fix copyRowsFunc hangs bug

Author: wangfei

go/logic/migrator.go

@@ -1087,6 +1087,8 @@ func (this *Migrator) iterateChunks() error {
 		log.Debugf("No rows found in table. Rowcopy will be implicitly empty")
 		return terminateRowIteration(nil)
 	}
+
+	hasFurtherRange := true
 	// Iterate per chunk:
 	for {
 		if atomic.LoadInt64(&this.rowCopyCompleteFlag) == 1 {
@@ -1100,7 +1102,10 @@ func (this *Migrator) iterateChunks() error {
 				// There's another such check down the line
 				return nil
 			}
-			hasFurtherRange, err := this.applier.CalculateNextIterationRangeEndValues()
+
+			// When hasFurtherRange is false, original table might be write locked and CalculateNextIterationRangeEndValues would hangs forever
+			var err error
+			hasFurtherRange, err = this.applier.CalculateNextIterationRangeEndValues()
 			if err != nil {
 				return terminateRowIteration(err)
 			}
@@ -1135,6 +1140,9 @@ func (this *Migrator) iterateChunks() error {
 		}
 		// Enqueue copy operation; to be executed by executeWriteFuncs()
 		this.copyRowsQueue <- copyRowsFunc
+		if !hasFurtherRange {
+			break
+		}
 	}
 	return nil
 }