fix(Blockchain): sync chain when we receive new blocks in 'inv' messages

This got broken somehow. Now chains should keep emitting 'block' events after the initial sync.

lib/blockchain.js

@@ -103,6 +103,7 @@ Blockchain.prototype.close = function (cb) {
 
 Blockchain.prototype.sync = function (opts, cb) {
   var self = this
+  cb = cb || noop
   if (this.syncing) return cb(null)
   if (typeof opts === 'function') {
     cb = opts
@@ -113,6 +114,7 @@ Blockchain.prototype.sync = function (opts, cb) {
 
   this.syncing = true
   if (opts.to || this.to) this.syncHeight = opts.to || this.to
+  if (opts.peer) this._setDownloadPeer(opts.peer)
 
   var syncDone = function () {
     var done = false
@@ -321,6 +323,7 @@ Blockchain.prototype._setDownloadPeer = function (peer) {
   }
 
   this.downloadPeer = peer
+  this.syncing = !!peer
   if (peer) {
     peer.on('headers', this._onHeaders)
     peer.on('disconnect', this._onDownloadPeerDisconnect)
@@ -342,7 +345,7 @@ Blockchain.prototype._getHeaders = function (opts) {
 
   opts = opts || {}
   this.getLocator(opts.from || this.tip.hash, function (err, locator) {
-    if (err) throw err
+    if (err) return self._error(err)
     if (!self.downloadPeer) return
     var message = self.downloadPeer.messages.GetHeaders({
       starts: locator
@@ -378,7 +381,7 @@ Blockchain.prototype._listenForUpdates = function () {
       item.type === Inventory.TYPE.FILTERED_BLOCK) {
         if (self.lastInv && self.lastInv.compare(item.hash) === 0) return
         self.lastInv = item.hash
-        self._getHeaders()
+        self.sync({ peer: peer })
       }
     })
   })