downloader: improved downloading and synchronisation

* Downloader's peers keeps track of peer's previously requested hashes
  so that we don't have to re-request
* Changed `AddBlock` to be fully synchronous

eth/downloader/synchronous.go

@@ -0,0 +1,77 @@
+package downloader
+
+import (
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/logger"
+	"github.com/ethereum/go-ethereum/logger/glog"
+)
+
+// THIS IS PENDING AND TO DO CHANGES FOR MAKING THE DOWNLOADER SYNCHRONOUS
+
+// SynchroniseWithPeer will select the peer and use it for synchronising. If an empty string is given
+// it will use the best peer possible and synchronise if it's TD is higher than our own. If any of the
+// checks fail an error will be returned. This method is synchronous
+func (d *Downloader) SynchroniseWithPeer(id string) (types.Blocks, error) {
+	// Check if we're busy
+	if d.isBusy() {
+		return nil, errBusy
+	}
+
+	// Attempt to select a peer. This can either be nothing, which returns, best peer
+	// or selected peer. If no peer could be found an error will be returned
+	var p *peer
+	if len(id) == 0 {
+		p = d.peers[id]
+		if p == nil {
+			return nil, errUnknownPeer
+		}
+	} else {
+		p = d.peers.bestPeer()
+	}
+
+	// Make sure our td is lower than the peer's td
+	if p.td.Cmp(d.currentTd()) <= 0 || d.hasBlock(p.recentHash) {
+		return nil, errLowTd
+	}
+
+	// Get the hash from the peer and initiate the downloading progress.
+	err := d.getFromPeer(p, p.recentHash, false)
+	if err != nil {
+		return nil, err
+	}
+
+	return d.queue.blocks, nil
+}
+
+// Synchronise will synchronise using the best peer.
+func (d *Downloader) Synchronise() (types.Blocks, error) {
+	return d.SynchroniseWithPeer("")
+}
+
+func (d *Downloader) getFromPeer(p *peer, hash common.Hash, ignoreInitial bool) error {
+	glog.V(logger.Detail).Infoln("Synchronising with the network using:", p.id)
+	// Start the fetcher. This will block the update entirely
+	// interupts need to be send to the appropriate channels
+	// respectively.
+	if err := d.startFetchingHashes(p, hash, ignoreInitial); err != nil {
+		// handle error
+		glog.V(logger.Debug).Infoln("Error fetching hashes:", err)
+		// XXX Reset
+		return err
+	}
+
+	// Start fetching blocks in paralel. The strategy is simple
+	// take any available peers, seserve a chunk for each peer available,
+	// let the peer deliver the chunkn and periodically check if a peer
+	// has timedout. When done downloading, process blocks.
+	if err := d.startFetchingBlocks(p); err != nil {
+		glog.V(logger.Debug).Infoln("Error downloading blocks:", err)
+		// XXX reset
+		return err
+	}
+
+	glog.V(logger.Detail).Infoln("Sync completed")
+
+	return nil
+}