Discourage fee sniping with nLockTime

1 files changed, 27 insertions, 1 deletions

diff --git a/src/wallet.cpp b/src/wallet.cpp
index 026c53aa2e..a64d27a941 100644
--- a/src/wallet.cpp
+++ b/src/wallet.cpp
@@ -1347,6 +1347,28 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, CAmount> >& vecSend,
     wtxNew.BindWallet(this);
     CMutableTransaction txNew;
 
+    // Discourage fee sniping.
+    //
+    // However because of a off-by-one-error in previous versions we need to
+    // neuter it by setting nLockTime to at least one less than nBestHeight.
+    // Secondly currently propagation of transactions created for block heights
+    // corresponding to blocks that were just mined may be iffy - transactions
+    // aren't re-accepted into the mempool - we additionally neuter the code by
+    // going ten blocks back. Doesn't yet do anything for sniping, but does act
+    // to shake out wallet bugs like not showing nLockTime'd transactions at
+    // all.
+    txNew.nLockTime = std::max(0, chainActive.Height() - 10);
+
+    // Secondly occasionally randomly pick a nLockTime even further back, so
+    // that transactions that are delayed after signing for whatever reason,
+    // e.g. high-latency mix networks and some CoinJoin implementations, have
+    // better privacy.
+    if (GetRandInt(10) == 0)
+        txNew.nLockTime = std::max(0, (int)txNew.nLockTime - GetRandInt(100));
+
+    assert(txNew.nLockTime <= (unsigned int)chainActive.Height());
+    assert(txNew.nLockTime < LOCKTIME_THRESHOLD);
+
     {
         LOCK2(cs_main, cs_wallet);
         {
@@ -1440,8 +1462,12 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, CAmount> >& vecSend,
                     reservekey.ReturnKey();
 
                 // Fill vin
+                //
+                // Note how the sequence number is set to max()-1 so that the
+                // nLockTime set above actually works.
                 BOOST_FOREACH(const PAIRTYPE(const CWalletTx*,unsigned int)& coin, setCoins)
-                    txNew.vin.push_back(CTxIn(coin.first->GetHash(),coin.second));
+                    txNew.vin.push_back(CTxIn(coin.first->GetHash(),coin.second,CScript(),
+                                              std::numeric_limits<unsigned int>::max()-1));
 
                 // Sign
                 int nIn = 0;