store/tikv: call Next() after copIterator Closed lead to goroutine leak

After Close(), worker groutine receive from copIterator.finished and
exit directly, without writing any thing to taskCh.
Next() receives from taskCh and may hang forever, cause the caller goroutine
leak.

store/tikv/coprocessor.go

@@ -393,6 +393,15 @@ func (it *copIterator) run(ctx goctx.Context) {
 	})
 }
 
+func recvFromRespCh(respCh <-chan copResponse, finished <-chan struct{}) (resp copResponse, ok bool, exit bool) {
+	select {
+	case resp, ok = <-respCh:
+	case <-finished:
+		exit = true
+	}
+	return
+}
+
 func (it *copIterator) sendToTaskCh(ctx goctx.Context, t *copTask, taskCh chan<- *copTask) (finished bool, canceled bool) {
 	select {
 	case taskCh <- t:
@@ -421,13 +430,18 @@ func (it *copIterator) Next() ([]byte, error) {
 			return nil, nil
 		}
 	} else {
+		var closed bool
 		for {
 			if it.curr >= len(it.tasks) {
 				// Resp will be nil if iterator is finished.
 				return nil, nil
 			}
 			task := it.tasks[it.curr]
-			resp, ok = <-task.respChan
+			resp, ok, closed = recvFromRespCh(task.respChan, it.finished)
+			if closed {
+				// Close() is already called, so Next() is invalid.
+				return nil, nil
+			}
 			if ok {
 				break
 			}