dblink, postgres_fdw: Handle interrupts during connection establishment

Until now dblink and postgres_fdw did not process interrupts during connection
establishment. Besides preventing query cancellations etc, this can lead to
undetected deadlocks, as global barriers are not processed.

These aforementioned undetected deadlocks are the reason for the spate of CI
test failures in the FreeBSD 'test_running' step.

Fix the bug by using the helper from libpq-be-fe-helpers.h, introduced in a
prior commit. Besides fixing the bug, this also removes duplicated code
around reserving file descriptors.

As the change is relatively large and there are no field reports of the
problem, don't backpatch for now.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/20220925232237.p6uskba2dw6fnwj2@awork3.anarazel.de
Backpatch:

1 files changed, 6 insertions, 36 deletions

diff --git a/contrib/postgres_fdw/connection.c b/contrib/postgres_fdw/connection.c
index ed75ce3f79c..7760380f00d 100644
--- a/contrib/postgres_fdw/connection.c
+++ b/contrib/postgres_fdw/connection.c
@@ -17,6 +17,7 @@
 #include "catalog/pg_user_mapping.h"
 #include "commands/defrem.h"
 #include "funcapi.h"
+#include "libpq/libpq-be-fe-helpers.h"
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"
 #include "pgstat.h"
@@ -446,35 +447,10 @@ connect_pg_server(ForeignServer *server, UserMapping *user)
 		/* verify the set of connection parameters */
 		check_conn_params(keywords, values, user);
 
-		/*
-		 * We must obey fd.c's limit on non-virtual file descriptors.  Assume
-		 * that a PGconn represents one long-lived FD.  (Doing this here also
-		 * ensures that VFDs are closed if needed to make room.)
-		 */
-		if (!AcquireExternalFD())
-		{
-#ifndef WIN32					/* can't write #if within ereport() macro */
-			ereport(ERROR,
-					(errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),
-					 errmsg("could not connect to server \"%s\"",
-							server->servername),
-					 errdetail("There are too many open files on the local server."),
-					 errhint("Raise the server's max_files_per_process and/or \"ulimit -n\" limits.")));
-#else
-			ereport(ERROR,
-					(errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),
-					 errmsg("could not connect to server \"%s\"",
-							server->servername),
-					 errdetail("There are too many open files on the local server."),
-					 errhint("Raise the server's max_files_per_process setting.")));
-#endif
-		}
-
 		/* OK to make connection */
-		conn = PQconnectdbParams(keywords, values, false);
-
-		if (!conn)
-			ReleaseExternalFD();	/* because the PG_CATCH block won't */
+		conn = libpqsrv_connect_params(keywords, values,
+									    /* expand_dbname = */ false,
+									   PG_WAIT_EXTENSION);
 
 		if (!conn || PQstatus(conn) != CONNECTION_OK)
 			ereport(ERROR,
@@ -507,12 +483,7 @@ connect_pg_server(ForeignServer *server, UserMapping *user)
 	}
 	PG_CATCH();
 	{
-		/* Release PGconn data structure if we managed to create one */
-		if (conn)
-		{
-			PQfinish(conn);
-			ReleaseExternalFD();
-		}
+		libpqsrv_disconnect(conn);
 		PG_RE_THROW();
 	}
 	PG_END_TRY();
@@ -528,9 +499,8 @@ disconnect_pg_server(ConnCacheEntry *entry)
 {
 	if (entry->conn != NULL)
 	{
-		PQfinish(entry->conn);
+		libpqsrv_disconnect(entry->conn);
 		entry->conn = NULL;
-		ReleaseExternalFD();
 	}
 }