postgres_fdw: stabilize terminated-connection regression tests

The regression test for postgres_fdw_get_connections(true) assumed that
a terminated remote connection would still remain visible in the FDW
connection cache long enough to be reported as closed with a nonzero
remote_backend_pid.

That assumption is not always valid. postgres_fdw_get_connections()
reports only entries that are still present in ConnectionHash, while
pgfdw_inval_callback() may immediately discard an idle cached connection
(xact_depth == 0) when a relevant invalidation arrives. In CI, that can
happen between terminating the remote backend and querying
postgres_fdw_get_connections(true), causing the function to return no
rows.

Adjust the idle-connection test to accept either outcome: if the cache
entry is still present, verify that it reports the expected server name,
closed status, and nonzero remote backend PID; otherwise treat zero rows
as a legitimate result.

To preserve coverage of the terminated-backend reporting path, add a
separate check inside an explicit transaction. In that case, concurrent
invalidation may mark the connection invalid but cannot discard it
before transaction end, so postgres_fdw_get_connections(true) should
still report the terminated connection as in-use, closed, and associated
with a nonzero remote backend PID.

Backpatch to v18, where the affected postgres_fdw_get_connections(true)
test was introduced.

Reported-by: Robert Haas <robertmhaas@gmail.com>
Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CA+Tgmoax3cHXHsm9OidN4F-xiu16y8q2W8T5dTNFic1Zoo2cOw@mail.gmail.com
Backpatch-through: 18

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -13221,23 +13221,67 @@ SELECT server_name,
  loopback    | f      | t
 (1 row)
 
--- After terminating the remote backend, since the connection is closed,
--- "closed" should be TRUE, or NULL if the connection status check
--- is not available. Despite the termination, remote_backend_pid should
--- still show the non-zero PID of the terminated remote backend.
+-- After terminating the remote backend, if the connection entry is still in
+-- the cache, "closed" should be TRUE, or NULL if the connection status check
+-- is not available, and remote_backend_pid should still show the non-zero PID
+-- of the terminated remote backend. Concurrent invalidation can remove the
+-- idle cached connection before the next statement, in which case
+-- postgres_fdw_get_connections(true) can legitimately return no rows.
 DO $$ BEGIN
 PERFORM pg_terminate_backend(pid, 180000) FROM pg_stat_activity
   WHERE application_name = 'fdw_conn_check';
 END $$;
-SELECT server_name,
+WITH terminated_conn AS (
+  SELECT server_name,
+    CASE WHEN closed IS NOT false THEN true ELSE false END AS closed,
+    remote_backend_pid <> 0 AS remote_backend_pid
+    FROM postgres_fdw_get_connections(true)
+)
+SELECT CASE
+  WHEN count(*) = 0 THEN true
+  WHEN count(*) = 1 THEN bool_and(server_name = 'loopback'
+                                  AND closed
+                                  AND remote_backend_pid)
+  ELSE false
+END AS ok
+FROM terminated_conn;
+ ok 
+----
+ t
+(1 row)
+
+-- In an explicit transaction, concurrent invalidation may mark the
+-- connection invalid but cannot discard it before transaction end, so the
+-- terminated connection should remain visible in the cache.
+SELECT 1 FROM postgres_fdw_disconnect_all();
+ ?column? 
+----------
+        1
+(1 row)
+
+SET client_min_messages = 'ERROR';
+BEGIN;
+SELECT 1 FROM ft1 LIMIT 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+DO $$ BEGIN
+PERFORM pg_terminate_backend(pid, 180000) FROM pg_stat_activity
+  WHERE application_name = 'fdw_conn_check';
+END $$;
+SELECT server_name, used_in_xact,
   CASE WHEN closed IS NOT false THEN true ELSE false END AS closed,
   remote_backend_pid <> 0 AS remote_backend_pid
   FROM postgres_fdw_get_connections(true);
- server_name | closed | remote_backend_pid 
--------------+--------+--------------------
- loopback    | t      | t
+ server_name | used_in_xact | closed | remote_backend_pid 
+-------------+--------------+--------+--------------------
+ loopback    | t            | t      | t
 (1 row)
 
+ABORT;
+RESET client_min_messages;
 -- Clean up
 \set VERBOSITY default
 RESET debug_discard_caches;

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -4748,18 +4748,48 @@ SELECT server_name,
   WHERE application_name = 'fdw_conn_check') AS remote_backend_pid
   FROM postgres_fdw_get_connections(true);
 
--- After terminating the remote backend, since the connection is closed,
--- "closed" should be TRUE, or NULL if the connection status check
--- is not available. Despite the termination, remote_backend_pid should
--- still show the non-zero PID of the terminated remote backend.
+-- After terminating the remote backend, if the connection entry is still in
+-- the cache, "closed" should be TRUE, or NULL if the connection status check
+-- is not available, and remote_backend_pid should still show the non-zero PID
+-- of the terminated remote backend. Concurrent invalidation can remove the
+-- idle cached connection before the next statement, in which case
+-- postgres_fdw_get_connections(true) can legitimately return no rows.
 DO $$ BEGIN
 PERFORM pg_terminate_backend(pid, 180000) FROM pg_stat_activity
   WHERE application_name = 'fdw_conn_check';
 END $$;
-SELECT server_name,
+WITH terminated_conn AS (
+  SELECT server_name,
+    CASE WHEN closed IS NOT false THEN true ELSE false END AS closed,
+    remote_backend_pid <> 0 AS remote_backend_pid
+    FROM postgres_fdw_get_connections(true)
+)
+SELECT CASE
+  WHEN count(*) = 0 THEN true
+  WHEN count(*) = 1 THEN bool_and(server_name = 'loopback'
+                                  AND closed
+                                  AND remote_backend_pid)
+  ELSE false
+END AS ok
+FROM terminated_conn;
+
+-- In an explicit transaction, concurrent invalidation may mark the
+-- connection invalid but cannot discard it before transaction end, so the
+-- terminated connection should remain visible in the cache.
+SELECT 1 FROM postgres_fdw_disconnect_all();
+SET client_min_messages = 'ERROR';
+BEGIN;
+SELECT 1 FROM ft1 LIMIT 1;
+DO $$ BEGIN
+PERFORM pg_terminate_backend(pid, 180000) FROM pg_stat_activity
+  WHERE application_name = 'fdw_conn_check';
+END $$;
+SELECT server_name, used_in_xact,
   CASE WHEN closed IS NOT false THEN true ELSE false END AS closed,
   remote_backend_pid <> 0 AS remote_backend_pid
   FROM postgres_fdw_get_connections(true);
+ABORT;
+RESET client_min_messages;
 
 -- Clean up
 \set VERBOSITY default