eth/downloader: resolve local header by hash for beacon sync

eth/downloader/beaconsync.go

@@ -254,7 +254,7 @@ func (d *Downloader) findBeaconAncestor() (uint64, error) {
 // fetchBeaconHeaders feeds skeleton headers to the downloader queue for scheduling
 // until sync errors or is finished.
 func (d *Downloader) fetchBeaconHeaders(from uint64) error {
-	head, _, err := d.skeleton.Bounds()
+	head, tail, err := d.skeleton.Bounds()
 	if err != nil {
 		return err
 	}
@@ -266,8 +266,17 @@ func (d *Downloader) fetchBeaconHeaders(from uint64) error {
 		)
 		for i := 0; i < maxHeadersProcess && from <= head.Number.Uint64(); i++ {
 			header := d.skeleton.Header(from)
+
+			// The header is not found in skeleton space, try to resolve the header
+			// reversely via skeleton tail. Note we can't retrieve the header by
+			// number directly, since there is no guarantee it's the header we want.
+			if header == nil {
+				header = d.getAncestorHeader(from, tail)
+			}
+			// The header is still missing, the beacon sync is corrupted and bail out
+			// the error here.
 			if header == nil {
-				header = d.lightchain.GetHeaderByNumber(from)
+				return fmt.Errorf("missing beacon header %d", from)
 			}
 			headers = append(headers, header)
 			hashes = append(hashes, headers[i].Hash())

eth/downloader/downloader.go

@@ -159,9 +159,6 @@ type LightChain interface {
 	// GetHeaderByHash retrieves a header from the local chain.
 	GetHeaderByHash(common.Hash) *types.Header
 
-	// GetHeaderByNumber retrieves a block header from the local chain by number.
-	GetHeaderByNumber(number uint64) *types.Header
-
 	// CurrentHeader retrieves the head header from the local chain.
 	CurrentHeader() *types.Header
 
@@ -486,7 +483,8 @@ func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td, ttd *
 			// Retrieve the pivot header from the skeleton chain segment but
 			// fallback to local chain if it's not found in skeleton space.
 			if pivot = d.skeleton.Header(number); pivot == nil {
-				pivot = d.lightchain.GetHeaderByNumber(number)
+				_, tail, _ := d.skeleton.Bounds() // error is already checked
+				pivot = d.getAncestorHeader(number, tail)
 			}
 			// Print an error log and return directly in case the pivot header
 			// is still not found. It means the skeleton chain is not linked
@@ -1772,3 +1770,24 @@ func (d *Downloader) DeliverSnapPacket(peer *snap.Peer, packet snap.Packet) erro
 		return fmt.Errorf("unexpected snap packet type: %T", packet)
 	}
 }
+
+// getAncestorHeader retrieves the header with given number. Different from obtaining
+// the canonical header through the number directly, a verified tail header is used
+// here as the base point and obtain the header eventually by resolving the parent.
+func (d *Downloader) getAncestorHeader(number uint64, last *types.Header) *types.Header {
+	var current = last
+	for {
+		parent := d.lightchain.GetHeaderByHash(current.ParentHash)
+		if parent == nil {
+			break // The chain is not continuous
+		}
+		if parent.Number.Uint64() < number {
+			break // It shouldn't happen though
+		}
+		if parent.Number.Uint64() == number {
+			return parent
+		}
+		current = parent
+	}
+	return nil
+}