Disallow MERGE cleanly for foreign partitions

While directly targetting a foreign table with MERGE was already
expressly forbidden, we failed to catch the case of a partitioned table
that has a foreign table as a partition; and the result if you try is an
incomprehensible error.  Fix that by adding a specific check.

Backpatch to 15.

Reported-by: Tatsuhiro Nakamori <bt22nakamorit@oss.nttdata.com>
Discussion: https://postgr.es/m/bt22nakamorit@oss.nttdata.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -8284,6 +8284,11 @@ select tableoid::regclass, * FROM remp2;
 (3 rows)
 
 delete from itrtest;
+-- MERGE ought to fail cleanly
+merge into itrtest using (select 1, 'foo') as source on (true)
+  when matched then do nothing;
+ERROR:  cannot execute MERGE on relation "remp1"
+DETAIL:  This operation is not supported for foreign tables.
 create unique index loct1_idx on loct1 (a);
 -- DO NOTHING without an inference specification is supported
 insert into itrtest values (1, 'foo') on conflict do nothing returning *;

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -2207,6 +2207,10 @@ select tableoid::regclass, * FROM remp2;
 
 delete from itrtest;
 
+-- MERGE ought to fail cleanly
+merge into itrtest using (select 1, 'foo') as source on (true)
+  when matched then do nothing;
+
 create unique index loct1_idx on loct1 (a);
 
 -- DO NOTHING without an inference specification is supported

src/backend/optimizer/plan/createplan.c

@@ -7078,12 +7078,32 @@ make_modifytable(PlannerInfo *root, Plan *subplan,
 			RelOptInfo *resultRel = root->simple_rel_array[rti];
 
 			fdwroutine = resultRel->fdwroutine;
+
+			/*
+			 * MERGE is not currently supported for foreign tables and we
+			 * already checked when the table mentioned in the query is
+			 * foreign; but we can still get here if a partitioned table has a
+			 * foreign table as partition.  Disallow that now, to avoid an
+			 * uglier error message later.
+			 */
+			if (operation == CMD_MERGE && fdwroutine != NULL)
+			{
+				RangeTblEntry *rte = root->simple_rte_array[rti];
+
+				ereport(ERROR,
+						errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						errmsg("cannot execute MERGE on relation \"%s\"",
+							   get_rel_name(rte->relid)),
+						errdetail_relkind_not_supported(rte->relkind));
+			}
+
 		}
 		else
 		{
 			RangeTblEntry *rte = planner_rt_fetch(rti, root);
 
 			Assert(rte->rtekind == RTE_RELATION);
+			Assert(operation != CMD_MERGE);
 			if (rte->relkind == RELKIND_FOREIGN_TABLE)
 				fdwroutine = GetFdwRoutineByRelId(rte->relid);
 			else