Add opt-in full-RBF to mempool
Replaces transactions already in the mempool if a new transaction seen with a higher fee, specifically both a higher fee per KB and a higher absolute fee. Children are evaluateed for replacement as well, using the mempool package tracking to calculate replaced fees/size. Transactions can opt-out of transaction replacement by setting nSequence >= maxint-1 on all inputs. (which all wallets do already)
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1 changed files with 121 additions and 5 deletions
126
src/main.cpp
126
src/main.cpp
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@ -831,15 +831,42 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
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return state.Invalid(false, REJECT_ALREADY_KNOWN, "txn-already-in-mempool");
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// Check for conflicts with in-memory transactions
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set<uint256> setConflicts;
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{
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LOCK(pool.cs); // protect pool.mapNextTx
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for (unsigned int i = 0; i < tx.vin.size(); i++)
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BOOST_FOREACH(const CTxIn &txin, tx.vin)
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{
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COutPoint outpoint = tx.vin[i].prevout;
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if (pool.mapNextTx.count(outpoint))
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if (pool.mapNextTx.count(txin.prevout))
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{
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// Disable replacement feature for now
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return state.Invalid(false, REJECT_CONFLICT, "txn-mempool-conflict");
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const CTransaction *ptxConflicting = pool.mapNextTx[txin.prevout].ptx;
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if (!setConflicts.count(ptxConflicting->GetHash()))
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{
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// Allow opt-out of transaction replacement by setting
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// nSequence >= maxint-1 on all inputs.
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//
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// maxint-1 is picked to still allow use of nLockTime by
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// non-replacable transactions. All inputs rather than just one
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// is for the sake of multi-party protocols, where we don't
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// want a single party to be able to disable replacement.
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//
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// The opt-out ignores descendants as anyone relying on
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// first-seen mempool behavior should be checking all
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// unconfirmed ancestors anyway; doing otherwise is hopelessly
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// insecure.
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bool fReplacementOptOut = true;
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BOOST_FOREACH(const CTxIn &txin, ptxConflicting->vin)
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{
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if (txin.nSequence < std::numeric_limits<unsigned int>::max()-1)
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{
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fReplacementOptOut = false;
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break;
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}
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}
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if (fReplacementOptOut)
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return state.Invalid(false, REJECT_CONFLICT, "txn-mempool-conflict");
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setConflicts.insert(ptxConflicting->GetHash());
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}
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}
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}
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}
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@ -957,6 +984,82 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
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return state.DoS(0, false, REJECT_NONSTANDARD, "too-long-mempool-chain", false, errString);
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}
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// A transaction that spends outputs that would be replaced by it is invalid. Now
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// that we have the set of all ancestors we can detect this
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// pathological case by making sure setConflicts and setAncestors don't
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// intersect.
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BOOST_FOREACH(CTxMemPool::txiter ancestorIt, setAncestors)
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{
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const uint256 &hashAncestor = ancestorIt->GetTx().GetHash();
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if (setConflicts.count(hashAncestor))
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{
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return state.DoS(10, error("AcceptToMemoryPool: %s spends conflicting transaction %s",
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hash.ToString(),
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hashAncestor.ToString()),
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REJECT_INVALID, "bad-txns-spends-conflicting-tx");
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}
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}
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// Check if it's economically rational to mine this transaction rather
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// than the ones it replaces.
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CAmount nConflictingFees = 0;
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size_t nConflictingSize = 0;
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if (setConflicts.size())
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{
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LOCK(pool.cs);
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// For efficiency we simply sum up the pre-calculated
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// fees/size-with-descendants values from the mempool package
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// tracking; this does mean the pathological case of diamond tx
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// graphs will be overcounted.
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BOOST_FOREACH(const uint256 hashConflicting, setConflicts)
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{
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CTxMemPool::txiter mi = pool.mapTx.find(hashConflicting);
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if (mi == pool.mapTx.end())
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continue;
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nConflictingFees += mi->GetFeesWithDescendants();
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nConflictingSize += mi->GetSizeWithDescendants();
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}
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// First of all we can't allow a replacement unless it pays greater
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// fees than the transactions it conflicts with - if we did the
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// bandwidth used by those conflicting transactions would not be
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// paid for
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if (nFees < nConflictingFees)
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{
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return state.DoS(0, error("AcceptToMemoryPool: rejecting replacement %s, less fees than conflicting txs; %s < %s",
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hash.ToString(), FormatMoney(nFees), FormatMoney(nConflictingFees)),
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REJECT_INSUFFICIENTFEE, "insufficient fee");
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}
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// Secondly in addition to paying more fees than the conflicts the
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// new transaction must additionally pay for its own bandwidth.
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CAmount nDeltaFees = nFees - nConflictingFees;
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if (nDeltaFees < ::minRelayTxFee.GetFee(nSize))
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{
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return state.DoS(0,
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error("AcceptToMemoryPool: rejecting replacement %s, not enough additional fees to relay; %s < %s",
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hash.ToString(),
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FormatMoney(nDeltaFees),
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FormatMoney(::minRelayTxFee.GetFee(nSize))),
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REJECT_INSUFFICIENTFEE, "insufficient fee");
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}
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// Finally replace only if we end up with a larger fees-per-kb than
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// the replacements.
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CFeeRate oldFeeRate(nConflictingFees, nConflictingSize);
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CFeeRate newFeeRate(nFees, nSize);
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if (newFeeRate <= oldFeeRate)
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{
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return state.DoS(0,
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error("AcceptToMemoryPool: rejecting replacement %s; new feerate %s <= old feerate %s",
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hash.ToString(),
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newFeeRate.ToString(),
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oldFeeRate.ToString()),
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REJECT_INSUFFICIENTFEE, "insufficient fee");
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}
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}
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// Check against previous transactions
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// This is done last to help prevent CPU exhaustion denial-of-service attacks.
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if (!CheckInputs(tx, state, view, true, STANDARD_SCRIPT_VERIFY_FLAGS, true))
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@ -977,6 +1080,19 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
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__func__, hash.ToString(), FormatStateMessage(state));
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}
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// Remove conflicting transactions from the mempool
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list<CTransaction> ltxConflicted;
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pool.removeConflicts(tx, ltxConflicted);
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BOOST_FOREACH(const CTransaction &txConflicted, ltxConflicted)
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{
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LogPrint("mempool", "replacing tx %s with %s for %s BTC additional fees, %d delta bytes\n",
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txConflicted.GetHash().ToString(),
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hash.ToString(),
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FormatMoney(nFees - nConflictingFees),
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(int)nSize - (int)nConflictingSize);
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}
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// Store transaction in memory
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pool.addUnchecked(hash, entry, setAncestors, !IsInitialBlockDownload());
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