downloader: return an error for peer.fetch and return chunk to queue

If a peer was somehow already fetching and somehow managed to end up in
the `available` pool it should return it's work.

eth/downloader/downloader.go

@@ -268,8 +268,14 @@ out:
 					chunk := d.queue.get(peer, maxBlockFetch)
 					if chunk != nil {
 						//fmt.Println("fetching for", peer.id)
-						// Fetch the chunk.
-						peer.fetch(chunk)
+						// Fetch the chunk and check for error. If the peer was somehow
+						// already fetching a chunk due to a bug, it will be returned to
+						// the queue
+						if err := peer.fetch(chunk); err != nil {
+							// log for tracing
+							glog.V(logger.Debug).Infof("peer %s received double work (state = %v)\n", peer.id, peer.state)
+							d.queue.put(chunk.hashes)
+						}
 					}
 				}
 				atomic.StoreInt32(&d.downloadingBlocks, 1)

eth/downloader/peer.go

 package downloader
 
 import (
+	"errors"
 	"math/big"
 	"sync"
 
@@ -31,10 +32,14 @@ func newPeer(id string, td *big.Int, hash common.Hash, getHashes hashFetcherFn,
 }
 
 // fetch a chunk using the peer
-func (p *peer) fetch(chunk *chunk) {
+func (p *peer) fetch(chunk *chunk) error {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 
+	if p.state == workingState {
+		return errors.New("peer already fetching chunk")
+	}
+
 	// set working state
 	p.state = workingState
 	// convert the set to a fetchable slice
@@ -45,4 +50,6 @@ func (p *peer) fetch(chunk *chunk) {
 		return true
 	})
 	p.getBlocks(hashes)
+
+	return nil
 }