eth/downloader: bypass peer validation if remote peer is far away

eth/downloader/downloader.go

@@ -894,13 +894,6 @@
 // In the rare scenario when we ended up on a long reorganisation (i.e. none of
 // the head links match), we do a binary search to find the common ancestor.
 func (d *Downloader) findAncestor(p *peerConnection, remoteHeader *types.Header) (uint64, error) {
-	// Check the validity of peer from which the chain is to be downloaded
-	if d.ChainValidator != nil {
-		if _, err := d.IsValidPeer(d.getFetchHeadersByNumber(p)); err != nil {
-			return 0, err
-		}
-	}
-
 	// Figure out the valid ancestor range to prevent rewrite attacks
 	var (
 		floor        = int64(-1)
@@ -918,6 +911,23 @@
 		localHeight = d.lightchain.CurrentHeader().Number.Uint64()
 	}
 
+	// Check the validity of peer from which the chain is to be downloaded
+	if d.ChainValidator != nil {
+		_, err := d.IsValidPeer(d.getFetchHeadersByNumber(p))
+		if errors.Is(err, whitelist.ErrMismatch) {
+			return 0, err
+		}
+
+		// Assuming that `remoteHeight` is always greater than `localHeight`, we won't
+		// check peer validity if the remote header is far ahead of us.
+		if errors.Is(err, whitelist.ErrNoRemote) && localHeight > remoteHeight-1024 {
+			log.Info("Remote peer didn't respond but is far ahead, skipping validation", "id", p.id, "local", localHeight, "remote", remoteHeight, "err", err)
+		} else if errors.Is(err, whitelist.ErrNoRemote) {
+			log.Info("Remote peer didn't respond", "id", p.id, "local", localHeight, "remote", remoteHeight, "err", err)
+			return 0, err
+		}
+	}
+
 	p.log.Debug("Looking for common ancestor", "local", localHeight, "remote", remoteHeight)
 
 	// Recap floor value for binary search