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// Copyright (c) 2016 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 "consensus/consensus.h"
#include "consensus/validation.h"
#include "chainparams.h"
#include "hash.h"
#include "random.h"
#include "streams.h"
#include "txmempool.h"
#include "main.h"
#include "util.h"
#include <unordered_map>
#define MIN_TRANSACTION_BASE_SIZE (::GetSerializeSize(CTransaction(), SER_NETWORK, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS))
CBlockHeaderAndShortTxIDs::CBlockHeaderAndShortTxIDs(const CBlock& block, bool fUseWTXID) :
nonce(GetRand(std::numeric_limits<uint64_t>::max())),
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(fUseWTXID ? tx.GetWitnessHash() : tx.GetHash());
}
}
void CBlockHeaderAndShortTxIDs::FillShortTxIDSelector() const {
CDataStream stream(SER_NETWORK, PROTOCOL_VERSION);
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 uint256& txhash) const {
static_assert(SHORTTXIDS_LENGTH == 6, "shorttxids calculation assumes 6-byte shorttxids");
return SipHashUint256(shorttxidk0, shorttxidk1, txhash) & 0xffffffffffffL;
}
ReadStatus PartiallyDownloadedBlock::InitData(const CBlockHeaderAndShortTxIDs& cmpctblock) {
if (cmpctblock.header.IsNull() || (cmpctblock.shorttxids.empty() && cmpctblock.prefilledtxn.empty()))
return READ_STATUS_INVALID;
if (cmpctblock.shorttxids.size() + cmpctblock.prefilledtxn.size() > MAX_BLOCK_BASE_SIZE / MIN_TRANSACTION_BASE_SIZE)
return READ_STATUS_INVALID;
assert(header.IsNull() && txn_available.empty());
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);
const std::vector<std::pair<uint256, CTxMemPool::txiter> >& vTxHashes = pool->vTxHashes;
for (size_t i = 0; i < vTxHashes.size(); i++) {
uint64_t shortid = cmpctblock.GetShortID(vTxHashes[i].first);
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] = vTxHashes[i].second->GetSharedTx();
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;
}
LogPrint("cmpctblock", "Initialized PartiallyDownloadedBlock for block %s using a cmpctblock of size %lu\n", cmpctblock.header.GetHash().ToString(), GetSerializeSize(cmpctblock, SER_NETWORK, PROTOCOL_VERSION));
return READ_STATUS_OK;
}
bool PartiallyDownloadedBlock::IsTxAvailable(size_t index) const {
assert(!header.IsNull());
assert(index < txn_available.size());
return txn_available[index] ? true : false;
}
ReadStatus PartiallyDownloadedBlock::FillBlock(CBlock& block, const std::vector<CTransactionRef>& vtx_missing) const {
assert(!header.IsNull());
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] = txn_available[i];
}
if (vtx_missing.size() != tx_missing_offset)
return READ_STATUS_INVALID;
CValidationState state;
if (!CheckBlock(block, state, Params().GetConsensus())) {
// 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.CorruptionPossible())
return READ_STATUS_FAILED; // Possible Short ID collision
return READ_STATUS_CHECKBLOCK_FAILED;
}
LogPrint("cmpctblock", "Successfully reconstructed block %s with %lu txn prefilled, %lu txn from mempool and %lu txn requested\n", header.GetHash().ToString(), prefilled_count, mempool_count, vtx_missing.size());
if (vtx_missing.size() < 5) {
for (const auto& tx : vtx_missing)
LogPrint("cmpctblock", "Reconstructed block %s required tx %s\n", header.GetHash().ToString(), tx->GetHash().ToString());
}
return READ_STATUS_OK;
}