Merge pull request #6216

db6047d Take the training wheels off anti-fee-sniping (Peter Todd)

Author: laanwj

src/wallet/wallet.cpp

@@ -1871,15 +1871,25 @@ bool CWallet::CreateTransaction(const vector<CRecipient>& vecSend, CWalletTx& wt
 
     // 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);
+    // For a large miner the value of the transactions in the best block and
+    // the mempool can exceed the cost of delibrately attempting to mine two
+    // blocks to orphan the current best block. By setting nLockTime such that
+    // only the next block can include the transaction, we discourage this
+    // practice as the height restricted and limited blocksize gives miners
+    // considering fee sniping fewer options for pulling off this attack.
+    //
+    // A simple way to think about this is from the wallet's point of view we
+    // always want the blockchain to move forward. By setting nLockTime this
+    // way we're basically making the statement that we only want this
+    // transaction to appear in the next block; we don't want to potentially
+    // encourage reorgs by allowing transactions to appear at lower heights
+    // than the next block in forks of the best chain.
+    //
+    // Of course, the subsidy is high enough, and transaction volume low
+    // enough, that fee sniping isn't a problem yet, but by implementing a fix
+    // now we ensure code won't be written that makes assumptions about
+    // nLockTime that preclude a fix later.
+    txNew.nLockTime = chainActive.Height();
 
     // Secondly occasionally randomly pick a nLockTime even further back, so
     // that transactions that are delayed after signing for whatever reason,