MDEV-29873 MSAN uninitialized value errors in bcmp /

	   prep_alter_part_table upon re-partitioning by system time

memcmp() tries to compare beyond the last member of interval because
sizeof(Vers_part_info::interval) is 80. It is sizeof of variable,
sizeof of type is 76.

Now we compare interval_t struct C++ way.

mysql-test/suite/versioning/r/partition.result

@@ -3406,4 +3406,11 @@ f()
 drop procedure p;
 drop function f;
 drop table t;
+#
+# MDEV-29873 MSAN uninitialized value errors in bcmp /
+#            prep_alter_part_table upon re-partitioning by system time
+#
+create table t (a int) with system versioning partition by system_time interval 5 week;
+alter table t partition by system_time interval 10 week;
+drop table t;
 set global innodb_stats_persistent= @save_persistent;

mysql-test/suite/versioning/t/partition.test

@@ -2624,6 +2624,15 @@ drop procedure p;
 drop function f;
 drop table t;
 
+--echo #
+--echo # MDEV-29873 MSAN uninitialized value errors in bcmp /
+--echo #            prep_alter_part_table upon re-partitioning by system time
+--echo #
+create table t (a int) with system versioning partition by system_time interval 5 week;
+alter table t partition by system_time interval 10 week;
+# cleanup
+drop table t;
+
 --disable_prepare_warnings
 set global innodb_stats_persistent= @save_persistent;
 --source suite/versioning/common_finish.inc

sql/partition_info.h

@@ -78,11 +78,16 @@ struct Vers_part_info : public Sql_alloc
     }
     return false;
   }
-  struct {
+  struct interval_t {
     my_time_t start;
     INTERVAL step;
     enum interval_type type;
     bool is_set() const { return type < INTERVAL_LAST; }
+    bool operator==(const interval_t &rhs) const
+    {
+      /* TODO: equivalent intervals like 1 hour and 60 mins should be considered equal */
+      return start == rhs.start && type == rhs.type && !memcmp(&step, &rhs.step, sizeof(INTERVAL));
+    }
   } interval;
   ulonglong limit;
   bool auto_hist;

sql/sql_partition.cc

@@ -6035,10 +6035,7 @@ the generated partition syntax in a correct manner.
       {
         if (part_info->vers_info->interval.is_set() && (
             !tab_part_info->vers_info->interval.is_set() ||
-            /* TODO: equivalent intervals like 1 hour and 60 mins should be considered equal */
-            memcmp(&part_info->vers_info->interval,
-                   &tab_part_info->vers_info->interval,
-                   sizeof(Vers_part_info::interval))))
+            part_info->vers_info->interval == tab_part_info->vers_info->interval))
         {
           /* If interval is changed we can not do fast alter */
           tab_part_info= tab_part_info->get_clone(thd);