ddl: remove the function of Start (#3288)

ddl/ddl.go

@@ -126,8 +126,6 @@ type DDL interface {
 	GetScope(status string) variable.ScopeFlag
 	// Stop stops DDL worker.
 	Stop() error
-	// Start starts DDL worker.
-	Start() error
 	// RegisterEventCh registers event channel for ddl.
 	RegisterEventCh(chan<- *Event)
 }
@@ -297,19 +295,6 @@ func (d *ddl) Stop() error {
 	return errors.Trace(err)
 }
 
-func (d *ddl) Start() error {
-	d.m.Lock()
-	defer d.m.Unlock()
-
-	if !d.isClosed() {
-		return nil
-	}
-
-	d.start()
-
-	return nil
-}
-
 func (d *ddl) start() {
 	d.quitCh = make(chan struct{})
 	d.wait.Add(2)

ddl/ddl_worker_test.go

@@ -74,10 +74,8 @@ func (s *testDDLSuite) TestCheckOwner(c *C) {
 	d2.SetLease(1 * time.Second)
 	err := d2.Stop()
 	c.Assert(err, IsNil)
-	err = d1.Start()
-	c.Assert(err, IsNil)
-	err = d1.Start()
-	c.Assert(err, IsNil)
+	d1.start()
+	d2.start()
 	testCheckOwner(c, d1, true, ddlJobFlag)
 	testCheckOwner(c, d1, true, bgJobFlag)