Fix handling of pending inserts in nodeModifyTable.c.

Commit b663a4136, which allowed FDWs to INSERT rows in bulk, added to
nodeModifyTable.c code to flush pending inserts to the foreign-table
result relation(s) before completing processing of the ModifyTable node,
but the code failed to take into account the case where the INSERT query
has modifying CTEs, leading to incorrect results.

Also, that commit failed to flush pending inserts before firing BEFORE
ROW triggers so that rows are visible to such triggers.

In that commit we scanned through EState's
es_tuple_routing_result_relations or es_opened_result_relations list to
find the foreign-table result relations to which pending inserts are
flushed, but that would be inefficient in some cases.  So to fix, 1) add
a List member to EState to record the insert-pending result relations,
and 2) modify nodeModifyTable.c so that it adds the foreign-table result
relation to the list in ExecInsert() if appropriate, and flushes pending
inserts properly using the list where needed.

While here, fix a copy-and-pasteo in a comment in ExecBatchInsert(),
which was added by that commit.

Back-patch to v14 where that commit appeared.

Discussion: https://postgr.es/m/CAPmGK16qutyCmyJJzgQOhfBq%3DNoGDqTB6O0QBZTihrbqre%2BoxA%40mail.gmail.com

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -3261,8 +3261,94 @@ INSERT INTO batch_table SELECT i, 'test'||i, 'test'|| i FROM generate_series(1,
 SELECT COUNT(*) FROM batch_table;
 SELECT * FROM batch_table ORDER BY x;
 
+-- Clean up
+DROP TABLE batch_table;
+DROP TABLE batch_table_p0;
+DROP TABLE batch_table_p1;
+
 ALTER SERVER loopback OPTIONS (DROP batch_size);
 
+-- Test that pending inserts are handled properly when needed
+CREATE TABLE batch_table (a text, b int);
+CREATE FOREIGN TABLE ftable (a text, b int)
+	SERVER loopback
+	OPTIONS (table_name 'batch_table', batch_size '2');
+CREATE TABLE ltable (a text, b int);
+CREATE FUNCTION ftable_rowcount_trigf() RETURNS trigger LANGUAGE plpgsql AS
+$$
+begin
+	raise notice '%: there are % rows in ftable',
+		TG_NAME, (SELECT count(*) FROM ftable);
+	if TG_OP = 'DELETE' then
+		return OLD;
+	else
+		return NEW;
+	end if;
+end;
+$$;
+CREATE TRIGGER ftable_rowcount_trigger
+BEFORE INSERT OR UPDATE OR DELETE ON ltable
+FOR EACH ROW EXECUTE PROCEDURE ftable_rowcount_trigf();
+
+WITH t AS (
+	INSERT INTO ltable VALUES ('AAA', 42), ('BBB', 42) RETURNING *
+)
+INSERT INTO ftable SELECT * FROM t;
+
+SELECT * FROM ltable;
+SELECT * FROM ftable;
+DELETE FROM ftable;
+
+WITH t AS (
+	UPDATE ltable SET b = b + 100 RETURNING *
+)
+INSERT INTO ftable SELECT * FROM t;
+
+SELECT * FROM ltable;
+SELECT * FROM ftable;
+DELETE FROM ftable;
+
+WITH t AS (
+	DELETE FROM ltable RETURNING *
+)
+INSERT INTO ftable SELECT * FROM t;
+
+SELECT * FROM ltable;
+SELECT * FROM ftable;
+DELETE FROM ftable;
+
+-- Clean up
+DROP FOREIGN TABLE ftable;
+DROP TABLE batch_table;
+DROP TRIGGER ftable_rowcount_trigger ON ltable;
+DROP TABLE ltable;
+
+CREATE TABLE parent (a text, b int) PARTITION BY LIST (a);
+CREATE TABLE batch_table (a text, b int);
+CREATE FOREIGN TABLE ftable
+	PARTITION OF parent
+	FOR VALUES IN ('AAA')
+	SERVER loopback
+	OPTIONS (table_name 'batch_table', batch_size '2');
+CREATE TABLE ltable
+	PARTITION OF parent
+	FOR VALUES IN ('BBB');
+CREATE TRIGGER ftable_rowcount_trigger
+BEFORE INSERT ON ltable
+FOR EACH ROW EXECUTE PROCEDURE ftable_rowcount_trigf();
+
+INSERT INTO parent VALUES ('AAA', 42), ('BBB', 42), ('AAA', 42), ('BBB', 42);
+
+SELECT tableoid::regclass, * FROM parent;
+
+-- Clean up
+DROP FOREIGN TABLE ftable;
+DROP TABLE batch_table;
+DROP TRIGGER ftable_rowcount_trigger ON ltable;
+DROP TABLE ltable;
+DROP TABLE parent;
+DROP FUNCTION ftable_rowcount_trigf;
+
 -- ===================================================================
 -- test asynchronous execution
 -- ===================================================================