Implement a fast path for the Initial Block Download.

It is not necessary to do all of the transaction validation on
blocks if they have been confirmed to be in the block chain leading
up to the final checkpoint in a given blockschain.

This algorithm fetches block headers from the peer, then once it has
established the full blockchain connection, it requests blocks.
Any blocks before the final checkpoint pass true for fastAdd on
btcchain operation, which causes it to do less valiation on the block.

blockmanager.go

@@ -47,6 +47,13 @@ type invMsg struct {
 	peer *peer
 }
 
+// blockMsg packages a bitcoin block message and the peer it came from together
+// so the block handler has access to that information.
+type headersMsg struct {
+	headers *btcwire.MsgHeaders
+	peer    *peer
+}
+
 // donePeerMsg signifies a newly disconnected peer to the block handler.
 type donePeerMsg struct {
 	peer *peer
@@ -77,6 +84,21 @@ type blockManager struct {
 	msgChan           chan interface{}
 	wg                sync.WaitGroup
 	quit              chan bool
+
+	headerPool       map[btcwire.ShaHash]*headerstr
+	headerOrphan     map[btcwire.ShaHash]*headerstr
+	fetchingHeaders  bool
+	startBlock       *btcwire.ShaHash
+	fetchBlock       *btcwire.ShaHash
+	lastBlock        *btcwire.ShaHash
+	latestCheckpoint *btcchain.Checkpoint
+}
+
+type headerstr struct {
+	header *btcwire.BlockHeader
+	next   *headerstr
+	height int
+	sha    btcwire.ShaHash
 }
 
 // startSync will choose the best peer among the available candidate peers to
@@ -129,7 +151,17 @@ func (b *blockManager) startSync(peers *list.List) {
 
 		bmgrLog.Infof("Syncing to block height %d from peer %v",
 			bestPeer.lastBlock, bestPeer.addr)
-		bestPeer.PushGetBlocksMsg(locator, &zeroHash)
+
+		// if starting from the beginning fetch headers and
+		// download blocks based on that, otherwise compute
+		// the block download via inv messages.
+
+		if height == 0 {
+			bestPeer.PushGetHeadersMsg(locator)
+			b.fetchingHeaders = true
+		} else {
+			bestPeer.PushGetBlocksMsg(locator, &zeroHash)
+		}
 		b.syncPeer = bestPeer
 	} else {
 		bmgrLog.Warnf("No sync peer candidates available")
@@ -331,6 +363,16 @@ func (b *blockManager) current() bool {
 
 // handleBlockMsg handles block messages from all peers.
 func (b *blockManager) handleBlockMsg(bmsg *blockMsg) {
+
+	defer func() {
+		if b.startBlock != nil &&
+			len(bmsg.peer.requestedBlocks) < 10 {
+
+			// block queue getting short, ask for more.
+			b.fetchHeaderBlocks()
+		}
+
+	}()
 	// Keep track of which peer the block was sent from so the notification
 	// handler can request the parent blocks from the appropriate peer.
 	blockSha, _ := bmsg.block.Sha()
@@ -351,9 +393,37 @@ func (b *blockManager) handleBlockMsg(bmsg *blockMsg) {
 	}
 	b.blockPeer[*blockSha] = bmsg.peer
 
+	fastAdd := false
+	if b.fetchBlock != nil && blockSha.IsEqual(b.fetchBlock) {
+		firstblock, ok := b.headerPool[*blockSha]
+		if ok {
+			if b.latestCheckpoint == nil {
+				b.latestCheckpoint =
+					b.blockChain.LatestCheckpoint()
+			}
+			if int64(firstblock.height) <=
+				b.latestCheckpoint.Height {
+
+				fastAdd = true
+			}
+			if firstblock.next != nil {
+				b.fetchBlock = &firstblock.next.sha
+			}
+		}
+	}
 	// Process the block to include validation, best chain selection, orphan
 	// handling, etc.
-	err := b.blockChain.ProcessBlock(bmsg.block)
+	err := b.blockChain.ProcessBlock(bmsg.block, fastAdd)
+
+	if fastAdd && blockSha.IsEqual(b.lastBlock) {
+		// have processed all blocks, switch to normal handling
+		b.fetchingHeaders = false
+		b.startBlock = nil
+		b.fetchBlock = nil
+		b.lastBlock = nil
+		b.headerPool = make(map[btcwire.ShaHash]*headerstr)
+		b.headerOrphan = make(map[btcwire.ShaHash]*headerstr)
+	}
 
 	// Remove block from request maps. Either chain knows about it and such
 	// we shouldn't have any more instances of trying to fetch it, or we
@@ -459,6 +529,14 @@ func (b *blockManager) handleInvMsg(imsg *invMsg) {
 		// for the peer.
 		imsg.peer.addKnownInventory(iv)
 
+		if b.fetchingHeaders {
+			// if we are fetching headers and already know
+			// about a block, do not add process it.
+			if _, ok := b.headerPool[iv.Hash]; ok {
+				continue
+			}
+		}
+
 		// Request the inventory if we don't already have it.
 		if !b.haveInventory(iv) {
 			// Add it to the request queue.
@@ -574,6 +652,9 @@ out:
 			case *invMsg:
 				b.handleInvMsg(msg)
 
+			case *headersMsg:
+				b.handleHeadersMsg(msg)
+
 			case *donePeerMsg:
 				b.handleDonePeerMsg(candidatePeers, msg.peer)
 
@@ -734,6 +815,17 @@ func (b *blockManager) QueueInv(inv *btcwire.MsgInv, p *peer) {
 	b.msgChan <- &invMsg{inv: inv, peer: p}
 }
 
+// QueueInv adds the passed headers message and peer to the block handling queue.
+func (b *blockManager) QueueHeaders(headers *btcwire.MsgHeaders, p *peer) {
+	// No channel handling here because peers do not need to block on inv
+	// messages.
+	if atomic.LoadInt32(&b.shutdown) != 0 {
+		return
+	}
+
+	b.msgChan <- &headersMsg{headers: headers, peer: p}
+}
+
 // DonePeer informs the blockmanager that a peer has disconnected.
 func (b *blockManager) DonePeer(p *peer) {
 	// Ignore if we are shutting down.
@@ -781,6 +873,8 @@ func newBlockManager(s *server) (*blockManager, error) {
 		requestedBlocks:  make(map[btcwire.ShaHash]bool),
 		lastBlockLogTime: time.Now(),
 		msgChan:          make(chan interface{}, cfg.MaxPeers*3),
+		headerPool:       make(map[btcwire.ShaHash]*headerstr),
+		headerOrphan:     make(map[btcwire.ShaHash]*headerstr),
 		quit:             make(chan bool),
 	}
 	bm.blockChain = btcchain.New(s.db, s.btcnet, bm.handleNotifyMsg)
@@ -927,3 +1021,116 @@ func loadBlockDB() (btcdb.Db, error) {
 	btcdLog.Infof("Block database loaded with block height %d", height)
 	return db, nil
 }
+
+// handleHeadersMsg is invoked when a peer receives a headers bitcoin
+// message.
+func (b *blockManager) handleHeadersMsg(bmsg *headersMsg) {
+	msg := bmsg.headers
+
+	nheaders := len(msg.Headers)
+	if nheaders == 0 {
+		bmgrLog.Infof("Received %v0 block headers: Fetching blocks",
+			len(b.headerPool))
+		b.fetchHeaderBlocks()
+		return
+	}
+	var blockhash btcwire.ShaHash
+
+	if b.latestCheckpoint == nil {
+		b.latestCheckpoint = b.blockChain.LatestCheckpoint()
+	}
+
+	for hdridx := range msg.Headers {
+		blockhash, _ = msg.Headers[hdridx].BlockSha()
+		var headerst headerstr
+		headerst.header = msg.Headers[hdridx]
+		headerst.sha = blockhash
+		prev, ok := b.headerPool[headerst.header.PrevBlock]
+		if ok {
+			if prev.next == nil {
+				prev.next = &headerst
+			} else {
+				bmgrLog.Infof("two children of the same block ??? %v %v %v", prev.sha, prev.next.sha, blockhash)
+			}
+			headerst.height = prev.height + 1
+		} else if headerst.header.PrevBlock.IsEqual(activeNetParams.genesisHash) {
+			ok = true
+			headerst.height = 1
+			b.startBlock = &headerst.sha
+		}
+		if int64(headerst.height) == b.latestCheckpoint.Height {
+			if headerst.sha.IsEqual(b.latestCheckpoint.Hash) {
+				// we can trust this header first download
+				// TODO flag this?
+			} else {
+				// XXX marker does not match, must throw
+				// away headers !?!?!
+				// XXX dont trust peer?
+			}
+		}
+		if ok {
+			b.headerPool[blockhash] = &headerst
+			b.lastBlock = &blockhash
+		} else {
+			bmgrLog.Infof("found orphan block %v", blockhash)
+			b.headerOrphan[headerst.header.PrevBlock] = &headerst
+		}
+	}
+
+	// Construct the getheaders request and queue it to be sent.
+	ghmsg := btcwire.NewMsgGetHeaders()
+	err := ghmsg.AddBlockLocatorHash(&blockhash)
+	if err != nil {
+		bmgrLog.Infof("msgheaders bad addheaders", blockhash)
+		return
+	}
+
+	b.syncPeer.QueueMessage(ghmsg, nil)
+}
+
+// fetchHeaderBlocks is creates and sends a request to the syncPeer for
+// the next list of blocks to downloaded.
+func (b *blockManager) fetchHeaderBlocks() {
+	gdmsg := btcwire.NewMsgGetData()
+	numRequested := 0
+	startBlock := b.startBlock
+	for {
+		if b.startBlock == nil {
+			break
+		}
+		blockhash := b.startBlock
+		firstblock, ok := b.headerPool[*blockhash]
+		if !ok {
+			bmgrLog.Warnf("current fetch block %v missing from headerPool", blockhash)
+			break
+		}
+		var iv btcwire.InvVect
+		iv.Hash = *blockhash
+		iv.Type = btcwire.InvTypeBlock
+		if !b.haveInventory(&iv) {
+			b.requestedBlocks[*blockhash] = true
+			b.syncPeer.requestedBlocks[*blockhash] = true
+			gdmsg.AddInvVect(&iv)
+			numRequested++
+		}
+
+		if b.fetchBlock == nil {
+			b.fetchBlock = b.startBlock
+		}
+		if firstblock.next == nil {
+			b.startBlock = nil
+			break
+		} else {
+			b.startBlock = &firstblock.next.sha
+		}
+
+		if numRequested >= btcwire.MaxInvPerMsg {
+			break
+		}
+	}
+	if len(gdmsg.InvList) > 0 {
+		bmgrLog.Debugf("requesting block %v len %v\n", startBlock, len(gdmsg.InvList))
+
+		b.syncPeer.QueueMessage(gdmsg, nil)
+	}
+}

peer.go

@@ -431,6 +431,43 @@ func (p *peer) PushGetBlocksMsg(locator btcchain.BlockLocator, stopHash *btcwire
 	return nil
 }
 
+// PushGetHeadersMsg sends a getblocks message for the provided block locator
+// and stop hash.  It will ignore back-to-back duplicate requests.
+func (p *peer) PushGetHeadersMsg(locator btcchain.BlockLocator) error {
+	// Extract the begin hash from the block locator, if one was specified,
+	// to use for filtering duplicate getblocks requests.
+	// request.
+	var beginHash *btcwire.ShaHash
+	if len(locator) > 0 {
+		beginHash = locator[0]
+	}
+
+	// Filter duplicate getblocks requests.
+	if p.prevGetBlocksBegin != nil &&
+		beginHash != nil &&
+		beginHash.IsEqual(p.prevGetBlocksBegin) {
+
+		peerLog.Tracef("PEER: Filtering duplicate [getblocks] with begin "+
+			"hash %v", beginHash)
+		return nil
+	}
+
+	// Construct the getheaders request and queue it to be sent.
+	msg := btcwire.NewMsgGetHeaders()
+	for _, hash := range locator {
+		err := msg.AddBlockLocatorHash(hash)
+		if err != nil {
+			return err
+		}
+	}
+	p.QueueMessage(msg, nil)
+
+	// Update the previous getblocks request information for filtering
+	// duplicates.
+	p.prevGetBlocksBegin = beginHash
+	return nil
+}
+
 // handleMemPoolMsg is invoked when a peer receives a mempool bitcoin message.
 // It creates and sends an inventory message with the contents of the memory
 // pool up to the maximum inventory allowed per message.
@@ -520,6 +557,14 @@ func (p *peer) handleInvMsg(msg *btcwire.MsgInv) {
 	p.server.blockManager.QueueInv(msg, p)
 }
 
+// handleHeadersMsg is invoked when a peer receives an inv bitcoin message and
+// is used to examine the inventory being advertised by the remote peer and
+// react accordingly. We pass the message down to blockmanager which will call
+// QueueMessage with any appropriate responses.
+func (p *peer) handleHeadersMsg(msg *btcwire.MsgHeaders) {
+	p.server.blockManager.QueueHeaders(msg, p)
+}
+
 // handleGetData is invoked when a peer receives a getdata bitcoin message and
 // is used to deliver block and transaction information.
 func (p *peer) handleGetDataMsg(msg *btcwire.MsgGetData) {
@@ -1044,6 +1089,9 @@ out:
 		case *btcwire.MsgGetHeaders:
 			p.handleGetHeadersMsg(msg)
 
+		case *btcwire.MsgHeaders:
+			p.handleHeadersMsg(msg)
+
 		default:
 			peerLog.Debugf("Received unhandled message of type %v: Fix Me",
 				rmsg.Command())