Make log_pos be non-zero in the binary log, for BEGIN and COMMIT Query_log_events.

Now log_pos is the real position of the event. For events that are between BEGIN and COMMIT,
log_pos is still the BEGIN's log_pos.
This fixes bug #52 (If only transactions are run on the master then t

sql/log.cc

@@ -1211,6 +1211,14 @@ bool MYSQL_LOG::write(THD *thd, IO_CACHE *cache)
     */
     {
       Query_log_event qinfo(thd, "BEGIN", 5, TRUE);
+      /*
+        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.
+      */
+      qinfo.set_log_pos(this);
       if (qinfo.write(&log_file))
 	goto err;
     }
@@ -1233,6 +1241,7 @@ bool MYSQL_LOG::write(THD *thd, IO_CACHE *cache)
 
     {
       Query_log_event qinfo(thd, "COMMIT", 6, TRUE);
+      qinfo.set_log_pos(this);
       if (qinfo.write(&log_file) || flush_io_cache(&log_file))
 	goto err;
     }