From 82199692b04ba0fa9976977e282080c727828c52 Mon Sep 17 00:00:00 2001
From: Henning Diedrich <hdiedrich@de.ibm.com>
Date: Thu, 21 Jul 2016 11:52:18 +0200
Subject: [PATCH] eth/downloader: fix #2841 spikes by block fetches

Ignore a reportedly higher total difficulty when a
peer's best block is already present in local chain.
Makes robust against inaccurately reported height or
total difficulty.
---
 eth/downloader/downloader.go | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index 01c0818a01..1dafb46dc0 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -412,6 +412,14 @@ func (d *Downloader) syncWithPeer(p *peer, hash common.Hash, td *big.Int) (err e
 		)
 
 	case p.version >= 62:
+		// essentially double check the peer-reported height. If its
+		// block at that height turns out to already be in our chain,
+		// stop search and ignore the alleged higher total difficulty
+		if d.hasBlockAndState(p.head) {
+			glog.Infof("%v: advertised a head already in our chain. Ignoring reported higher total difficulty", p)
+			return nil
+		}
+
 		// Look up the sync boundaries: the common ancestor and the target block
 		latest, err := d.fetchHeight(p)
 		if err != nil {