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

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.

mysql-test/r/mysql_client_test.result

@@ -126,6 +126,11 @@ Data:
 EOF
 mysql_stmt_next_result(): 0; field_count: 0
 # ------------------------------------
+# cat MYSQL_TMP_DIR/test_mdev26145.out.log
+# ------------------------------------
+Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
+def				MAX(a)	MAX(a)	3	11	0	Y	32768	0	63
+# ------------------------------------
 
 SET @@global.general_log= @old_general_log;
 SET @@global.slow_query_log= @old_slow_query_log;

mysql-test/t/mysql_client_test.test

@@ -35,6 +35,10 @@ echo ok;
 --echo # ------------------------------------
 --cat_file $MYSQL_TMP_DIR/test_wl4435.out.log
 --echo # ------------------------------------
+--echo # cat MYSQL_TMP_DIR/test_mdev26145.out.log
+--echo # ------------------------------------
+--cat_file $MYSQL_TMP_DIR/test_mdev26145.out.log
+--echo # ------------------------------------
 --echo
 
 SET @@global.general_log= @old_general_log;

sql/sql_prepare.cc

@@ -1674,7 +1674,12 @@ 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->select_lex.item_list);
+    SELECT_LEX_UNIT* master_unit= unit->first_select()->master_unit();
+    bool is_union_op=
+      master_unit->is_union() || master_unit->fake_select_lex;
+
+    List<Item> fields(is_union_op ? unit->item_list :
+                                    lex->select_lex.item_list);
 
     /* Change columns if a procedure like analyse() */
     if (unit->last_procedure && unit->last_procedure->change_columns(thd, fields))

tests/mysql_client_test.c

@@ -19896,6 +19896,71 @@ static void test_ps_params_in_ctes()
   myquery(rc);
 }
 
+void display_result_metadata(MYSQL_FIELD *field,
+                             uint num_fields)
+{
+  MYSQL_FIELD* field_end;
+
+  mct_log("Catalog\tDatabase\tTable\tTable_alias\tColumn\t"
+          "Column_alias\tType\tLength\tMax length\tIs_null\t"
+          "Flags\tDecimals\tCharsetnr\n");
+  for (field_end= field+num_fields; field < field_end; field++)
+  {
+    mct_log("%s\t", field->catalog);
+    mct_log("%s\t", field->db);
+    mct_log("%s\t", field->org_table);
+    mct_log("%s\t", field->table);
+    mct_log("%s\t", field->org_name);
+    mct_log("%s\t", field->name);
+    mct_log("%u\t", field->type);
+    mct_log("%lu\t", field->length);
+    mct_log("%lu\t", field->max_length);
+    mct_log("%s\t", (IS_NOT_NULL(field->flags) ?  "N" : "Y"));
+    mct_log("%u\t", field->flags);
+    mct_log("%u\t", field->decimals);
+    mct_log("%u\n", field->charsetnr);
+  }
+}
+
+static void test_mdev_26145()
+{
+  MYSQL_STMT *stmt;
+  MYSQL_RES *result;
+  MYSQL_FIELD *fields;
+  int        rc, num_fields;
+
+  myheader("test_mdev_26145");
+
+  rc= mysql_query(mysql, "DROP TABLE IF EXISTS t1");
+  myquery(rc);
+
+  rc= mysql_query(mysql, "CREATE TABLE t1(a INT)");
+  myquery(rc);
+
+  stmt= mysql_simple_prepare(
+    mysql, "(SELECT MAX(a) FROM t1) UNION (SELECT MAX(a) FROM t1)");
+  check_stmt(stmt);
+
+  rc= mysql_stmt_execute(stmt);
+  check_execute(stmt, rc);
+
+  result= mysql_stmt_result_metadata(stmt);
+  DIE_UNLESS(result);
+
+  num_fields= mysql_stmt_field_count(stmt);
+  fields= mysql_fetch_fields(result);
+
+  mct_start_logging("test_mdev26145");
+  display_result_metadata(fields, num_fields);
+  mct_close_log();
+
+  mysql_free_result(result);
+  mysql_stmt_close(stmt);
+
+  rc= mysql_query(mysql, "DROP TABLE t1");
+
+  myquery(rc);
+}
 
 #ifndef EMBEDDED_LIBRARY
 #define MDEV19838_MAX_PARAM_COUNT 32
@@ -20047,6 +20112,7 @@ static void test_mdev19838()
 #endif // EMBEDDED_LIBRARY
 
 static struct my_tests_st my_tests[]= {
+  { "test_mdev_26145", test_mdev_26145 },
   { "disable_query_logs", disable_query_logs },
   { "test_view_sp_list_fields", test_view_sp_list_fields },
   { "client_query", client_query },