From 84462625975bd1a34dadc47c062a9999c6000203 Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Thu, 11 Dec 2014 13:35:14 +0100
Subject: Reject headers that build on an invalid parent

Rebased-From: 34970223472c9e83689a1c710eebc7c16f152b02
Github-Pull: #5459
---
 src/main.cpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/main.cpp b/src/main.cpp
index a3c65dbb93..4a0008726e 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -442,6 +442,10 @@ void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<CBl
         // are not yet downloaded and not in flight to vBlocks. In the mean time, update
         // pindexLastCommonBlock as long as all ancestors are already downloaded.
         BOOST_FOREACH(CBlockIndex* pindex, vToFetch) {
+            if (!pindex->IsValid(BLOCK_VALID_TREE)) {
+                // We consider the chain that this peer is on invalid.
+                return;
+            }
             if (pindex->nStatus & BLOCK_HAVE_DATA) {
                 if (pindex->nChainTx)
                     state->pindexLastCommonBlock = pindex;
@@ -2559,6 +2563,8 @@ bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state, CBloc
         if (mi == mapBlockIndex.end())
             return state.DoS(10, error("%s : prev block not found", __func__), 0, "bad-prevblk");
         pindexPrev = (*mi).second;
+        if (pindexPrev->nStatus & BLOCK_FAILED_MASK)
+            return state.DoS(100, error("%s : prev block invalid", __func__), REJECT_INVALID, "bad-prevblk");
     }
 
     if (!ContextualCheckBlockHeader(block, state, pindexPrev))
-- 
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