eth, eth/downloader: better remote head tracking

eth/downloader/downloader.go

@@ -236,12 +236,12 @@ func (d *Downloader) Synchronising() bool {
 
 // RegisterPeer injects a new download peer into the set of block source to be
 // used for fetching hashes and blocks from.
-func (d *Downloader) RegisterPeer(id string, version int, head common.Hash,
+func (d *Downloader) RegisterPeer(id string, version int, currentHead currentHeadRetrievalFn,
 	getRelHeaders relativeHeaderFetcherFn, getAbsHeaders absoluteHeaderFetcherFn, getBlockBodies blockBodyFetcherFn,
 	getReceipts receiptFetcherFn, getNodeData stateFetcherFn) error {
 
 	glog.V(logger.Detail).Infoln("Registering peer", id)
-	if err := d.peers.Register(newPeer(id, version, head, getRelHeaders, getAbsHeaders, getBlockBodies, getReceipts, getNodeData)); err != nil {
+	if err := d.peers.Register(newPeer(id, version, currentHead, getRelHeaders, getAbsHeaders, getBlockBodies, getReceipts, getNodeData)); err != nil {
 		glog.V(logger.Error).Infoln("Register failed:", err)
 		return err
 	}
@@ -501,7 +501,8 @@ func (d *Downloader) fetchHeight(p *peer) (*types.Header, error) {
 	glog.V(logger.Debug).Infof("%v: retrieving remote chain height", p)
 
 	// Request the advertised remote head block and wait for the response
-	go p.getRelHeaders(p.head, 1, 0, false)
+	head, _ := p.currentHead()
+	go p.getRelHeaders(head, 1, 0, false)
 
 	timeout := time.After(d.requestTTL())
 	for {

eth/downloader/downloader_test.go

@@ -400,11 +400,11 @@ func (dl *downloadTester) newSlowPeer(id string, version int, hashes []common.Ha
 	var err error
 	switch version {
 	case 62:
-		err = dl.downloader.RegisterPeer(id, version, hashes[0], dl.peerGetRelHeadersFn(id, delay), dl.peerGetAbsHeadersFn(id, delay), dl.peerGetBodiesFn(id, delay), nil, nil)
+		err = dl.downloader.RegisterPeer(id, version, dl.peerCurrentHeadFn(id), dl.peerGetRelHeadersFn(id, delay), dl.peerGetAbsHeadersFn(id, delay), dl.peerGetBodiesFn(id, delay), nil, nil)
 	case 63:
-		err = dl.downloader.RegisterPeer(id, version, hashes[0], dl.peerGetRelHeadersFn(id, delay), dl.peerGetAbsHeadersFn(id, delay), dl.peerGetBodiesFn(id, delay), dl.peerGetReceiptsFn(id, delay), dl.peerGetNodeDataFn(id, delay))
+		err = dl.downloader.RegisterPeer(id, version, dl.peerCurrentHeadFn(id), dl.peerGetRelHeadersFn(id, delay), dl.peerGetAbsHeadersFn(id, delay), dl.peerGetBodiesFn(id, delay), dl.peerGetReceiptsFn(id, delay), dl.peerGetNodeDataFn(id, delay))
 	case 64:
-		err = dl.downloader.RegisterPeer(id, version, hashes[0], dl.peerGetRelHeadersFn(id, delay), dl.peerGetAbsHeadersFn(id, delay), dl.peerGetBodiesFn(id, delay), dl.peerGetReceiptsFn(id, delay), dl.peerGetNodeDataFn(id, delay))
+		err = dl.downloader.RegisterPeer(id, version, dl.peerCurrentHeadFn(id), dl.peerGetRelHeadersFn(id, delay), dl.peerGetAbsHeadersFn(id, delay), dl.peerGetBodiesFn(id, delay), dl.peerGetReceiptsFn(id, delay), dl.peerGetNodeDataFn(id, delay))
 	}
 	if err == nil {
 		// Assign the owned hashes, headers and blocks to the peer (deep copy)
@@ -463,6 +463,17 @@ func (dl *downloadTester) dropPeer(id string) {
 	dl.downloader.UnregisterPeer(id)
 }
 
+// peerCurrentHeadFn constructs a function to retrieve a peer's current head hash
+// and total difficulty.
+func (dl *downloadTester) peerCurrentHeadFn(id string) func() (common.Hash, *big.Int) {
+	return func() (common.Hash, *big.Int) {
+		dl.lock.RLock()
+		defer dl.lock.RUnlock()
+
+		return dl.peerHashes[id][0], nil
+	}
+}
+
 // peerGetRelHeadersFn constructs a GetBlockHeaders function based on a hashed
 // origin; associated with a particular peer in the download tester. The returned
 // function can be used to retrieve batches of headers from the particular peer.

eth/downloader/peer.go

@@ -23,6 +23,7 @@ import (
 	"errors"
 	"fmt"
 	"math"
+	"math/big"
 	"sort"
 	"strings"
 	"sync"
@@ -37,6 +38,9 @@ const (
 	measurementImpact = 0.1  // The impact a single measurement has on a peer's final throughput value.
 )
 
+// Head hash and total difficulty retriever for
+type currentHeadRetrievalFn func() (common.Hash, *big.Int)
+
 // Block header and body fetchers belonging to eth/62 and above
 type relativeHeaderFetcherFn func(common.Hash, int, int, bool) error
 type absoluteHeaderFetcherFn func(uint64, int, int, bool) error
@@ -52,8 +56,7 @@ var (
 
 // peer represents an active peer from which hashes and blocks are retrieved.
 type peer struct {
-	id   string      // Unique identifier of the peer
-	head common.Hash // Hash of the peers latest known block
+	id string // Unique identifier of the peer
 
 	headerIdle  int32 // Current header activity state of the peer (idle = 0, active = 1)
 	blockIdle   int32 // Current block activity state of the peer (idle = 0, active = 1)
@@ -74,6 +77,8 @@ type peer struct {
 
 	lacking map[common.Hash]struct{} // Set of hashes not to request (didn't have previously)
 
+	currentHead currentHeadRetrievalFn // Method to fetch the currently known head of the peer
+
 	getRelHeaders  relativeHeaderFetcherFn // [eth/62] Method to retrieve a batch of headers from an origin hash
 	getAbsHeaders  absoluteHeaderFetcherFn // [eth/62] Method to retrieve a batch of headers from an absolute position
 	getBlockBodies blockBodyFetcherFn      // [eth/62] Method to retrieve a batch of block bodies
@@ -87,14 +92,14 @@ type peer struct {
 
 // newPeer create a new downloader peer, with specific hash and block retrieval
 // mechanisms.
-func newPeer(id string, version int, head common.Hash,
+func newPeer(id string, version int, currentHead currentHeadRetrievalFn,
 	getRelHeaders relativeHeaderFetcherFn, getAbsHeaders absoluteHeaderFetcherFn, getBlockBodies blockBodyFetcherFn,
 	getReceipts receiptFetcherFn, getNodeData stateFetcherFn) *peer {
 	return &peer{
 		id:      id,
-		head:    head,
 		lacking: make(map[common.Hash]struct{}),
 
+		currentHead:    currentHead,
 		getRelHeaders:  getRelHeaders,
 		getAbsHeaders:  getAbsHeaders,
 		getBlockBodies: getBlockBodies,