eth/downloader: fix for Issue #16539 (#16546)

eth/downloader/downloader.go

@@ -306,7 +306,7 @@ func (d *Downloader) UnregisterPeer(id string) error {
  d.cancelLock.RUnlock()
 
  if master {
- d.Cancel()
+ d.cancel()
  }
  return nil
 }
@@ -501,8 +501,10 @@ func (d *Downloader) spawnSync(fetchers []func() error) error {
  return err
 }
 
-// Cancel cancels all of the operations and resets the queue.
-func (d *Downloader) Cancel() {
+// cancel aborts all of the operations and resets the queue. However, cancel does
+// not wait for the running download goroutines to finish. This method should be
+// used when cancelling the downloads from inside the downloader.
+func (d *Downloader) cancel() {
  // Close the current cancel channel
  d.cancelLock.Lock()
  if d.cancelCh != nil {
@@ -514,6 +516,12 @@ func (d *Downloader) Cancel() {
  }
  }
  d.cancelLock.Unlock()
+}
+
+// Cancel aborts all of the operations and waits for all download goroutines to
+// finish before returning.
+func (d *Downloader) Cancel() {
+ d.cancel()
  d.cancelWg.Wait()
 }