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Diffstat (limited to 'src/headerssync.cpp')
| -rw-r--r-- | src/headerssync.cpp | 317 |
1 files changed, 317 insertions, 0 deletions
diff --git a/src/headerssync.cpp b/src/headerssync.cpp new file mode 100644 index 0000000000..3eca492d81 --- /dev/null +++ b/src/headerssync.cpp @@ -0,0 +1,317 @@ +// Copyright (c) 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 <headerssync.h> +#include <logging.h> +#include <pow.h> +#include <timedata.h> +#include <util/check.h> + +// The two constants below are computed using the simulation script on +// https://gist.github.com/sipa/016ae445c132cdf65a2791534dfb7ae1 + +//! Store a commitment to a header every HEADER_COMMITMENT_PERIOD blocks. +constexpr size_t HEADER_COMMITMENT_PERIOD{584}; + +//! Only feed headers to validation once this many headers on top have been +//! received and validated against commitments. +constexpr size_t REDOWNLOAD_BUFFER_SIZE{13959}; // 13959/584 = ~23.9 commitments + +// Our memory analysis assumes 48 bytes for a CompressedHeader (so we should +// re-calculate parameters if we compress further) +static_assert(sizeof(CompressedHeader) == 48); + +HeadersSyncState::HeadersSyncState(NodeId id, const Consensus::Params& consensus_params, + const CBlockIndex* chain_start, const arith_uint256& minimum_required_work) : + m_id(id), m_consensus_params(consensus_params), + m_chain_start(chain_start), + m_minimum_required_work(minimum_required_work), + m_current_chain_work(chain_start->nChainWork), + m_commit_offset(GetRand<unsigned>(HEADER_COMMITMENT_PERIOD)), + m_last_header_received(m_chain_start->GetBlockHeader()), + m_current_height(chain_start->nHeight) +{ + // Estimate the number of blocks that could possibly exist on the peer's + // chain *right now* using 6 blocks/second (fastest blockrate given the MTP + // rule) times the number of seconds from the last allowed block until + // today. This serves as a memory bound on how many commitments we might + // store from this peer, and we can safely give up syncing if the peer + // exceeds this bound, because it's not possible for a consensus-valid + // chain to be longer than this (at the current time -- in the future we + // could try again, if necessary, to sync a longer chain). + m_max_commitments = 6*(Ticks<std::chrono::seconds>(GetAdjustedTime() - NodeSeconds{std::chrono::seconds{chain_start->GetMedianTimePast()}}) + MAX_FUTURE_BLOCK_TIME) / HEADER_COMMITMENT_PERIOD; + + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync started with peer=%d: height=%i, max_commitments=%i, min_work=%s\n", m_id, m_current_height, m_max_commitments, m_minimum_required_work.ToString()); +} + +/** Free any memory in use, and mark this object as no longer usable. This is + * required to guarantee that we won't reuse this object with the same + * SaltedTxidHasher for another sync. */ +void HeadersSyncState::Finalize() +{ + Assume(m_download_state != State::FINAL); + m_header_commitments = {}; + m_last_header_received.SetNull(); + m_redownloaded_headers = {}; + m_redownload_buffer_last_hash.SetNull(); + m_redownload_buffer_first_prev_hash.SetNull(); + m_process_all_remaining_headers = false; + m_current_height = 0; + + m_download_state = State::FINAL; +} + +/** Process the next batch of headers received from our peer. + * Validate and store commitments, and compare total chainwork to our target to + * see if we can switch to REDOWNLOAD mode. */ +HeadersSyncState::ProcessingResult HeadersSyncState::ProcessNextHeaders(const + std::vector<CBlockHeader>& received_headers, const bool full_headers_message) +{ + ProcessingResult ret; + + Assume(!received_headers.empty()); + if (received_headers.empty()) return ret; + + Assume(m_download_state != State::FINAL); + if (m_download_state == State::FINAL) return ret; + + if (m_download_state == State::PRESYNC) { + // During PRESYNC, we minimally validate block headers and + // occasionally add commitments to them, until we reach our work + // threshold (at which point m_download_state is updated to REDOWNLOAD). + ret.success = ValidateAndStoreHeadersCommitments(received_headers); + if (ret.success) { + if (full_headers_message || m_download_state == State::REDOWNLOAD) { + // A full headers message means the peer may have more to give us; + // also if we just switched to REDOWNLOAD then we need to re-request + // headers from the beginning. + ret.request_more = true; + } else { + Assume(m_download_state == State::PRESYNC); + // If we're in PRESYNC and we get a non-full headers + // message, then the peer's chain has ended and definitely doesn't + // have enough work, so we can stop our sync. + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: incomplete headers message at height=%i (presync phase)\n", m_id, m_current_height); + } + } + } else if (m_download_state == State::REDOWNLOAD) { + // During REDOWNLOAD, we compare our stored commitments to what we + // receive, and add headers to our redownload buffer. When the buffer + // gets big enough (meaning that we've checked enough commitments), + // we'll return a batch of headers to the caller for processing. + ret.success = true; + for (const auto& hdr : received_headers) { + if (!ValidateAndStoreRedownloadedHeader(hdr)) { + // Something went wrong -- the peer gave us an unexpected chain. + // We could consider looking at the reason for failure and + // punishing the peer, but for now just give up on sync. + ret.success = false; + break; + } + } + + if (ret.success) { + // Return any headers that are ready for acceptance. + ret.pow_validated_headers = PopHeadersReadyForAcceptance(); + + // If we hit our target blockhash, then all remaining headers will be + // returned and we can clear any leftover internal state. + if (m_redownloaded_headers.empty() && m_process_all_remaining_headers) { + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync complete with peer=%d: releasing all at height=%i (redownload phase)\n", m_id, m_redownload_buffer_last_height); + } else if (full_headers_message) { + // If the headers message is full, we need to request more. + ret.request_more = true; + } else { + // For some reason our peer gave us a high-work chain, but is now + // declining to serve us that full chain again. Give up. + // Note that there's no more processing to be done with these + // headers, so we can still return success. + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: incomplete headers message at height=%i (redownload phase)\n", m_id, m_redownload_buffer_last_height); + } + } + } + + if (!(ret.success && ret.request_more)) Finalize(); + return ret; +} + +bool HeadersSyncState::ValidateAndStoreHeadersCommitments(const std::vector<CBlockHeader>& headers) +{ + // The caller should not give us an empty set of headers. + Assume(headers.size() > 0); + if (headers.size() == 0) return true; + + Assume(m_download_state == State::PRESYNC); + if (m_download_state != State::PRESYNC) return false; + + if (headers[0].hashPrevBlock != m_last_header_received.GetHash()) { + // Somehow our peer gave us a header that doesn't connect. + // This might be benign -- perhaps our peer reorged away from the chain + // they were on. Give up on this sync for now (likely we will start a + // new sync with a new starting point). + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: non-continuous headers at height=%i (presync phase)\n", m_id, m_current_height); + return false; + } + + // If it does connect, (minimally) validate and occasionally store + // commitments. + for (const auto& hdr : headers) { + if (!ValidateAndProcessSingleHeader(hdr)) { + return false; + } + } + + if (m_current_chain_work >= m_minimum_required_work) { + m_redownloaded_headers.clear(); + m_redownload_buffer_last_height = m_chain_start->nHeight; + m_redownload_buffer_first_prev_hash = m_chain_start->GetBlockHash(); + m_redownload_buffer_last_hash = m_chain_start->GetBlockHash(); + m_redownload_chain_work = m_chain_start->nChainWork; + m_download_state = State::REDOWNLOAD; + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync transition with peer=%d: reached sufficient work at height=%i, redownloading from height=%i\n", m_id, m_current_height, m_redownload_buffer_last_height); + } + return true; +} + +bool HeadersSyncState::ValidateAndProcessSingleHeader(const CBlockHeader& current) +{ + Assume(m_download_state == State::PRESYNC); + if (m_download_state != State::PRESYNC) return false; + + int next_height = m_current_height + 1; + + // Verify that the difficulty isn't growing too fast; an adversary with + // limited hashing capability has a greater chance of producing a high + // work chain if they compress the work into as few blocks as possible, + // so don't let anyone give a chain that would violate the difficulty + // adjustment maximum. + if (!PermittedDifficultyTransition(m_consensus_params, next_height, + m_last_header_received.nBits, current.nBits)) { + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: invalid difficulty transition at height=%i (presync phase)\n", m_id, next_height); + return false; + } + + if (next_height % HEADER_COMMITMENT_PERIOD == m_commit_offset) { + // Add a commitment. + m_header_commitments.push_back(m_hasher(current.GetHash()) & 1); + if (m_header_commitments.size() > m_max_commitments) { + // The peer's chain is too long; give up. + // It's possible the chain grew since we started the sync; so + // potentially we could succeed in syncing the peer's chain if we + // try again later. + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: exceeded max commitments at height=%i (presync phase)\n", m_id, next_height); + return false; + } + } + + m_current_chain_work += GetBlockProof(CBlockIndex(current)); + m_last_header_received = current; + m_current_height = next_height; + + return true; +} + +bool HeadersSyncState::ValidateAndStoreRedownloadedHeader(const CBlockHeader& header) +{ + Assume(m_download_state == State::REDOWNLOAD); + if (m_download_state != State::REDOWNLOAD) return false; + + int64_t next_height = m_redownload_buffer_last_height + 1; + + // Ensure that we're working on a header that connects to the chain we're + // downloading. + if (header.hashPrevBlock != m_redownload_buffer_last_hash) { + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: non-continuous headers at height=%i (redownload phase)\n", m_id, next_height); + return false; + } + + // Check that the difficulty adjustments are within our tolerance: + uint32_t previous_nBits{0}; + if (!m_redownloaded_headers.empty()) { + previous_nBits = m_redownloaded_headers.back().nBits; + } else { + previous_nBits = m_chain_start->nBits; + } + + if (!PermittedDifficultyTransition(m_consensus_params, next_height, + previous_nBits, header.nBits)) { + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: invalid difficulty transition at height=%i (redownload phase)\n", m_id, next_height); + return false; + } + + // Track work on the redownloaded chain + m_redownload_chain_work += GetBlockProof(CBlockIndex(header)); + + if (m_redownload_chain_work >= m_minimum_required_work) { + m_process_all_remaining_headers = true; + } + + // If we're at a header for which we previously stored a commitment, verify + // it is correct. Failure will result in aborting download. + // Also, don't check commitments once we've gotten to our target blockhash; + // it's possible our peer has extended its chain between our first sync and + // our second, and we don't want to return failure after we've seen our + // target blockhash just because we ran out of commitments. + if (!m_process_all_remaining_headers && next_height % HEADER_COMMITMENT_PERIOD == m_commit_offset) { + if (m_header_commitments.size() == 0) { + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: commitment overrun at height=%i (redownload phase)\n", m_id, next_height); + // Somehow our peer managed to feed us a different chain and + // we've run out of commitments. + return false; + } + bool commitment = m_hasher(header.GetHash()) & 1; + bool expected_commitment = m_header_commitments.front(); + m_header_commitments.pop_front(); + if (commitment != expected_commitment) { + LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: commitment mismatch at height=%i (redownload phase)\n", m_id, next_height); + return false; + } + } + + // Store this header for later processing. + m_redownloaded_headers.push_back(header); + m_redownload_buffer_last_height = next_height; + m_redownload_buffer_last_hash = header.GetHash(); + + return true; +} + +std::vector<CBlockHeader> HeadersSyncState::PopHeadersReadyForAcceptance() +{ + std::vector<CBlockHeader> ret; + + Assume(m_download_state == State::REDOWNLOAD); + if (m_download_state != State::REDOWNLOAD) return ret; + + while (m_redownloaded_headers.size() > REDOWNLOAD_BUFFER_SIZE || + (m_redownloaded_headers.size() > 0 && m_process_all_remaining_headers)) { + ret.emplace_back(m_redownloaded_headers.front().GetFullHeader(m_redownload_buffer_first_prev_hash)); + m_redownloaded_headers.pop_front(); + m_redownload_buffer_first_prev_hash = ret.back().GetHash(); + } + return ret; +} + +CBlockLocator HeadersSyncState::NextHeadersRequestLocator() const +{ + Assume(m_download_state != State::FINAL); + if (m_download_state == State::FINAL) return {}; + + auto chain_start_locator = LocatorEntries(m_chain_start); + std::vector<uint256> locator; + + if (m_download_state == State::PRESYNC) { + // During pre-synchronization, we continue from the last header received. + locator.push_back(m_last_header_received.GetHash()); + } + + if (m_download_state == State::REDOWNLOAD) { + // During redownload, we will download from the last received header that we stored. + locator.push_back(m_redownload_buffer_last_hash); + } + + locator.insert(locator.end(), chain_start_locator.begin(), chain_start_locator.end()); + + return CBlockLocator{std::move(locator)}; +} |