BUG#26395: if crash during autocommit update to transactional table on master, slave fails

Now, every transaction (including autocommit transactions) start with
a BEGIN and end with a COMMIT/ROLLBACK in the binlog.
Added a test case, and updated lots of test case result files.

mysql-test/r/mix_innodb_myisam_binlog.result

@@ -100,9 +100,10 @@ insert into t1 values(9);
 insert into t2 select * from t1;
 show binlog events from 98;
 Log_name	Pos	Event_type	Server_id	End_log_pos	Info
-master-bin.000001	98	Query	1	#	use `test`; insert into t1 values(9)
-master-bin.000001	185	Xid	1	#	COMMIT /* XID */
-master-bin.000001	212	Query	1	#	use `test`; insert into t2 select * from t1
+master-bin.000001	98	Query	1	#	use `test`; BEGIN
+master-bin.000001	166	Query	1	#	use `test`; insert into t1 values(9)
+master-bin.000001	253	Xid	1	#	COMMIT /* XID */
+master-bin.000001	280	Query	1	#	use `test`; insert into t2 select * from t1
 delete from t1;
 delete from t2;
 reset master;
@@ -111,19 +112,21 @@ begin;
 insert into t2 select * from t1;
 show binlog events from 98;
 Log_name	Pos	Event_type	Server_id	End_log_pos	Info
-master-bin.000001	98	Query	1	#	use `test`; insert into t1 values(10)
-master-bin.000001	186	Xid	1	#	COMMIT /* XID */
-master-bin.000001	213	Query	1	#	use `test`; insert into t2 select * from t1
+master-bin.000001	98	Query	1	#	use `test`; BEGIN
+master-bin.000001	166	Query	1	#	use `test`; insert into t1 values(10)
+master-bin.000001	254	Xid	1	#	COMMIT /* XID */
+master-bin.000001	281	Query	1	#	use `test`; insert into t2 select * from t1
 insert into t1 values(11);
 commit;
 show binlog events from 98;
 Log_name	Pos	Event_type	Server_id	End_log_pos	Info
-master-bin.000001	98	Query	1	#	use `test`; insert into t1 values(10)
-master-bin.000001	186	Xid	1	#	COMMIT /* XID */
-master-bin.000001	213	Query	1	#	use `test`; insert into t2 select * from t1
-master-bin.000001	307	Query	1	#	use `test`; BEGIN
-master-bin.000001	375	Query	1	#	use `test`; insert into t1 values(11)
-master-bin.000001	463	Xid	1	#	COMMIT /* XID */
+master-bin.000001	98	Query	1	#	use `test`; BEGIN
+master-bin.000001	166	Query	1	#	use `test`; insert into t1 values(10)
+master-bin.000001	254	Xid	1	#	COMMIT /* XID */
+master-bin.000001	281	Query	1	#	use `test`; insert into t2 select * from t1
+master-bin.000001	375	Query	1	#	use `test`; BEGIN
+master-bin.000001	443	Query	1	#	use `test`; insert into t1 values(11)
+master-bin.000001	531	Xid	1	#	COMMIT /* XID */
 alter table t2 engine=INNODB;
 delete from t1;
 delete from t2;
@@ -235,25 +238,29 @@ master-bin.000001	98	Query	1	#	use `test`; BEGIN
 master-bin.000001	166	Query	1	#	use `test`; insert into t1 values(16)
 master-bin.000001	254	Query	1	#	use `test`; insert into t1 values(18)
 master-bin.000001	342	Xid	1	#	COMMIT /* XID */
-master-bin.000001	369	Query	1	#	use `test`; delete from t1
-master-bin.000001	446	Xid	1	#	COMMIT /* XID */
-master-bin.000001	473	Query	1	#	use `test`; delete from t2
-master-bin.000001	550	Xid	1	#	COMMIT /* XID */
-master-bin.000001	577	Query	1	#	use `test`; alter table t2 type=MyISAM
-master-bin.000001	666	Query	1	#	use `test`; insert into t1 values (1)
-master-bin.000001	754	Xid	1	#	COMMIT /* XID */
-master-bin.000001	781	Query	1	#	use `test`; insert into t2 values (20)
-master-bin.000001	870	Query	1	#	use `test`; drop table t1,t2
-master-bin.000001	949	Query	1	#	use `test`; create temporary table ti (a int) engine=innodb
-master-bin.000001	1059	Query	1	#	use `test`; insert into ti values(1)
-master-bin.000001	1146	Xid	1	#	COMMIT /* XID */
-master-bin.000001	1173	Query	1	#	use `test`; create temporary table t1 (a int) engine=myisam
-master-bin.000001	1283	Query	1	#	use `test`; insert t1 values (1)
-master-bin.000001	1366	Query	1	#	use `test`; create table t0 (n int)
-master-bin.000001	1452	Query	1	#	use `test`; insert t0 select * from t1
-master-bin.000001	1541	Query	1	#	use `test`; insert into t0 select GET_LOCK("lock1",null)
-master-bin.000001	1648	Query	1	#	use `test`; create table t2 (n int) engine=innodb
-master-bin.000001	1748	Query	1	#	use `test`; DROP /*!40005 TEMPORARY */ TABLE IF EXISTS `test`.`t1`,`test`.`ti`
+master-bin.000001	369	Query	1	#	use `test`; BEGIN
+master-bin.000001	437	Query	1	#	use `test`; delete from t1
+master-bin.000001	514	Xid	1	#	COMMIT /* XID */
+master-bin.000001	541	Query	1	#	use `test`; BEGIN
+master-bin.000001	609	Query	1	#	use `test`; delete from t2
+master-bin.000001	686	Xid	1	#	COMMIT /* XID */
+master-bin.000001	713	Query	1	#	use `test`; alter table t2 type=MyISAM
+master-bin.000001	802	Query	1	#	use `test`; BEGIN
+master-bin.000001	870	Query	1	#	use `test`; insert into t1 values (1)
+master-bin.000001	958	Xid	1	#	COMMIT /* XID */
+master-bin.000001	985	Query	1	#	use `test`; insert into t2 values (20)
+master-bin.000001	1074	Query	1	#	use `test`; drop table t1,t2
+master-bin.000001	1153	Query	1	#	use `test`; create temporary table ti (a int) engine=innodb
+master-bin.000001	1263	Query	1	#	use `test`; BEGIN
+master-bin.000001	1331	Query	1	#	use `test`; insert into ti values(1)
+master-bin.000001	1418	Xid	1	#	COMMIT /* XID */
+master-bin.000001	1445	Query	1	#	use `test`; create temporary table t1 (a int) engine=myisam
+master-bin.000001	1555	Query	1	#	use `test`; insert t1 values (1)
+master-bin.000001	1638	Query	1	#	use `test`; create table t0 (n int)
+master-bin.000001	1724	Query	1	#	use `test`; insert t0 select * from t1
+master-bin.000001	1813	Query	1	#	use `test`; insert into t0 select GET_LOCK("lock1",null)
+master-bin.000001	1920	Query	1	#	use `test`; create table t2 (n int) engine=innodb
+master-bin.000001	2020	Query	1	#	use `test`; DROP /*!40005 TEMPORARY */ TABLE IF EXISTS `test`.`t1`,`test`.`ti`
 do release_lock("lock1");
 drop table t0,t2;
 reset master;
@@ -402,7 +409,7 @@ insert into t2 values (bug27417(1));
 ERROR 23000: Duplicate entry '1' for key 1
 show master status /* the offset must denote there is the query */;
 File	Position	Binlog_Do_DB	Binlog_Ignore_DB
-master-bin.000001	267		
+master-bin.000001	335		
 select count(*) from t1 /* must be 1 */;
 count(*)
 1
@@ -414,7 +421,7 @@ insert into t2 select bug27417(1) union select bug27417(2);
 ERROR 23000: Duplicate entry '2' for key 1
 show master status /* the offset must denote there is the query */;
 File	Position	Binlog_Do_DB	Binlog_Ignore_DB
-master-bin.000001	290		
+master-bin.000001	358		
 select count(*) from t1 /* must be 2 */;
 count(*)
 2
@@ -438,7 +445,7 @@ UPDATE t4,t3 SET t4.a=t3.a + bug27417(1) /* top level non-ta table */;
 ERROR 23000: Duplicate entry '2' for key 1
 show master status /* the offset must denote there is the query */;
 File	Position	Binlog_Do_DB	Binlog_Ignore_DB
-master-bin.000001	301		
+master-bin.000001	369		
 select count(*) from t1 /* must be 4 */;
 count(*)
 4
@@ -466,7 +473,7 @@ delete from t2;
 ERROR 23000: Duplicate entry '1' for key 1
 show master status /* the offset must denote there is the query */;
 File	Position	Binlog_Do_DB	Binlog_Ignore_DB
-master-bin.000001	246		
+master-bin.000001	314		
 select count(*) from t1 /* must be 1 */;
 count(*)
 1
@@ -483,7 +490,7 @@ delete t2.* from t2,t5 where t2.a=t5.a + 1;
 ERROR 23000: Duplicate entry '1' for key 1
 show master status /* the offset must denote there is the query */;
 File	Position	Binlog_Do_DB	Binlog_Ignore_DB
-master-bin.000001	274		
+master-bin.000001	342		
 select count(*) from t1 /* must be 1 */;
 count(*)
 1
@@ -501,7 +508,7 @@ count(*)
 2
 show master status /* the offset must denote there is the query */;
 File	Position	Binlog_Do_DB	Binlog_Ignore_DB
-master-bin.000001	376		
+master-bin.000001	444		
 drop trigger trg_del_t2;
 drop table t1,t2,t3,t4,t5;
 drop function bug27417;

mysql-test/r/multi_update.result

@@ -545,7 +545,7 @@ a	b
 4	4
 show master status /* there must be the UPDATE query event */;
 File	Position	Binlog_Do_DB	Binlog_Ignore_DB
-master-bin.000001	260		
+master-bin.000001	328		
 delete from t1;
 delete from t2;
 insert into t1 values (1,2),(3,4),(4,4);
@@ -555,7 +555,7 @@ UPDATE t2,t1  SET t2.a=t2.b where t2.a=t1.a;
 ERROR 23000: Duplicate entry '4' for key 1
 show master status /* there must be the UPDATE query event */;
 File	Position	Binlog_Do_DB	Binlog_Ignore_DB
-master-bin.000001	275		
+master-bin.000001	343		
 drop table t1, t2;
 drop table if exists t1, t2, t3;
 CREATE TABLE t1 (a int, PRIMARY KEY (a));

mysql-test/r/rpl_transaction.result

+stop slave;
+drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
+reset master;
+reset slave;
+drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
+start slave;
+CREATE TABLE tmyisam (a int) ENGINE = MYISAM;
+CREATE TABLE tinnodb (a int) ENGINE = INNODB;
+SHOW CREATE TABLE tmyisam;
+Table	Create Table
+tmyisam	CREATE TABLE `tmyisam` (
+  `a` int(11) default NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+SHOW CREATE TABLE tinnodb;
+Table	Create Table
+tinnodb	CREATE TABLE `tinnodb` (
+  `a` int(11) default NULL
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+==== Test 1: Non-XA Engines ====
+--- on master ---
+SET AUTOCOMMIT = 1;
+INSERT INTO tmyisam VALUES (1);
+BEGIN;
+INSERT INTO tmyisam VALUES (2);
+INSERT INTO tmyisam VALUES (3);
+COMMIT;
+BEGIN;
+INSERT INTO tmyisam VALUES (5);
+INSERT INTO tmyisam VALUES (6);
+ROLLBACK;
+Warnings:
+Warning	1196	Some non-transactional changed tables couldn't be rolled back
+SELECT * FROM tmyisam ORDER BY a;
+a
+1
+2
+3
+5
+6
+--- on slave ---
+SELECT * FROM tmyisam ORDER BY a;
+a
+1
+2
+3
+5
+6
+==== Test 2: Master crash before writing XID event on XA engine ====
+--- on master ---
+INSERT INTO tinnodb VALUES (1);
+SELECT * FROM tinnodb ORDER BY a;
+a
+1
+--- on slave ---
+STOP SLAVE;
+SHOW SLAVE STATUS;
+Slave_IO_State	
+Master_Host	127.0.0.1
+Master_User	root
+Master_Port	#
+Connect_Retry	1
+Master_Log_File	master-bin.000001
+Read_Master_Log_Pos	#
+Relay_Log_File	#
+Relay_Log_Pos	#
+Relay_Master_Log_File	master-bin.000001
+Slave_IO_Running	No
+Slave_SQL_Running	No
+Replicate_Do_DB	
+Replicate_Ignore_DB	
+Replicate_Do_Table	
+Replicate_Ignore_Table	#
+Replicate_Wild_Do_Table	
+Replicate_Wild_Ignore_Table	
+Last_Errno	0
+Last_Error	
+Skip_Counter	0
+Exec_Master_Log_Pos	#
+Relay_Log_Space	#
+Until_Condition	None
+Until_Log_File	
+Until_Log_Pos	0
+Master_SSL_Allowed	No
+Master_SSL_CA_File	
+Master_SSL_CA_Path	
+Master_SSL_Cert	
+Master_SSL_Cipher	
+Master_SSL_Key	
+Seconds_Behind_Master	#
+SELECT * FROM tinnodb ORDER BY a;
+a
+DROP TABLE tmyisam;
+DROP TABLE tinnodb;
+DROP TABLE tmyisam;
+DROP TABLE tinnodb;

mysql-test/r/sp_trans_log.result

@@ -16,6 +16,7 @@ show binlog events from 98 /* with fixes for #23333 will show there are 2 querie
 Log_name	Pos	Event_type	Server_id	End_log_pos	Info
 master-bin.000001	#	Query	1	#	#
 master-bin.000001	#	Query	1	#	#
+master-bin.000001	#	Query	1	#	#
 select count(*),@a from t1 /* must be 1,1 */|
 count(*)	@a
 1	1

mysql-test/r/variables-big.result

 set session transaction_prealloc_size=1024*1024*1024*1;
 show processlist;
 Id	User	Host	db	Command	Time	State	Info
-1	root	localhost	test	Query	0	NULL	show processlist
+6	root	localhost	test	Query	0	NULL	show processlist
 set session transaction_prealloc_size=1024*1024*1024*2;
 show processlist;
 Id	User	Host	db	Command	Time	State	Info
-1	root	localhost	test	Query	2	NULL	show processlist
+6	root	localhost	test	Query	1	NULL	show processlist
 set session transaction_prealloc_size=1024*1024*1024*3;
 show processlist;
 Id	User	Host	db	Command	Time	State	Info
-1	root	localhost	test	Query	0	NULL	show processlist
+6	root	localhost	test	Query	0	NULL	show processlist
 set session transaction_prealloc_size=1024*1024*1024*4;
+Warnings:
+Warning	1292	Truncated incorrect transaction_prealloc_size value: '4294967296'
 show processlist;
 Id	User	Host	db	Command	Time	State	Info
-1	root	localhost	test	Query	0	NULL	show processlist
+6	root	localhost	test	Query	0	NULL	show processlist
 set session transaction_prealloc_size=1024*1024*1024*5;
+Warnings:
+Warning	1292	Truncated incorrect transaction_prealloc_size value: '5368709120'
 show processlist;
 Id	User	Host	db	Command	Time	State	Info
-1	root	localhost	test	Query	0	NULL	show processlist
+6	root	localhost	test	Query	0	NULL	show processlist

mysql-test/t/rpl_transaction-master.opt

+--innodb --debug=d,do_not_write_xid

mysql-test/t/rpl_transaction-slave.opt

+--innodb

mysql-test/t/rpl_transaction.test

+# Tests that transactions are replicated correctly, with various
+# combinations of non-transactional and transactional non-XA tables.
+# Also tests that an XA transaction where the master crashes just
+# before writing the XID log event is executed correctly.  See below
+# for implementation details.
+
+# Note: this test should not exist in 5.1 or higher.  It has been
+# replaced by rpl_ndb_transaction.test, which tests a superset of what
+# this test tests.
+
+source include/have_innodb.inc;
+source include/master-slave.inc;
+
+
+CREATE TABLE tmyisam (a int) ENGINE = MYISAM;
+CREATE TABLE tinnodb (a int) ENGINE = INNODB;
+
+SHOW CREATE TABLE tmyisam;
+SHOW CREATE TABLE tinnodb;
+
+
+--echo ==== Test 1: Non-XA Engines ====
+# Test that everything works fine with non-XA engines.  We just try
+# all ways to do transactions involving ndb and/or myisam, with
+# rollback or commit.
+
+--echo --- on master ---
+
+SET AUTOCOMMIT = 1;
+
+INSERT INTO tmyisam VALUES (1);
+
+BEGIN;
+INSERT INTO tmyisam VALUES (2);
+INSERT INTO tmyisam VALUES (3);
+COMMIT;
+
+BEGIN;
+INSERT INTO tmyisam VALUES (5);
+INSERT INTO tmyisam VALUES (6);
+--warning 1196
+ROLLBACK;
+
+SELECT * FROM tmyisam ORDER BY a;
+
+--echo --- on slave ---
+--sync_slave_with_master
+SELECT * FROM tmyisam ORDER BY a;
+
+
+--echo ==== Test 2: Master crash before writing XID event on XA engine ====
+# We now want to test the following scenario, to verify that BUG#26395
+# has been fixed:
+
+#   "master and slave have a transactional table that uses XA. Master
+#    has AUTOCOMMIT on and executes a statement (in this case an
+#    INSERT). Master crashes just before writing the XID event."
+
+# In this scenario, master will roll back, so slave should not execute
+# the statement, and slave should roll back later when master is
+# restarted.
+
+# However, we the master to be alive so that we are sure it replicates
+# the statement to the slave. So in the test case, we must therefore
+# not crash the master. Instead, we fake the crash by just not writing
+# the XID event to the binlog. This is done by the
+# --debug=d,do_not_write_xid flag in the .opt file.
+
+# So, unlike if the master had crashed, the master *will* execute the
+# statement. But the slave should not execute it. Hence, after the
+# first test is executed, the expected result on master is a table
+# with one row, and on slave a table with no rows.
+
+# To simulate the slave correctly, we wait until everything up to the
+# XID is replicated. We cannot sync_slave_with_master, because that
+# would wait for the transaction to end. Instead, we wait for
+# "sufficiently long time". Then we stop the slave.
+
+# Note: since this puts the master binlog in an inconsistent state,
+# this should be the last test of the file.
+
+--echo --- on master ---
+--connection master
+
+INSERT INTO tinnodb VALUES (1);
+SELECT * FROM tinnodb ORDER BY a;
+
+--echo --- on slave ---
+--connection slave
+--sleep 3
+STOP SLAVE;
+source include/wait_for_slave_to_stop.inc;
+--replace_column 4 # 7 # 8 # 9 # 16 # 22 # 23 # 33 #
+query_vertical SHOW SLAVE STATUS;
+# the following statement should show that nothing has been replicated
+SELECT * FROM tinnodb ORDER BY a;
+
+
+# clean up
+connection master;
+DROP TABLE tmyisam;
+DROP TABLE tinnodb;
+
+connection slave;
+DROP TABLE tmyisam;
+DROP TABLE tinnodb;

sql/log.cc

@@ -122,6 +122,20 @@ static int binlog_prepare(THD *thd, bool all)
   return 0;
 }
 
+/**
+  This function is called once after each statement.
+
+  It has the responsibility to flush the transaction cache to the
+  binlog file on commits.
+
+  @param thd   The client thread that executes the transaction.
+  @param all   true if this is the last statement before a COMMIT
+               statement; false if either this is a statement in a
+               transaction but not the last, or if this is a statement
+               not inside a BEGIN block and autocommit is on.
+
+  @see handlerton::commit
+*/
 static int binlog_commit(THD *thd, bool all)
 {
   IO_CACHE *trans_log= (IO_CACHE*)thd->ha_data[binlog_hton.slot];
@@ -134,7 +148,15 @@ static int binlog_commit(THD *thd, bool all)
     // we're here because trans_log was flushed in MYSQL_LOG::log_xid()
     DBUG_RETURN(0);
   }
-  if (all)
+  /*
+    Write commit event if at least one of the following holds:
+     - the user sends an explicit COMMIT; or
+     - the autocommit flag is on, and we are not inside a BEGIN.
+    However, if the user has not sent an explicit COMMIT, and we are
+    either inside a BEGIN or run with autocommit off, then this is not
+    the end of a transaction and we should not write a commit event.
+  */
+  if (all || !(thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN)))
   {
     Query_log_event qev(thd, STRING_WITH_LEN("COMMIT"), TRUE, FALSE);
     qev.error_code= 0; // see comment in MYSQL_LOG::write(THD, IO_CACHE)
@@ -144,6 +166,22 @@ static int binlog_commit(THD *thd, bool all)
     DBUG_RETURN(binlog_end_trans(thd, trans_log, &invisible_commit));
 }
 
+/**
+  This function is called when a transaction involving a transactional
+  table is rolled back.
+
+  It has the responsibility to flush the transaction cache to the
+  binlog file. However, if the transaction does not involve
+  non-transactional tables, nothing needs to be logged.
+
+  @param thd   The client thread that executes the transaction.
+  @param all   true if this is the last statement before a COMMIT
+               statement; false if either this is a statement in a
+               transaction but not the last, or if this is a statement
+               not inside a BEGIN block and autocommit is on.
+
+  @see handlerton::rollback
+*/
 static int binlog_rollback(THD *thd, bool all)
 {
   int error=0;
@@ -1817,9 +1855,11 @@ uint MYSQL_LOG::next_file_id()
   IMPLEMENTATION
     - To support transaction over replication, we wrap the transaction
       with BEGIN/COMMIT or BEGIN/ROLLBACK in the binary log.
-      We want to write a BEGIN/ROLLBACK block when a non-transactional table
-      was updated in a transaction which was rolled back. This is to ensure
-      that the same updates are run on the slave.
+      If a transaction that only involves transactional tables is
+      rolled back, we do not binlog it. However, we write a
+      BEGIN/ROLLBACK block when a non-transactional table was updated
+      in a transaction which was rolled back. This is to ensure that
+      the same updates are run on the slave.
 */
 
 bool MYSQL_LOG::write(THD *thd, IO_CACHE *cache, Log_event *commit_event)
@@ -1837,32 +1877,34 @@ bool MYSQL_LOG::write(THD *thd, IO_CACHE *cache, Log_event *commit_event)
     byte header[LOG_EVENT_HEADER_LEN];
 
     /*
-      Log "BEGIN" at the beginning of the transaction.
-      which may contain more than 1 SQL statement.
+      Log "BEGIN" at the beginning of every transaction.  Here, a
+      transaction is either a BEGIN..COMMIT block or a single
+      statement in autocommit mode.
     */
-    if (thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN))
-    {
     Query_log_event qinfo(thd, STRING_WITH_LEN("BEGIN"), TRUE, FALSE);
     /*
-        Imagine this is rollback due to net timeout, after all statements of
-        the transaction succeeded. Then we want a zero-error code in BEGIN.
-        In other words, if there was a really serious error code it's already
-        in the statement's events, there is no need to put it also in this
-        internally generated event, and as this event is generated late it
-        would lead to false alarms.
+      Imagine this is rollback due to net timeout, after all
+      statements of the transaction succeeded. Then we want a
+      zero-error code in BEGIN.  In other words, if there was a
+      really serious error code it's already in the statement's
+      events, there is no need to put it also in this internally
+      generated event, and as this event is generated late it would
+      lead to false alarms.
+
       This is safer than thd->clear_error() against kills at shutdown.
     */
     qinfo.error_code= 0;
     /*
-        Now this Query_log_event has artificial log_pos 0. It must be adjusted
-        to reflect the real position in the log. Not doing it would confuse the
-	slave: it would prevent this one from knowing where he is in the
-	master's binlog, which would result in wrong positions being shown to
-	the user, MASTER_POS_WAIT undue waiting etc.
+      Now this Query_log_event has artificial log_pos 0. It must be
+      adjusted to reflect the real position in the log. Not doing it
+      would confuse the slave: it would prevent this one from
+      knowing where he is in the master's binlog, which would result
+      in wrong positions being shown to the user, MASTER_POS_WAIT
+      undue waiting etc.
     */
     if (qinfo.write(&log_file))
       goto err;
-    }
+
     /* Read from the file used to cache the queries .*/
     if (reinit_io_cache(cache, READ_CACHE, 0, 0, 0))
       goto err;

sql/log_event.cc

@@ -3793,6 +3793,7 @@ Xid_log_event(const char* buf,
 #ifndef MYSQL_CLIENT
 bool Xid_log_event::write(IO_CACHE* file)
 {
+  DBUG_EXECUTE_IF("do_not_write_xid", return 0;);
   return write_header(file, sizeof(xid)) ||
          my_b_safe_write(file, (byte*) &xid, sizeof(xid));
 }