MDEV-32803 Assertion `total == 0' failed in Event_log::write_cache_raw

A second DML in a transaction for a table of non-rollbackable engine
leads to a cache corruption, because the cache wasn't reset after a
statement end, but also wasn't destroyed.

This patch resets the cache for a reuse by subsequent statements in
current transaction.

mysql-test/main/alter_table_online_debug.result

@@ -1785,6 +1785,28 @@ a	d
 0	qwe
 0	qwe
 drop table t;
+#
+# MDEV-32803 Assertion `total == 0' failed in Event_log::write_cache_raw
+#
+create or replace table t1 (a int) engine=aria;
+insert t1 values (5);
+set debug_sync= 'alter_table_copy_end SIGNAL ended WAIT_FOR end';
+alter table t1 add b int NULL, algorithm= copy, lock= none;
+connection con2;
+set debug_sync= 'now WAIT_FOR ended';
+begin;
+insert into t1 values (123);
+insert into t1 values (456), (789);
+commit;
+set debug_sync= 'now SIGNAL end';
+connection default;
+select * from t1;
+a	b
+5	NULL
+123	NULL
+456	NULL
+789	NULL
+drop table t1;
 set global default_storage_engine= MyISAM;
 disconnect con1;
 disconnect con2;

mysql-test/main/alter_table_online_debug.test

@@ -2041,6 +2041,31 @@ select * from t;
 drop table t;
 
 
+--echo #
+--echo # MDEV-32803 Assertion `total == 0' failed in Event_log::write_cache_raw
+--echo #
+
+create or replace table t1 (a int) engine=aria;
+insert t1 values (5);
+set debug_sync= 'alter_table_copy_end SIGNAL ended WAIT_FOR end';
+
+send alter table t1 add b int NULL, algorithm= copy, lock= none;
+
+--connection con2
+set debug_sync= 'now WAIT_FOR ended';
+begin;
+insert into t1 values (123);
+insert into t1 values (456), (789);
+commit;
+set debug_sync= 'now SIGNAL end';
+
+--connection default
+--reap
+select * from t1;
+
+drop table t1;
+
+
 eval set global default_storage_engine= $default_storage_engine;
 
 --disconnect con1

sql/online_alter.cc

@@ -152,6 +152,7 @@ int online_alter_log_row(TABLE* table, const uchar *before_record,
   if (!table->online_alter_cache)
   {
     table->online_alter_cache= get_cache_data(thd, table);
+    DBUG_ASSERT(table->online_alter_cache->cache_log.type == WRITE_CACHE);
     trans_register_ha(thd, false, online_alter_hton, 0);
     if (thd->in_multi_stmt_transaction_mode())
       trans_register_ha(thd, true, online_alter_hton, 0);
@@ -237,6 +238,8 @@ int online_alter_end_trans(Online_alter_cache_list &cache_list, THD *thd,
         mysql_mutex_lock(binlog->get_log_lock());
         error= binlog->write_cache_raw(thd, &cache.cache_log);
         mysql_mutex_unlock(binlog->get_log_lock());
+        if (!is_ending_transaction)
+          cache.reset();
       }
     }
     else if (!commit) // rollback