record=0x6344c3d7f788) at
/pgpro/postgres/src/backend/replication/logical/decode.c:116
#6 0x00006344a8e8577b in pg_logical_slot_get_changes_guts
(fcinfo=0x6344c3d83410, confirm=confirm(at)entry=true,
binary=binary(at)entry=false) at
/pgpro/postgres/src/backend/replication/logical/logicalfuncs.c:266
#7 0x00006344a8e858f4 in pg_logical_slot_get_changes (fcinfo=<optimized
out>) at /pgpro/postgres/src/backend/replication/logical/logicalfuncs.c:333
#8 0x00006344a8d3666d in ExecMakeTableFunctionResult
(setexpr=0x6344c3d5ffc8, econtext=0x6344c3d5fe18, argContext=<optimized
out>, expectedDesc=0x6344c3d49868, randomAccess=false)
at /pgpro/postgres/src/backend/executor/execSRF.c:234
#9 0x00006344a8d4a14b in FunctionNext (node=node(at)entry=0x6344c3d5fc08) at
/pgpro/postgres/src/backend/executor/nodeFunctionscan.c:94
#10 0x00006344a8d36f4c in ExecScanFetch (node=<optimized out>,
epqstate=<optimized out>, accessMtd=<optimized out>, recheckMtd=<optimized
out>) at /pgpro/postgres/src/include/executor/execScan.h:126
#11 ExecScanExtended (node=<optimized out>, accessMtd=0x6344a8d49e10
<FunctionNext>, recheckMtd=0x6344a8d49e00 <FunctionRecheck>, epqstate=0x0,
qual=0x0, projInfo=0x6344c3d49ea8)
at /pgpro/postgres/src/include/executor/execScan.h:187
#12 ExecScan (node=0x6344c3d5fc08, accessMtd=0x6344a8d49e10 <FunctionNext>,
recheckMtd=0x6344a8d49e00 <FunctionRecheck>) at
/pgpro/postgres/src/backend/executor/execScan.c:59
#13 0x00006344a8d2c17b in ExecProcNode (node=0x6344c3d5fc08) at
/pgpro/postgres/src/include/executor/executor.h:272
#14 ExecutePlan (queryDesc=0x6344c3cd55d0, operation=CMD_SELECT,
sendTuples=true, numberTuples=0, direction=<optimized out>,
dest=0x6344c3d8b7b8) at /pgpro/postgres/src/backend/executor/execMain.c:1675
#15 standard_ExecutorRun (queryDesc=0x6344c3cd55d0, direction=<optimized
out>, count=0) at /pgpro/postgres/src/backend/executor/execMain.c:364
#16 0x00006344a8f11478 in PortalRunSelect
(portal=portal(at)entry=0x6344c3d018b0, forward=forward(at)entry=true, count=0,
count(at)entry=9223372036854775807, dest=dest(at)entry=0x6344c3d8b7b8)
at /pgpro/postgres/src/backend/tcop/pquery.c:920
#17 0x00006344a8f12c5e in PortalRun (portal=portal(at)entry=0x6344c3d018b0,
count=count(at)entry=9223372036854775807, isTopLevel=isTopLevel(at)entry=true,
dest=dest(at)entry=0x6344c3d8b7b8, altdest=altdest(at)entry=0x6344c3d8b7b8,
qc=qc(at)entry=0x7ffcb3621640) at
/pgpro/postgres/src/backend/tcop/pquery.c:764
#18 0x00006344a8f0e95e in exec_simple_query (query_string=0x6344c3c80f60
"SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL,
'include-xids', '0', 'skip-empty-xacts', '1');")
at /pgpro/postgres/src/backend/tcop/postgres.c:1271
#19 0x00006344a8f103e5 in PostgresMain (dbname=<optimized out>,
username=<optimized out>) at
/pgpro/postgres/src/backend/tcop/postgres.c:4691
#20 0x00006344a8f0aaa3 in BackendMain (startup_data=<optimized out>,
startup_data_len=<optimized out>) at
/pgpro/postgres/src/backend/tcop/backend_startup.c:107
#21 0x00006344a8e6184f in postmaster_child_launch (child_type=<optimized
out>, child_slot=1, startup_data=startup_data(at)entry=0x7ffcb3621aec "",
startup_data_len=startup_data_len(at)entry=4,
client_sock=client_sock(at)entry=0x7ffcb3621af0) at
/pgpro/postgres/src/backend/postmaster/launch_backend.c:274
#22 0x00006344a8e653ef in BackendStartup (client_sock=0x7ffcb3621af0) at
/pgpro/postgres/src/backend/postmaster/postmaster.c:3519
#23 ServerLoop () at
/pgpro/postgres/src/backend/postmaster/postmaster.c:1688
#24 0x00006344a8e66cbf in PostmasterMain (argc=argc(at)entry=3,
argv=argv(at)entry=0x6344c3c7b440) at
/pgpro/postgres/src/backend/postmaster/postmaster.c:1386
#25 0x00006344a8b3b35e in main (argc=3, argv=0x6344c3c7b440) at
/pgpro/postgres/src/backend/main/main.c:230

first bad commit 072ee847ad4c3fb52