eth/downloader: revert to demotion, use harsher penalty

eth/downloader/downloader.go

@@ -335,7 +335,7 @@ out:
 				// Deliver the received chunk of blocks, but drop the peer if invalid
 				if err := d.queue.Deliver(blockPack.peerId, blockPack.blocks); err != nil {
 					glog.V(logger.Debug).Infof("Failed delivery for peer %s: %v\n", blockPack.peerId, err)
-					d.peers.Unregister(blockPack.peerId)
+					peer.Demote()
 					break
 				}
 				if glog.V(logger.Debug) {
@@ -358,7 +358,9 @@ out:
 				// 1) Time for them to respond;
 				// 2) Measure their speed;
 				// 3) Amount and availability.
-				d.peers.Unregister(pid)
+				if peer := d.peers.Peer(pid); peer != nil {
+					peer.Demote()
+				}
 			}
 			// After removing bad peers make sure we actually have sufficient peer left to keep downloading
 			if d.peers.Peers() == 0 {
@@ -372,9 +374,13 @@ out:
 				if d.queue.Throttle() {
 					continue
 				}
-				// Send a download request to all idle peers
+				// Send a download request to all idle peers, until throttled
 				idlePeers := d.peers.IdlePeers()
 				for _, peer := range idlePeers {
+					// Short circuit if throttling activated since above
+					if d.queue.Throttle() {
+						break
+					}
 					// Get a possible chunk. If nil is returned no chunk
 					// could be returned due to no hashes available.
 					request := d.queue.Reserve(peer, maxBlockFetch)

eth/downloader/peer.go

@@ -86,10 +86,8 @@ func (p *peer) Demote() {
 	for {
 		// Calculate the new reputation value
 		prev := atomic.LoadInt32(&p.rep)
-		next := prev - 2
-		if next < 0 {
-			next = 0
-		}
+		next := prev / 2
+
 		// Try to update the old value
 		if atomic.CompareAndSwapInt32(&p.rep, prev, next) {
 			return
@@ -177,7 +175,7 @@ func (ps *peerSet) AllPeers() []*peer {
 }
 
 // IdlePeers retrieves a flat list of all the currently idle peers within the
-// active peer set.
+// active peer set, ordered by their reputation.
 func (ps *peerSet) IdlePeers() []*peer {
 	ps.lock.RLock()
 	defer ps.lock.RUnlock()
@@ -188,5 +186,12 @@ func (ps *peerSet) IdlePeers() []*peer {
 			list = append(list, p)
 		}
 	}
+	for i := 0; i < len(list); i++ {
+		for j := i + 1; j < len(list); j++ {
+			if atomic.LoadInt32(&list[i].rep) < atomic.LoadInt32(&list[j].rep) {
+				list[i], list[j] = list[j], list[i]
+			}
+		}
+	}
 	return list
 }