From 055eb26ae884fbe25b198fa011e9ff6f2ae4b1d3 Mon Sep 17 00:00:00 2001
From: Zsolt Felfoldi <zsfelfoldi@gmail.com>
Date: Thu, 15 Feb 2018 18:08:39 +0100
Subject: [PATCH] les: fix light fetcher database race

---
 les/fetcher.go | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/les/fetcher.go b/les/fetcher.go
index 9d224176fc..d0a74ad258 100644
--- a/les/fetcher.go
+++ b/les/fetcher.go
@@ -46,7 +46,7 @@ type lightFetcher struct {
 	peers           map[*peer]*fetcherPeerInfo
 	lastUpdateStats *updateStatsEntry
 
-	lock       sync.Mutex // qwerqwerqwe
+	lock       sync.Mutex
 	deliverChn chan fetchResponse
 	reqMu      sync.RWMutex
 	requested  map[uint64]fetchRequest
@@ -560,8 +560,13 @@ func (f *lightFetcher) checkAnnouncedHeaders(fp *fetcherPeerInfo, headers []*typ
 				return true
 			}
 			// we ran out of recently delivered headers but have not reached a node known by this peer yet, continue matching
-			td = f.chain.GetTd(header.ParentHash, header.Number.Uint64()-1)
-			header = f.chain.GetHeader(header.ParentHash, header.Number.Uint64()-1)
+			hash, number := header.ParentHash, header.Number.Uint64()-1
+			td = f.chain.GetTd(hash, number)
+			header = f.chain.GetHeader(hash, number)
+			if header == nil || td == nil {
+				log.Error("Missing parent of validated header", "hash", hash, "number", number)
+				return false
+			}
 		} else {
 			header = headers[i]
 			td = tds[i]
@@ -645,13 +650,18 @@ func (f *lightFetcher) checkKnownNode(p *peer, n *fetcherTreeNode) bool {
 	if td == nil {
 		return false
 	}
+	header := f.chain.GetHeader(n.hash, n.number)
+	// check the availability of both header and td because reads are not protected by chain db mutex
+	// Note: returning false is always safe here
+	if header == nil {
+		return false
+	}
 
 	fp := f.peers[p]
 	if fp == nil {
 		p.Log().Debug("Unknown peer to check known nodes")
 		return false
 	}
-	header := f.chain.GetHeader(n.hash, n.number)
 	if !f.checkAnnouncedHeaders(fp, []*types.Header{header}, []*big.Int{td}) {
 		p.Log().Debug("Inconsistent announcement")
 		go f.pm.removePeer(p.id)