update failover

Author: siddontang

README.md

@@ -123,6 +123,11 @@ mysql -h127.0.0.1 -P4000 -uroot -p
 //Becuase empty handler does nothing, so here the MySQL client can only connect the proxy server. :-) 
 ```
 
+## Failover
+
+Failover supports to promote a new master and let other slaves replicate from it automatically when the old master was down.
+
+
 ## Feedback
 
 go-mysql is still in development, your feedback is very welcome. 

failover/failover.go

@@ -16,18 +16,10 @@ import (
 //
 func Failover(slaves []*Server) ([]*Server, error) {
 	// First check slaves use gtid or not
-	gtidMode, err := slaves[0].GTIDUsed()
+	gtidMode, err := CheckGTIDMode(slaves)
 	if err != nil {
 		return nil, err
 	}
-	for i := 1; i < len(slaves); i++ {
-		mode, err := slaves[i].GTIDUsed()
-		if err != nil {
-			return nil, err
-		} else if gtidMode != mode {
-			return nil, fmt.Errorf("%s use GTID %s, but %s use GTID %s", slaves[0].Addr, gtidMode, slaves[i].Addr, mode)
-		}
-	}
 
 	var h Handler
 
@@ -63,3 +55,21 @@ func Failover(slaves []*Server) ([]*Server, error) {
 
 	return slaves, nil
 }
+
+// Check slaves have same GTID used or not
+func CheckGTIDMode(slaves []*Server) (string, error) {
+	gtidMode, err := slaves[0].GTIDMode()
+	if err != nil {
+		return GTIDModeOff, err
+	}
+	for i := 1; i < len(slaves); i++ {
+		mode, err := slaves[i].GTIDMode()
+		if err != nil {
+			return GTIDModeOff, err
+		} else if gtidMode != mode {
+			return GTIDModeOff, fmt.Errorf("%s use GTID %s, but %s use GTID %s", slaves[0].Addr, gtidMode, slaves[i].Addr, mode)
+		}
+	}
+
+	return gtidMode, nil
+}

failover/server.go

@@ -102,7 +102,7 @@ func (s *Server) ResetMaster() error {
 	return err
 }
 
-func (s *Server) GTIDUsed() (string, error) {
+func (s *Server) GTIDMode() (string, error) {
 	r, err := s.Execute("SELECT @@gtid_mode")
 	if err != nil {
 		return GTIDModeOff, err