Backporting parts of

ChangeSet 1.1620.12.1 and ChangeSet 1.1625.2.1
from 4.1. This makes the slave I/O thread flush the relay log 
after every event, which provides additional safety in case
of brutal crash (reduces chances to lose a part of the relay log).

sql/repl_failsafe.cc

@@ -894,7 +894,7 @@ int load_master_data(THD* thd)
 	if (active_mi->master_log_pos < BIN_LOG_HEADER_SIZE)
 	  active_mi->master_log_pos = BIN_LOG_HEADER_SIZE;
 	active_mi->rli.pending = 0;
-	flush_master_info(active_mi);
+	flush_master_info(active_mi, 0);
       }
       mc_mysql_free_result(master_status_res);
     }

sql/slave.cc

@@ -1657,7 +1657,7 @@ file '%s')", fname);
   mi->inited = 1;
   // now change cache READ -> WRITE - must do this before flush_master_info
   reinit_io_cache(&mi->file, WRITE_CACHE,0L,0,1);
-  if ((error=test(flush_master_info(mi))))
+  if ((error= test(flush_master_info(mi, 1))))
     sql_print_error("Failed to flush master info file");
   pthread_mutex_unlock(&mi->data_lock);
   DBUG_RETURN(error);
@@ -1784,13 +1784,15 @@ int show_master_info(THD* thd, MASTER_INFO* mi)
 }
 
 
-bool flush_master_info(MASTER_INFO* mi)
+bool flush_master_info(MASTER_INFO* mi, bool flush_relay_log_cache)
 {
   IO_CACHE* file = &mi->file;
   char lbuf[22];
   DBUG_ENTER("flush_master_info");
   DBUG_PRINT("enter",("master_pos: %ld", (long) mi->master_log_pos));
 
+  if (flush_relay_log_cache)   /* Comments for this are in MySQL 4.1 */
+    flush_io_cache(mi->rli.relay_log.get_log_file());
   my_b_seek(file, 0L);
   my_b_printf(file, "%s\n%s\n%s\n%s\n%s\n%d\n%d\n",
 	      mi->master_log_name, llstr(mi->master_log_pos, lbuf),
@@ -2542,7 +2544,7 @@ reconnect done to recover from failed read");
 	sql_print_error("Slave I/O thread could not queue event from master");
 	goto err;
       }
-      flush_master_info(mi);
+      flush_master_info(mi, 1);
       /*
         See if the relay logs take too much space.
         We don't lock mi->rli.log_space_lock here; this dirty read saves time

sql/slave.h

@@ -361,7 +361,7 @@ typedef struct st_table_rule_ent
 
 int init_slave();
 void init_slave_skip_errors(const char* arg);
-bool flush_master_info(MASTER_INFO* mi);
+bool flush_master_info(MASTER_INFO* mi, bool flush_relay_log_cache);
 bool flush_relay_log_info(RELAY_LOG_INFO* rli);
 int register_slave_on_master(MYSQL* mysql);
 int terminate_slave_threads(MASTER_INFO* mi, int thread_mask,

sql/sql_repl.cc

@@ -958,7 +958,7 @@ int change_master(THD* thd, MASTER_INFO* mi)
              sizeof(mi->master_log_name)-1);
   }
 
-  flush_master_info(mi);
+  flush_master_info(mi, 0);
   if (need_relay_log_purge)
   {
     mi->rli.skip_log_purge= 0;