MDEV-14750 Valgrind Invalid read, ASAN heap-use-after-free in Item_ident::print upon SHOW CREATE on partitioned table

items in the partitioning function were taking
the table name from the table's field
(in set_field(from_field) in Item_field::fix_fields)
and field's table_name is TABLE::alias.

But alias is changed for every statement, and
can be realloced if next statement uses a longer
alias. But partitioning items are fixed once
and live as long as the TABLE does. So if
an alias is realloced, pointers to the old
alias string will become invalid.

Fix partitioning item table_name to point to
the actual table name instead.

Author: vuvova

mysql-test/suite/parts/r/quoting.result renamed to mysql-test/suite/parts/r/show_create.result

@@ -91,3 +91,15 @@ t2	CREATE TABLE "t2" (
  PARTITION BY RANGE ("f1")
 (PARTITION "p1" VALUES LESS THAN MAXVALUE ENGINE = MyISAM)
 drop table t1, t2;
+set sql_mode=default;
+create table t_partition (f1 int) partition by hash(f1) partitions 2;
+select * from t_partition as tbl;
+f1
+show create table t_partition;
+Table	Create Table
+t_partition	CREATE TABLE `t_partition` (
+  `f1` int(11) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+ PARTITION BY HASH (`f1`)
+PARTITIONS 2
+drop table t_partition;

mysql-test/suite/parts/t/quoting.test renamed to mysql-test/suite/parts/t/show_create.test

@@ -30,3 +30,12 @@ set sql_mode=ansi_quotes;
 show create table t1;
 show create table t2;
 drop table t1, t2;
+set sql_mode=default;
+
+#
+# MDEV-14750 Valgrind Invalid read, ASAN heap-use-after-free in Item_ident::print upon SHOW CREATE on partitioned table
+#
+create table t_partition (f1 int) partition by hash(f1) partitions 2;
+select * from t_partition as tbl;
+show create table t_partition;
+drop table t_partition;

sql/item.cc

@@ -5766,6 +5766,10 @@ bool Item_field::post_fix_fields_part_expr_processor(void *int_arg)
   DBUG_ASSERT(fixed);
   if (field->vcol_info)
     field->vcol_info->mark_as_in_partitioning_expr();
+  /*
+    Update table_name to be real table name, not the alias. Because alias is
+    reallocated for every statement, and this item has a long life time */
+  table_name= field->table->s->table_name.str;
   return FALSE;
 }