postgres_fdw: set search_path to 'pg_catalog' while deparsing constants.

The motivation for this is to ensure successful transmission of the
values of constants of regconfig and other reg* types.  The remote
will be reading them with search_path = 'pg_catalog', so schema
qualification is necessary when referencing objects in other schemas.

Per bug #17483 from Emmanuel Quincerot.  Back-patch to all supported
versions.  (There's some other stuff to do here, but it's less
back-patchable.)

Discussion: https://postgr.es/m/1423433.1652722406@sss.pgh.pa.us

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -1008,6 +1008,26 @@ SELECT count(c3) FROM ft1 t1 WHERE t1.c1 === t1.c2;
      9
 (1 row)
 
+-- check schema-qualification of regconfig constant
+CREATE TEXT SEARCH CONFIGURATION public.custom_search
+  (COPY = pg_catalog.english);
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT c1, to_tsvector('custom_search'::regconfig, c3) FROM ft1
+WHERE c1 = 642 AND length(to_tsvector('custom_search'::regconfig, c3)) > 0;
+                                                                  QUERY PLAN                                                                  
+----------------------------------------------------------------------------------------------------------------------------------------------
+ Foreign Scan on public.ft1
+   Output: c1, to_tsvector('custom_search'::regconfig, c3)
+   Remote SQL: SELECT "C 1", c3 FROM "S 1"."T 1" WHERE (("C 1" = 642)) AND ((length(to_tsvector('public.custom_search'::regconfig, c3)) > 0))
+(3 rows)
+
+SELECT c1, to_tsvector('custom_search'::regconfig, c3) FROM ft1
+WHERE c1 = 642 AND length(to_tsvector('custom_search'::regconfig, c3)) > 0;
+ c1  | to_tsvector 
+-----+-------------
+ 642 | '00642':1
+(1 row)
+
 -- ===================================================================
 -- JOIN queries
 -- ===================================================================

contrib/postgres_fdw/postgres_fdw.c

@@ -3114,6 +3114,14 @@ set_transmission_modes(void)
 								 PGC_USERSET, PGC_S_SESSION,
 								 GUC_ACTION_SAVE, true, 0, false);
 
+	/*
+	 * In addition force restrictive search_path, in case there are any
+	 * regproc or similar constants to be printed.
+	 */
+	(void) set_config_option("search_path", "pg_catalog",
+							 PGC_USERSET, PGC_S_SESSION,
+							 GUC_ACTION_SAVE, true, 0, false);
+
 	return nestlevel;
 }
 

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -364,6 +364,15 @@ EXPLAIN (VERBOSE, COSTS OFF)
   SELECT count(c3) FROM ft1 t1 WHERE t1.c1 === t1.c2;
 SELECT count(c3) FROM ft1 t1 WHERE t1.c1 === t1.c2;
 
+-- check schema-qualification of regconfig constant
+CREATE TEXT SEARCH CONFIGURATION public.custom_search
+  (COPY = pg_catalog.english);
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT c1, to_tsvector('custom_search'::regconfig, c3) FROM ft1
+WHERE c1 = 642 AND length(to_tsvector('custom_search'::regconfig, c3)) > 0;
+SELECT c1, to_tsvector('custom_search'::regconfig, c3) FROM ft1
+WHERE c1 = 642 AND length(to_tsvector('custom_search'::regconfig, c3)) > 0;
+
 -- ===================================================================
 -- JOIN queries
 -- ===================================================================