MDEV-26145: Incorrect metadata is sent on running query with union in PS modebb-10.6-MDEV-26145

Test cases like the following one produce different result sets if it's run
with and without th option --ps-protocol.

CREATE TABLE t1(a INT);
--enable_metadata
(SELECT MAX(a) FROM t1) UNION (SELECT MAX(a) FROM t1);
--disable_metadata
DROP TABLE t1;

Result sets differ in metadata for the query
  (SELECT MAX(a) FROM t1) UNION (SELECT MAX(a) FROM t1);

The reason for different content of query metadata is that for queries
with union the items being created on JOIN preparing phase is placed into
item_list from SELECT_LEX_UNIT whereas for queries without union item_list
from SELECT_LEX is used instead.

1 files changed, 7 insertions, 1 deletions

diff --git a/sql/sql_prepare.cc b/sql/sql_prepare.cc
index 2f5470db123..5b4e0698de4 100644
--- a/sql/sql_prepare.cc
+++ b/sql/sql_prepare.cc
@@ -1609,7 +1609,13 @@ static int mysql_test_select(Prepared_statement *stmt,
   if (!lex->describe && !thd->lex->analyze_stmt && !stmt->is_sql_prepare())
   {
     /* Make copy of item list, as change_columns may change it */
-    List<Item> fields(lex->first_select_lex()->item_list);
+
+    SELECT_LEX_UNIT* master_unit= lex->first_select_lex()->master_unit();
+    bool is_union_op=
+      master_unit->is_unit_op() || master_unit->fake_select_lex;
+
+    List<Item> fields(is_union_op ? unit->item_list :
+                                    lex->first_select_lex()->item_list);
 
     /* Change columns if a procedure like analyse() */
     if (unit->last_procedure && unit->last_procedure->change_columns(thd, fields))