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Merge pull request #6216

db6047d Take the training wheels off anti-fee-sniping (Peter Todd)
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laanwj committed Dec 2, 2015
2 parents 7c7a05d + db6047d commit 83f06ca937366bea5190705d1f6d7871e5bd110c
Showing with 19 additions and 9 deletions.
  1. +19 −9 src/wallet/wallet.cpp
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@@ -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,

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