Test for bug 578. And a comment in slave.cc.

sql/slave.cc:
  Comment to remember that size of relay logs on disk is not the total size
  of the relay log.

mysql-test/r/lock_tables_lost_commit.result

+drop table if exists t1;
+create table t1(a int) type=innodb;
+lock tables t1 write;
+insert into t1 values(10);
+select * from t1;
+a
+10
+drop table t1;

mysql-test/t/lock_tables_lost_commit-master.opt

+--binlog-ignore-db=test innodb
\ No newline at end of file

mysql-test/t/lock_tables_lost_commit.test

+# This is a test for bug 578
+
+connect (con1,localhost,root,,);
+connect (con2,localhost,root,,);
+
+connection con1;
+drop table if exists t1;
+create table t1(a int) type=innodb;
+lock tables t1 write;
+insert into t1 values(10);
+disconnect con1;
+
+connection con2;
+# The bug was that, because of the LOCK TABLES, the handler "forgot" to commit,
+# and the other commit when we write to the binlog was not done because of
+# binlog-ignore-db  
+select * from t1;
+drop table t1;

sql/slave.cc

@@ -1186,6 +1186,26 @@ int init_relay_log_info(RELAY_LOG_INFO* rli, const char* info_fname)
     strmov(strcend(tmp,'.'),"-relay-bin");
     opt_relay_logname=my_strdup(tmp,MYF(MY_WME));
   }
+
+  /*
+    The relay log will now be opened, as a SEQ_READ_APPEND IO_CACHE. It is
+    notable that the last kilobytes of it (8 kB for example) may live in memory,
+    not on disk (depending on what the thread using it does). While this is
+    efficient, it has a side-effect one must know: 
+    the size of the relay log on disk (displayed by 'ls -l' on Unix) can be a
+    few kilobytes less than one would expect by doing SHOW SLAVE STATUS; this
+    happens when only the IO thread is started (not the SQL thread). The
+    "missing" kilobytes are in memory, are preserved during 'STOP SLAVE; START
+    SLAVE IO_THREAD', and are flushed to disk when the slave's mysqld stops. So
+    this does not cause any bug. Example of how disk size grows by leaps:
+
+     Read_Master_Log_Pos: 7811 -rw-rw----    1 guilhem  qq              4 Jun  5 16:19 gbichot2-relay-bin.002
+     ...later...
+     Read_Master_Log_Pos: 9744 -rw-rw----    1 guilhem  qq           8192 Jun  5 16:27 gbichot2-relay-bin.002
+
+    See how 4 is less than 7811 and 8192 is less than 9744.
+  */
+
   if (open_log(&rli->relay_log, glob_hostname, opt_relay_logname,
 	       "-relay-bin", opt_relaylog_index_name,
 	       LOG_BIN, 1 /* read_append cache */,