MDEV-37328 Assertion failure in make_empty_rec upon CONVERT PARTITION

ALTER TABLE ... CONVERT invokes build_frm_image() three times on
the same data structures. The latter should not increment
create_info->null_bits on every invocation.

Author: vuvova

mysql-test/suite/parts/r/alter_table.result

@@ -383,4 +383,15 @@ disconnect con1;
 connection default;
 drop user u@localhost;
 drop database db;
+#
+# MDEV-37328 Assertion failure in make_empty_rec upon CONVERT PARTITION
+#
+create table t (f1 int, f2 int, f3 int, f4 int, f5 int, f6 int, f7 int) engine=myisam
+partition by list (f3) (
+partition p0 values in (null,0),
+partition p1 values in (1,2,3),
+partition p2 default
+);
+alter table t convert partition p0 to table tp;
+drop table if exists tp, t;
 # End of 10.11 tests

mysql-test/suite/parts/t/alter_table.test

@@ -347,4 +347,16 @@ alter table t1 convert table tp to partition p2 values less than (1000);
 drop user u@localhost;
 drop database db;
 
+--echo #
+--echo # MDEV-37328 Assertion failure in make_empty_rec upon CONVERT PARTITION
+--echo #
+create table t (f1 int, f2 int, f3 int, f4 int, f5 int, f6 int, f7 int) engine=myisam
+ partition by list (f3) (
+  partition p0 values in (null,0),
+  partition p1 values in (1,2,3),
+  partition p2 default
+);
+alter table t convert partition p0 to table tp;
+drop table if exists tp, t;
+
 --echo # End of 10.11 tests

sql/unireg.cc

@@ -288,9 +288,8 @@ LEX_CUSTRING build_frm_image(THD *thd, const LEX_CSTRING &table,
   DBUG_ENTER("build_frm_image");
 
  /* If fixed row records, we need one bit to check for deleted rows */
-  if (!(create_info->table_options & HA_OPTION_PACK_RECORD))
-    create_info->null_bits++;
-  data_offset= (create_info->null_bits + 7) / 8;
+  bool need_deleted_bit= !(create_info->table_options & HA_OPTION_PACK_RECORD);
+  data_offset= (create_info->null_bits + need_deleted_bit + 7) / 8;
 
   error= pack_vcols(thd, &vcols,
                     create_fields, create_info->check_constraint_list);