// 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 <util/check.h>

#include <util/time.h>

#include <util/vector.h>

// The two constants below are computed using the simulation script in

// contrib/devtools/headerssync-params.py.

//! Store one header commitment per HEADER_COMMITMENT_PERIOD blocks.

constexpr size_t HEADER_COMMITMENT_PERIOD{615};

//! Only feed headers to validation once this many headers on top have been

//! received and validated against commitments.

constexpr size_t REDOWNLOAD_BUFFER_SIZE{14621}; // 14621/615 = ~23.8 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_commit_offset(FastRandomContext().randrange<unsigned>(HEADER_COMMITMENT_PERIOD)),

    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_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>(NodeClock::now() - NodeSeconds{std::chrono::seconds{chain_start->GetMedianTimePast()}}) + MAX_FUTURE_BLOCK_TIME) / HEADER_COMMITMENT_PERIOD;

    LogDebug(BCLog::NET, "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);

    ClearShrink(m_header_commitments);

    m_last_header_received.SetNull();

    ClearShrink(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.

                LogDebug(BCLog::NET, "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) {

                LogDebug(BCLog::NET, "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.

                LogDebug(BCLog::NET, "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).

        LogDebug(BCLog::NET, "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;

        LogDebug(BCLog::NET, "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)) {

        LogDebug(BCLog::NET, "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.

            LogDebug(BCLog::NET, "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) {

        LogDebug(BCLog::NET, "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)) {

        LogDebug(BCLog::NET, "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) {

            LogDebug(BCLog::NET, "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) {

            LogDebug(BCLog::NET, "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.emplace_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)};

}