Fix for BUG#858 "CHANGE MASTER forgets to update relay-log.info";

just a flush_relay_log_info() at the end of CHANGE MASTER
(there was already flush_master_info()).


sql/sql_repl.cc:
  Comments.
  Flush relay-log.info after CHANGE MASTER, or the changes
  to the relay log parameters (relay log name and position, and
  the corresponding master's binlog name and position) will be
  lost if the slave mysqld is shutdown immediately after the 
  CHANGE MASTER (without the slave threads being started).

sql/sql_repl.cc

@@ -878,9 +878,14 @@ int change_master(THD* thd, MASTER_INFO* mi)
     and we have the hold on the run locks which will keep all threads that
     could possibly modify the data structures from running
   */
+
+  /*
+    If the user specified host or port without binlog or position, 
+    reset binlog's name to FIRST and position to 4.
+  */ 
+
   if ((lex_mi->host || lex_mi->port) && !lex_mi->log_file_name && !lex_mi->pos)
   {
-    // if we change host or port, we must reset the postion
     mi->master_log_name[0] = 0;
     mi->master_log_pos= BIN_LOG_HEADER_SIZE;
     mi->rli.pending = 0;
@@ -950,15 +955,24 @@ int change_master(THD* thd, MASTER_INFO* mi)
       DBUG_RETURN(1);
     }
   }
-  mi->rli.master_log_pos = mi->master_log_pos;
   DBUG_PRINT("info", ("master_log_pos: %d", (ulong) mi->master_log_pos));
+  /* If changing RELAY_LOG_FILE or RELAY_LOG_POS, this will be nonsense: */
+  mi->rli.master_log_pos = mi->master_log_pos;
   strmake(mi->rli.master_log_name,mi->master_log_name,
 	  sizeof(mi->rli.master_log_name)-1);
   if (!mi->rli.master_log_name[0]) // uninitialized case
     mi->rli.master_log_pos=0;
 
   pthread_mutex_lock(&mi->rli.data_lock);
-  mi->rli.abort_pos_wait++;
+  mi->rli.abort_pos_wait++; /* for MASTER_POS_WAIT() to abort */
+  /* 
+     If we don't write new coordinates to disk now, then old will remain in
+     relay-log.info until START SLAVE is issued; but if mysqld is shutdown
+     before START SLAVE, then old will remain in relay-log.info, and will be the
+     in-memory value at restart (thus causing errors, as the old relay log does
+     not exist anymore).
+  */
+  flush_relay_log_info(&mi->rli); 
   pthread_cond_broadcast(&mi->data_cond);
   pthread_mutex_unlock(&mi->rli.data_lock);