Replication: give the slave I/O thread a higher priority than the slave SQL thread

so that it has fetched the most from the master's binlog before the master
accidently dies. SCRUM task #756.

sql/slave.cc

@@ -492,7 +492,8 @@ int start_slave_thread(pthread_handler h_func, pthread_mutex_t *start_lock,
 		       pthread_cond_t *start_cond,
 		       volatile bool *slave_running,
 		       volatile ulong *slave_run_id,
-		       MASTER_INFO* mi)
+		       MASTER_INFO* mi,
+                       bool high_priority)
 {
   pthread_t th;
   ulong start_id;
@@ -521,6 +522,8 @@ int start_slave_thread(pthread_handler h_func, pthread_mutex_t *start_lock,
   }
   start_id= *slave_run_id;
   DBUG_PRINT("info",("Creating new slave thread"));
+  if (high_priority)
+    my_pthread_attr_setprio(&connection_attrib,CONNECT_PRIOR);
   if (pthread_create(&th, &connection_attrib, h_func, (void*)mi))
   {
     if (start_lock)
@@ -585,13 +588,13 @@ int start_slave_threads(bool need_slave_mutex, bool wait_for_start,
     error=start_slave_thread(handle_slave_io,lock_io,lock_cond_io,
 			     cond_io,
 			     &mi->slave_running, &mi->slave_run_id,
-			     mi);
+			     mi, 1); //high priority, to read the most possible
   if (!error && (thread_mask & SLAVE_SQL))
   {
     error=start_slave_thread(handle_slave_sql,lock_sql,lock_cond_sql,
 			     cond_sql,
 			     &mi->rli.slave_running, &mi->rli.slave_run_id,
-			     mi);
+			     mi, 0);
     if (error)
       terminate_slave_threads(mi, thread_mask & SLAVE_IO, 0);
   }

sql/slave.h

@@ -368,7 +368,8 @@ int start_slave_thread(pthread_handler h_func, pthread_mutex_t* start_lock,
 		       pthread_cond_t* start_cond,
 		       volatile bool *slave_running,
 		       volatile ulong *slave_run_id,
-		       MASTER_INFO* mi);
+		       MASTER_INFO* mi,
+                       bool high_priority);
 
 /* If fd is -1, dump to NET */
 int mysql_table_dump(THD* thd, const char* db,