Bugfix: only track UTXO modification after lookup

Otherwise, if CCoinsViewCache::ModifyCoins throws an exception in between
setting hasModifier and constructing the CCoinsModifier, the cache ends up
in an inconsistent state, resulting in an assert failure in the next
modification.

Bug discovered by Wladimir J. van der Laan.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/coins.cpp b/src/coins.cpp
index bc430c9c7e..4917de625c 100644
--- a/src/coins.cpp
+++ b/src/coins.cpp
@@ -106,7 +106,6 @@ bool CCoinsViewCache::GetCoins(const uint256 &txid, CCoins &coins) const {
 
 CCoinsModifier CCoinsViewCache::ModifyCoins(const uint256 &txid) {
     assert(!hasModifier);
-    hasModifier = true;
     std::pair<CCoinsMap::iterator, bool> ret = cacheCoins.insert(std::make_pair(txid, CCoinsCacheEntry()));
     if (ret.second) {
         if (!base->GetCoins(txid, ret.first->second.coins)) {
@@ -247,7 +246,10 @@ double CCoinsViewCache::GetPriority(const CTransaction &tx, int nHeight) const
     return tx.ComputePriority(dResult);
 }
 
-CCoinsModifier::CCoinsModifier(CCoinsViewCache& cache_, CCoinsMap::iterator it_) : cache(cache_), it(it_) {}
+CCoinsModifier::CCoinsModifier(CCoinsViewCache& cache_, CCoinsMap::iterator it_) : cache(cache_), it(it_) {
+    assert(!cache.hasModifier);
+    cache.hasModifier = true;
+}
 
 CCoinsModifier::~CCoinsModifier()
 {