BUG #19484: Segmentation fault triggered by FDW

The following bug has been logged on the website:

Bug reference: 19484
Logged by: Chi Zhang
Email address: 798604270(at)qq(dot)com
PostgreSQL version: 18.4
Operating system: Ubuntu 24.04
Description:

Hi,

I found the following test case triggers a segmentation fault:

```
\set ON_ERROR_STOP on

CREATE EXTENSION postgres_fdw;

CREATE SERVER loopback FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (
host '/path/to/pg_socket',
port '5432',
dbname :'dbname'
);

CREATE USER MAPPING FOR postgres SERVER loopback
OPTIONS (user 'postgres');

CREATE SCHEMA r;
CREATE TABLE r.remote_p2 (a int NOT NULL, b int);

CREATE TABLE pt (a int NOT NULL, b int) PARTITION BY LIST (a);
CREATE TABLE pt_p1 PARTITION OF pt FOR VALUES IN (1);
CREATE FOREIGN TABLE pt_p2 PARTITION OF pt FOR VALUES IN (2)
SERVER loopback
OPTIONS (schema_name 'r', table_name 'remote_p2');

INSERT INTO pt_p1 VALUES (1, 10);
INSERT INTO r.remote_p2 VALUES (2, 20);

SET plan_cache_mode = force_generic_plan;

PREPARE upd(int) AS
UPDATE pt
SET b = b + 1
WHERE a = $1
RETURNING tableoid::regclass, a, b;

EXPLAIN (costs off) EXECUTE upd(2);
EXECUTE upd(2);
SELECT * FROM r.remote_p2 ORDER BY a;

```

This is the log:

```
2026-05-18 13:40:41.888 CST [21729] LOG: database system is ready to accept
connections
2026-05-18 13:41:03.317 CST [21932] LOG: unexpected EOF on client
connection with an open transaction
2026-05-18 13:41:03.317 CST [21729] LOG: client backend (PID 21931) was
terminated by signal 11: Segmentation fault
2026-05-18 13:41:03.317 CST [21729] DETAIL: Failed process was running:
EXECUTE upd(2);
2026-05-18 13:41:03.317 CST [21729] LOG: terminating any other active
server processes
2026-05-18 13:41:03.319 CST [21729] LOG: all server processes terminated;
reinitializing
2026-05-18 13:41:03.345 CST [21936] LOG: database system was interrupted;
last known up at 2026-05-18 13:40:41 CST
2026-05-18 13:41:03.509 CST [21936] LOG: database system was not properly
shut down; automatic recovery in progress
2026-05-18 13:41:03.513 CST [21936] LOG: redo starts at 0/98371040
2026-05-18 13:41:03.531 CST [21936] LOG: invalid record length at
0/987B6E68: expected at least 24, got 0
2026-05-18 13:41:03.531 CST [21936] LOG: redo done at 0/987B6E40 system
usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.01 s
2026-05-18 13:41:03.537 CST [21937] LOG: checkpoint starting:
end-of-recovery fast wait
2026-05-18 13:41:03.654 CST [21937] LOG: checkpoint complete:
end-of-recovery fast wait: wrote 975 buffers (6.0%), wrote 3 SLRU buffers; 0
WAL file(s) added, 0 removed, 0
recycled; write=0.081 s, sync=0.030 s, total=0.121 s; sync files=325,
longest=0.005 s, average=0.001 s; distance=4375 kB, estimate=4375 kB;
lsn=0/987B6E68, redo lsn=0/987B6E68
2026-05-18 13:41:03.660 CST [21729] LOG: database system is ready to
accept connections
```

I built the Postgres from source code
901ed9b352b41f034e17bc540725082a488fce31 of github commit.