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// Copyright (c) 2016-2022 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 <blockencodings.h>
#include <chainparams.h>
#include <common/system.h>
#include <consensus/consensus.h>
#include <consensus/validation.h>
#include <crypto/sha256.h>
#include <crypto/siphash.h>
#include <logging.h>
#include <random.h>
#include <streams.h>
#include <txmempool.h>
#include <validation.h>
#include <unordered_map>
CBlockHeaderAndShortTxIDs::CBlockHeaderAndShortTxIDs(const CBlock& block, const uint64_t nonce) :
nonce(nonce),
shorttxids(block.vtx.size() - 1), prefilledtxn(1), header(block) {
FillShortTxIDSelector();
//TODO: Use our mempool prior to block acceptance to predictively fill more than just the coinbase
prefilledtxn[0] = {0, block.vtx[0]};
for (size_t i = 1; i < block.vtx.size(); i++) {
const CTransaction& tx = *block.vtx[i];
shorttxids[i - 1] = GetShortID(tx.GetWitnessHash());
}
}
void CBlockHeaderAndShortTxIDs::FillShortTxIDSelector() const {
DataStream stream{};
stream << header << nonce;
CSHA256 hasher;
hasher.Write((unsigned char*)&(*stream.begin()), stream.end() - stream.begin());
uint256 shorttxidhash;
hasher.Finalize(shorttxidhash.begin());
shorttxidk0 = shorttxidhash.GetUint64(0);
shorttxidk1 = shorttxidhash.GetUint64(1);
}
uint64_t CBlockHeaderAndShortTxIDs::GetShortID(const Wtxid& wtxid) const {
static_assert(SHORTTXIDS_LENGTH == 6, "shorttxids calculation assumes 6-byte shorttxids");
return SipHashUint256(shorttxidk0, shorttxidk1, wtxid) & 0xffffffffffffL;
}
ReadStatus PartiallyDownloadedBlock::InitData(const CBlockHeaderAndShortTxIDs& cmpctblock, const std::vector<CTransactionRef>& extra_txn) {
if (cmpctblock.header.IsNull() || (cmpctblock.shorttxids.empty() && cmpctblock.prefilledtxn.empty()))
return READ_STATUS_INVALID;
if (cmpctblock.shorttxids.size() + cmpctblock.prefilledtxn.size() > MAX_BLOCK_WEIGHT / MIN_SERIALIZABLE_TRANSACTION_WEIGHT)
return READ_STATUS_INVALID;
if (!header.IsNull() || !txn_available.empty()) return READ_STATUS_INVALID;
header = cmpctblock.header;
txn_available.resize(cmpctblock.BlockTxCount());
int32_t lastprefilledindex = -1;
for (size_t i = 0; i < cmpctblock.prefilledtxn.size(); i++) {
if (cmpctblock.prefilledtxn[i].tx->IsNull())
return READ_STATUS_INVALID;
lastprefilledindex += cmpctblock.prefilledtxn[i].index + 1; //index is a uint16_t, so can't overflow here
if (lastprefilledindex > std::numeric_limits<uint16_t>::max())
return READ_STATUS_INVALID;
if ((uint32_t)lastprefilledindex > cmpctblock.shorttxids.size() + i) {
// If we are inserting a tx at an index greater than our full list of shorttxids
// plus the number of prefilled txn we've inserted, then we have txn for which we
// have neither a prefilled txn or a shorttxid!
return READ_STATUS_INVALID;
}
txn_available[lastprefilledindex] = cmpctblock.prefilledtxn[i].tx;
}
prefilled_count = cmpctblock.prefilledtxn.size();
// Calculate map of txids -> positions and check mempool to see what we have (or don't)
// Because well-formed cmpctblock messages will have a (relatively) uniform distribution
// of short IDs, any highly-uneven distribution of elements can be safely treated as a
// READ_STATUS_FAILED.
std::unordered_map<uint64_t, uint16_t> shorttxids(cmpctblock.shorttxids.size());
uint16_t index_offset = 0;
for (size_t i = 0; i < cmpctblock.shorttxids.size(); i++) {
while (txn_available[i + index_offset])
index_offset++;
shorttxids[cmpctblock.shorttxids[i]] = i + index_offset;
// To determine the chance that the number of entries in a bucket exceeds N,
// we use the fact that the number of elements in a single bucket is
// binomially distributed (with n = the number of shorttxids S, and p =
// 1 / the number of buckets), that in the worst case the number of buckets is
// equal to S (due to std::unordered_map having a default load factor of 1.0),
// and that the chance for any bucket to exceed N elements is at most
// buckets * (the chance that any given bucket is above N elements).
// Thus: P(max_elements_per_bucket > N) <= S * (1 - cdf(binomial(n=S,p=1/S), N)).
// If we assume blocks of up to 16000, allowing 12 elements per bucket should
// only fail once per ~1 million block transfers (per peer and connection).
if (shorttxids.bucket_size(shorttxids.bucket(cmpctblock.shorttxids[i])) > 12)
return READ_STATUS_FAILED;
}
// TODO: in the shortid-collision case, we should instead request both transactions
// which collided. Falling back to full-block-request here is overkill.
if (shorttxids.size() != cmpctblock.shorttxids.size())
return READ_STATUS_FAILED; // Short ID collision
std::vector<bool> have_txn(txn_available.size());
{
LOCK(pool->cs);
for (const auto& tx : pool->txns_randomized) {
uint64_t shortid = cmpctblock.GetShortID(tx->GetWitnessHash());
std::unordered_map<uint64_t, uint16_t>::iterator idit = shorttxids.find(shortid);
if (idit != shorttxids.end()) {
if (!have_txn[idit->second]) {
txn_available[idit->second] = tx;
have_txn[idit->second] = true;
mempool_count++;
} else {
// If we find two mempool txn that match the short id, just request it.
// This should be rare enough that the extra bandwidth doesn't matter,
// but eating a round-trip due to FillBlock failure would be annoying
if (txn_available[idit->second]) {
txn_available[idit->second].reset();
mempool_count--;
}
}
}
// Though ideally we'd continue scanning for the two-txn-match-shortid case,
// the performance win of an early exit here is too good to pass up and worth
// the extra risk.
if (mempool_count == shorttxids.size())
break;
}
}
for (size_t i = 0; i < extra_txn.size(); i++) {
if (extra_txn[i] == nullptr) {
continue;
}
uint64_t shortid = cmpctblock.GetShortID(extra_txn[i]->GetWitnessHash());
std::unordered_map<uint64_t, uint16_t>::iterator idit = shorttxids.find(shortid);
if (idit != shorttxids.end()) {
if (!have_txn[idit->second]) {
txn_available[idit->second] = extra_txn[i];
have_txn[idit->second] = true;
mempool_count++;
extra_count++;
} else {
// If we find two mempool/extra txn that match the short id, just
// request it.
// This should be rare enough that the extra bandwidth doesn't matter,
// but eating a round-trip due to FillBlock failure would be annoying
// Note that we don't want duplication between extra_txn and mempool to
// trigger this case, so we compare witness hashes first
if (txn_available[idit->second] &&
txn_available[idit->second]->GetWitnessHash() != extra_txn[i]->GetWitnessHash()) {
txn_available[idit->second].reset();
mempool_count--;
extra_count--;
}
}
}
// Though ideally we'd continue scanning for the two-txn-match-shortid case,
// the performance win of an early exit here is too good to pass up and worth
// the extra risk.
if (mempool_count == shorttxids.size())
break;
}
LogDebug(BCLog::CMPCTBLOCK, "Initialized PartiallyDownloadedBlock for block %s using a cmpctblock of size %lu\n", cmpctblock.header.GetHash().ToString(), GetSerializeSize(cmpctblock));
return READ_STATUS_OK;
}
bool PartiallyDownloadedBlock::IsTxAvailable(size_t index) const
{
if (header.IsNull()) return false;
assert(index < txn_available.size());
return txn_available[index] != nullptr;
}
ReadStatus PartiallyDownloadedBlock::FillBlock(CBlock& block, const std::vector<CTransactionRef>& vtx_missing)
{
if (header.IsNull()) return READ_STATUS_INVALID;
uint256 hash = header.GetHash();
block = header;
block.vtx.resize(txn_available.size());
size_t tx_missing_offset = 0;
for (size_t i = 0; i < txn_available.size(); i++) {
if (!txn_available[i]) {
if (vtx_missing.size() <= tx_missing_offset)
return READ_STATUS_INVALID;
block.vtx[i] = vtx_missing[tx_missing_offset++];
} else
block.vtx[i] = std::move(txn_available[i]);
}
// Make sure we can't call FillBlock again.
header.SetNull();
txn_available.clear();
if (vtx_missing.size() != tx_missing_offset)
return READ_STATUS_INVALID;
BlockValidationState state;
CheckBlockFn check_block = m_check_block_mock ? m_check_block_mock : CheckBlock;
if (!check_block(block, state, Params().GetConsensus(), /*fCheckPoW=*/true, /*fCheckMerkleRoot=*/true)) {
// TODO: We really want to just check merkle tree manually here,
// but that is expensive, and CheckBlock caches a block's
// "checked-status" (in the CBlock?). CBlock should be able to
// check its own merkle root and cache that check.
if (state.GetResult() == BlockValidationResult::BLOCK_MUTATED)
return READ_STATUS_FAILED; // Possible Short ID collision
return READ_STATUS_CHECKBLOCK_FAILED;
}
LogDebug(BCLog::CMPCTBLOCK, "Successfully reconstructed block %s with %lu txn prefilled, %lu txn from mempool (incl at least %lu from extra pool) and %lu txn requested\n", hash.ToString(), prefilled_count, mempool_count, extra_count, vtx_missing.size());
if (vtx_missing.size() < 5) {
for (const auto& tx : vtx_missing) {
LogDebug(BCLog::CMPCTBLOCK, "Reconstructed block %s required tx %s\n", hash.ToString(), tx->GetHash().ToString());
}
}
return READ_STATUS_OK;
}
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