bitcoin/src/policy/rbf.cpp

// Copyright (c) 2016-2020 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include <policy/rbf.h>

#include <policy/settings.h>
#include <tinyformat.h>
#include <util/moneystr.h>
#include <util/rbf.h>

RBFTransactionState IsRBFOptIn(const CTransaction& tx, const CTxMemPool& pool)
{
    AssertLockHeld(pool.cs);

    CTxMemPool::setEntries ancestors;

    // First check the transaction itself.
    if (SignalsOptInRBF(tx)) {
        return RBFTransactionState::REPLACEABLE_BIP125;
    }

    // If this transaction is not in our mempool, then we can't be sure
    // we will know about all its inputs.
    if (!pool.exists(tx.GetHash())) {
        return RBFTransactionState::UNKNOWN;
    }

    // If all the inputs have nSequence >= maxint-1, it still might be
    // signaled for RBF if any unconfirmed parents have signaled.
    uint64_t noLimit = std::numeric_limits<uint64_t>::max();
    std::string dummy;
    CTxMemPoolEntry entry = *pool.mapTx.find(tx.GetHash());
    pool.CalculateMemPoolAncestors(entry, ancestors, noLimit, noLimit, noLimit, noLimit, dummy, false);

    for (CTxMemPool::txiter it : ancestors) {
        if (SignalsOptInRBF(it->GetTx())) {
            return RBFTransactionState::REPLACEABLE_BIP125;
        }
    }
    return RBFTransactionState::FINAL;
}

RBFTransactionState IsRBFOptInEmptyMempool(const CTransaction& tx)
{
    // If we don't have a local mempool we can only check the transaction itself.
    return SignalsOptInRBF(tx) ? RBFTransactionState::REPLACEABLE_BIP125 : RBFTransactionState::UNKNOWN;
}

std::optional<std::string> GetEntriesForConflicts(const CTransaction& tx,
                            CTxMemPool& pool,
                            const CTxMemPool::setEntries& iters_conflicting,
                            CTxMemPool::setEntries& all_conflicts)
{
    AssertLockHeld(pool.cs);
    const uint256 txid = tx.GetHash();
    uint64_t nConflictingCount = 0;
    for (const auto& mi : iters_conflicting) {
        nConflictingCount += mi->GetCountWithDescendants();
        // This potentially overestimates the number of actual descendants
        // but we just want to be conservative to avoid doing too much
        // work.
        if (nConflictingCount > MAX_BIP125_REPLACEMENT_CANDIDATES) {
            return strprintf("rejecting replacement %s; too many potential replacements (%d > %d)\n",
                        txid.ToString(),
                        nConflictingCount,
                        MAX_BIP125_REPLACEMENT_CANDIDATES);
        }
    }
    // If not too many to replace, then calculate the set of
    // transactions that would have to be evicted
    for (CTxMemPool::txiter it : iters_conflicting) {
        pool.CalculateDescendants(it, all_conflicts);
    }
    return std::nullopt;
}

std::optional<std::string> HasNoNewUnconfirmed(const CTransaction& tx,
                                               const CTxMemPool& pool,
                                               const CTxMemPool::setEntries& iters_conflicting)
{
    AssertLockHeld(pool.cs);
    std::set<uint256> parents_of_conflicts;
    for (const auto& mi : iters_conflicting) {
        for (const CTxIn &txin : mi->GetTx().vin)
        {
            parents_of_conflicts.insert(txin.prevout.hash);
        }
    }

    for (unsigned int j = 0; j < tx.vin.size(); j++)
    {
        // We don't want to accept replacements that require low
        // feerate junk to be mined first. Ideally we'd keep track of
        // the ancestor feerates and make the decision based on that,
        // but for now requiring all new inputs to be confirmed works.
        //
        // Note that if you relax this to make RBF a little more useful,
        // this may break the CalculateMempoolAncestors RBF relaxation,
        // above. See the comment above the first CalculateMempoolAncestors
        // call for more info.
        if (!parents_of_conflicts.count(tx.vin[j].prevout.hash))
        {
            // Rather than check the UTXO set - potentially expensive -
            // it's cheaper to just check if the new input refers to a
            // tx that's in the mempool.
            if (pool.exists(tx.vin[j].prevout.hash)) {
                return strprintf("replacement %s adds unconfirmed input, idx %d",
                                 tx.GetHash().ToString(), j);
            }
        }
    }
    return std::nullopt;
}

std::optional<std::string> EntriesAndTxidsDisjoint(const CTxMemPool::setEntries& ancestors,
                                                   const std::set<uint256>& direct_conflicts,
                                                   const uint256& txid)
{
    for (CTxMemPool::txiter ancestorIt : ancestors)
    {
        const uint256 &hashAncestor = ancestorIt->GetTx().GetHash();
        if (direct_conflicts.count(hashAncestor))
        {
            return strprintf("%s spends conflicting transaction %s",
                             txid.ToString(),
                             hashAncestor.ToString());
        }
    }
    return std::nullopt;
}

std::optional<std::string> PaysMoreThanConflicts(const CTxMemPool::setEntries& iters_conflicting,
                                                 CFeeRate replacement_feerate,
                                                 const uint256& txid)
{
    for (const auto& mi : iters_conflicting) {
        // Don't allow the replacement to reduce the feerate of the
        // mempool.
        //
        // We usually don't want to accept replacements with lower
        // feerates than what they replaced as that would lower the
        // feerate of the next block. Requiring that the feerate always
        // be increased is also an easy-to-reason about way to prevent
        // DoS attacks via replacements.
        //
        // We only consider the feerates of transactions being directly
        // replaced, not their indirect descendants. While that does
        // mean high feerate children are ignored when deciding whether
        // or not to replace, we do require the replacement to pay more
        // overall fees too, mitigating most cases.
        CFeeRate original_feerate(mi->GetModifiedFee(), mi->GetTxSize());
        if (replacement_feerate <= original_feerate)
        {
            return strprintf("rejecting replacement %s; new feerate %s <= old feerate %s",
                             txid.ToString(),
                             replacement_feerate.ToString(),
                             original_feerate.ToString());
        }
    }
    return std::nullopt;
}

std::optional<std::string> PaysForRBF(CAmount original_fees,
                                      CAmount replacement_fees,
                                      size_t replacement_vsize,
                                      const uint256& txid)
{
    // The replacement must pay greater fees than the transactions it
    // replaces - if we did the bandwidth used by those conflicting
    // transactions would not be paid for.
    if (replacement_fees < original_fees)
    {
        return strprintf("rejecting replacement %s, less fees than conflicting txs; %s < %s",
                         txid.ToString(), FormatMoney(replacement_fees), FormatMoney(original_fees));
    }

    // Finally in addition to paying more fees than the conflicts the
    // new transaction must pay for its own bandwidth.
    CAmount additional_fees = replacement_fees - original_fees;
    if (additional_fees < ::incrementalRelayFee.GetFee(replacement_vsize))
    {
        return strprintf("rejecting replacement %s, not enough additional fees to relay; %s < %s",
                         txid.ToString(),
                         FormatMoney(additional_fees),
                         FormatMoney(::incrementalRelayFee.GetFee(replacement_vsize)));
    }
    return std::nullopt;
}