eth/downloader: remove update channel in fetchParts

Almost all select cases sent a value to update, which would then be
received in the next iteration. Simplify it by doing the update on every
iteration.

eth/downloader/downloader.go

@@ -993,15 +993,12 @@ func (d *Downloader) fetchParts(errCancel error, deliveryCh chan dataPack, deliv
 	ticker := time.NewTicker(100 * time.Millisecond)
 	defer ticker.Stop()
 
-	update := make(chan struct{}, 1)
-
 	// Prepare the queue and fetch block parts until the block header fetcher's done
 	finished := false
 	for {
 		select {
 		case <-d.cancelCh:
 			return errCancel
-
 		case packet := <-deliveryCh:
 			// If the peer was previously banned and failed to deliver its pack
 			// in a reasonable time frame, ignore its message.
@@ -1027,31 +1024,14 @@ func (d *Downloader) fetchParts(errCancel error, deliveryCh chan dataPack, deliv
 					peer.log.Trace("Failed to deliver retrieved data", "type", kind, "err", err)
 				}
 			}
-			// Blocks assembled, try to update the progress
-			select {
-			case update <- struct{}{}:
-			default:
-			}
-
 		case cont := <-wakeCh:
 			// The header fetcher sent a continuation flag, check if it's done
 			if !cont {
 				finished = true
 			}
-			// Headers arrive, try to update the progress
-			select {
-			case update <- struct{}{}:
-			default:
-			}
-
 		case <-ticker.C:
-			// Sanity check update the progress
-			select {
-			case update <- struct{}{}:
-			default:
 		}
 
-		case <-update:
 		// Short circuit if we lost all our peers
 		if d.peers.Len() == 0 {
 			return errNoPeers
@@ -1081,12 +1061,11 @@ func (d *Downloader) fetchParts(errCancel error, deliveryCh chan dataPack, deliv
 				log.Debug("Data fetching completed", "type", kind)
 				return nil
 			}
-				break
+			continue
 		}
 		// Send a download request to all idle peers, until throttled
 		progressed, throttled, running := false, false, inFlight()
 		idles, total := idle()
-
 		for _, peer := range idles {
 			// Short circuit if throttling activated
 			if throttle() {
@@ -1138,7 +1117,6 @@ func (d *Downloader) fetchParts(errCancel error, deliveryCh chan dataPack, deliv
 		}
 	}
 }
-}
 
 // processHeaders takes batches of retrieved headers from an input channel and
 // keeps processing and scheduling them into the header chain and downloader's