MDEV-25607: Auto-generated DELETE from HEAP table can break replication

The special logic used by the memory storage engine
to keep slaves in sync with the master on a restart can
break replication. In particular, after a restart, the
master writes DELETE statements in the binlog for
each MEMORY-based table so the slave can empty its
data. If the DELETE is not executable, e.g. due to
invalid triggers, the slave will error and fail, whereas
the master will never see the problem.

Instead of DELETE statements, use TRUNCATE to
keep slaves in-sync with the master, thereby bypassing
triggers.

Reviewed By:
===========
Kristian Nielsen <knielsen@knielsen-hq.org>
Andrei Elkin <andrei.elkin@mariadb.com>

mysql-test/suite/rpl/r/rpl_mdev382.result

@@ -355,7 +355,7 @@ a`
 show binlog events in 'master-bin.000002' from <binlog_start>;
 Log_name	Pos	Event_type	Server_id	End_log_pos	Info
 master-bin.000002	#	Gtid	1	#	GTID #-#-#
-master-bin.000002	#	Query	1	#	DELETE FROM `db1``; select 'oops!'`.`t``1`
+master-bin.000002	#	Query	1	#	TRUNCATE TABLE `db1``; select 'oops!'`.`t``1`
 connection slave;
 include/start_slave.inc
 connection master;

mysql-test/suite/rpl/r/rpl_memory_engine_truncate_on_restart.result

+include/master-slave.inc
+[connection master]
+connection master;
+create table t (val int) engine=MEMORY;
+# DELETE trigger should never be activated
+create trigger tr after delete on t for each row update t2 set val = 1;
+insert into t values (1),(2);
+include/save_master_gtid.inc
+connection slave;
+include/sync_with_master_gtid.inc
+# Check pre-restart values
+include/diff_tables.inc [master:test.t,slave:test.t]
+# Restarting master should empty master and slave `t`
+connection master;
+include/rpl_restart_server.inc [server_number=1]
+connection master;
+# Validating MEMORY table on master is empty after restart
+# MYSQL_BINLOG datadir/binlog_file --result-file=assert_file
+include/assert_grep.inc [Query to truncate the MEMORY table should be the contents of the new event]
+# Ensuring slave MEMORY table is empty
+connection master;
+include/save_master_gtid.inc
+connection slave;
+include/sync_with_master_gtid.inc
+include/diff_tables.inc [master:test.t,slave:test.t]
+# Ensure new events replicate correctly
+connection master;
+insert into t values (3),(4);
+include/save_master_gtid.inc
+connection slave;
+include/sync_with_master_gtid.inc
+# Validate values on slave, after master restart, do not include those inserted previously
+include/diff_tables.inc [master:test.t,slave:test.t]
+#
+# Cleanup
+connection master;
+drop table t;
+include/rpl_end.inc
+# End of rpl_memory_engine_truncate_on_restart.test

mysql-test/suite/rpl/t/rpl_memory_engine_truncate_on_restart.test

+#
+#   This test ensures that a table with engine=memory is kept consistent with
+# the slave when the master restarts. That is, when the master is restarted, it
+# should binlog a new TRUNCATE TABLE command for tables with MEMORY engines,
+# such that after the slave executes these events, its MEMORY-engine tables
+# should be empty.
+#
+# References:
+#   MDEV-25607: Auto-generated DELETE from HEAP table can break replication
+#
+--source include/master-slave.inc
+
+--connection master
+create table t (val int) engine=MEMORY;
+
+-- echo # DELETE trigger should never be activated
+create trigger tr after delete on t for each row update t2 set val = 1;
+
+insert into t values (1),(2);
+--source include/save_master_gtid.inc
+--connection slave
+--source include/sync_with_master_gtid.inc
+
+-- echo # Check pre-restart values
+--let $diff_tables= master:test.t,slave:test.t
+--source include/diff_tables.inc
+
+--echo # Restarting master should empty master and slave `t`
+--connection master
+--let $seq_no_before_restart= `SELECT REGEXP_REPLACE(@@global.gtid_binlog_pos, "0-1-", "")`
+--let $rpl_server_number= 1
+--source include/rpl_restart_server.inc
+
+--connection master
+--echo # Validating MEMORY table on master is empty after restart
+--let $table_size= `select count(*) from t`
+if ($table_size)
+{
+  --echo # MEMORY table is not empty
+  --die MEMORY table is not empty
+}
+--let $seq_no_after_restart= `SELECT REGEXP_REPLACE(@@global.gtid_binlog_pos, "0-1-", "")`
+if ($seq_no_before_restart == $seq_no_after_restart)
+{
+  --echo # Event to empty MEMORY table was not binlogged
+  --die Event to empty MEMORY table was not binlogged
+}
+
+--let $binlog_file= query_get_value(SHOW MASTER STATUS, File, 1)
+--let $datadir=`select @@datadir`
+--let assert_file= $MYSQLTEST_VARDIR/tmp/binlog_decoded.out
+--echo # MYSQL_BINLOG datadir/binlog_file --result-file=assert_file
+--exec $MYSQL_BINLOG $datadir/$binlog_file --result-file=$assert_file
+
+--let assert_text= Query to truncate the MEMORY table should be the contents of the new event
+--let assert_count= 1
+--let assert_select= TRUNCATE TABLE
+--source include/assert_grep.inc
+
+--echo # Ensuring slave MEMORY table is empty
+--connection master
+--source include/save_master_gtid.inc
+--connection slave
+--source include/sync_with_master_gtid.inc
+--source include/diff_tables.inc
+
+--echo # Ensure new events replicate correctly
+--connection master
+insert into t values (3),(4);
+--source include/save_master_gtid.inc
+--connection slave
+--source include/sync_with_master_gtid.inc
+
+--echo # Validate values on slave, after master restart, do not include those inserted previously
+--source include/diff_tables.inc
+
+--echo #
+--echo # Cleanup
+--connection master
+drop table t;
+--source include/rpl_end.inc
+--echo # End of rpl_memory_engine_truncate_on_restart.test

sql/sql_base.cc

@@ -2993,7 +2993,7 @@ static bool open_table_entry_fini(THD *thd, TABLE_SHARE *share, TABLE *entry)
       String query(query_buf, sizeof(query_buf), system_charset_info);
 
       query.length(0);
-      query.append("DELETE FROM ");
+      query.append("TRUNCATE TABLE ");
       append_identifier(thd, &query, &share->db);
       query.append(".");
       append_identifier(thd, &query, &share->table_name);