eth: don't block sync goroutines that short circuit

eth/downloader/downloader.go

@@ -248,10 +248,11 @@ func (d *Downloader) UnregisterPeer(id string) error {
 
 // Synchronise tries to sync up our local block chain with a remote peer, both
 // adding various sanity checks as well as wrapping it with various log entries.
-func (d *Downloader) Synchronise(id string, head common.Hash, td *big.Int, mode SyncMode) {
+func (d *Downloader) Synchronise(id string, head common.Hash, td *big.Int, mode SyncMode) error {
 	glog.V(logger.Detail).Infof("Attempting synchronisation: %v, head [%x…], TD %v", id, head[:4], td)
 
-	switch err := d.synchronise(id, head, td, mode); err {
+	err := d.synchronise(id, head, td, mode)
+	switch err {
 	case nil:
 		glog.V(logger.Detail).Infof("Synchronisation completed")
 
@@ -268,6 +269,7 @@ func (d *Downloader) Synchronise(id string, head common.Hash, td *big.Int, mode
 	default:
 		glog.V(logger.Warn).Infof("Synchronisation failed: %v", err)
 	}
+	return err
 }
 
 // synchronise will select the peer and use it for synchronising. If an empty string is given

eth/sync.go

@@ -170,13 +170,16 @@ func (pm *ProtocolManager) synchronise(peer *peer) {
 	if pm.fastSync {
 		mode = downloader.FastSync
 	}
-	pm.downloader.Synchronise(peer.id, peer.Head(), peer.Td(), mode)
-
+	if err := pm.downloader.Synchronise(peer.id, peer.Head(), peer.Td(), mode); err != nil {
+		return
+	}
 	// If fast sync was enabled, and we synced up, disable it
 	if pm.fastSync {
+		// Wait until all pending imports finish processing
 		for pm.downloader.Synchronising() {
 			time.Sleep(100 * time.Millisecond)
 		}
+		// Disable fast sync if we indeed have something in our chain
 		if pm.blockchain.CurrentBlock().NumberU64() > 0 {
 			glog.V(logger.Info).Infof("fast sync complete, auto disabling")
 			pm.fastSync = false