ClickHouse/src/Storages/MergeTree/ReplicatedMergeTreeQueue.cpp

#include <Storages/MergeTree/ReplicatedMergeTreeQueue.h>
#include <Storages/StorageReplicatedMergeTree.h>
#include <Storages/MergeTree/IMergeTreeDataPart.h>
#include <Storages/MergeTree/MergeTreeDataMergerMutator.h>
#include <Storages/MergeTree/ReplicatedMergeTreeQuorumEntry.h>
#include <Storages/MergeTree/ReplicatedMergeTreeMergeStrategyPicker.h>
#include <IO/ReadHelpers.h>
#include <IO/WriteHelpers.h>
#include <Common/StringUtils/StringUtils.h>
#include <Common/CurrentMetrics.h>
#include <Parsers/formatAST.h>
#include <base/sort.h>


namespace DB
{

namespace ErrorCodes
{
    extern const int LOGICAL_ERROR;
    extern const int UNEXPECTED_NODE_IN_ZOOKEEPER;
    extern const int ABORTED;
}


ReplicatedMergeTreeQueue::ReplicatedMergeTreeQueue(StorageReplicatedMergeTree & storage_, ReplicatedMergeTreeMergeStrategyPicker & merge_strategy_picker_)
    : storage(storage_)
    , merge_strategy_picker(merge_strategy_picker_)
    , format_version(storage.format_version)
    , current_parts(format_version)
    , virtual_parts(format_version)
    , drop_parts(format_version)
{
    zookeeper_path = storage.zookeeper_path;
    replica_path = storage.replica_path;
    logger_name = storage.getStorageID().getFullTableName() + " (ReplicatedMergeTreeQueue)";
    log = &Poco::Logger::get(logger_name);
}


void ReplicatedMergeTreeQueue::clear()
{
    auto locks = lockQueue();
    chassert(future_parts.empty());
    current_parts.clear();
    virtual_parts.clear();
    queue.clear();
    inserts_by_time.clear();
    mutations_by_znode.clear();
    mutations_by_partition.clear();
    mutation_pointer.clear();
}

void ReplicatedMergeTreeQueue::setBrokenPartsToEnqueueFetchesOnLoading(Strings && parts_to_fetch)
{
    std::lock_guard lock(state_mutex);
    /// Can be called only before queue initialization
    assert(broken_parts_to_enqueue_fetches_on_loading.empty());
    assert(virtual_parts.size() == 0);
    broken_parts_to_enqueue_fetches_on_loading = std::move(parts_to_fetch);
}

void ReplicatedMergeTreeQueue::initialize(zkutil::ZooKeeperPtr zookeeper)
{
    clear();
    std::lock_guard lock(state_mutex);

    LOG_TRACE(log, "Initializing parts in queue");

    /// Get current parts state from zookeeper
    Strings parts = zookeeper->getChildren(replica_path + "/parts");
    for (const auto & part_name : parts)
    {
        LOG_TEST(log, "Adding part {} to current and virtual parts", part_name);
        current_parts.add(part_name, nullptr);
        virtual_parts.add(part_name, nullptr);
    }

    /// Drop parts can negatively affect virtual parts. So when we load parts
    /// from zookeeper we can break invariant with virtual parts. To fix this we
    /// have it here.
    for (const LogEntryPtr & entry : queue)
    {
        if (entry->isDropPart(format_version))
            virtual_parts.removePartAndCoveredParts(*entry->getDropRange(format_version));
    }

    LOG_TRACE(log, "Queue initialized");
}

bool ReplicatedMergeTreeQueue::isVirtualPart(const MergeTreeData::DataPartPtr & data_part) const
{
    std::lock_guard lock(state_mutex);
    auto virtual_part_name = virtual_parts.getContainingPart(data_part->info);
    return !virtual_part_name.empty() && virtual_part_name != data_part->name;
}

bool ReplicatedMergeTreeQueue::isGoingToBeDropped(const MergeTreePartInfo & part_info, MergeTreePartInfo * out_drop_range_info) const
{
    std::lock_guard lock(state_mutex);
    return isGoingToBeDroppedImpl(part_info, out_drop_range_info);
}

bool ReplicatedMergeTreeQueue::isGoingToBeDroppedImpl(const MergeTreePartInfo & part_info, MergeTreePartInfo * out_drop_range_info) const
{
    String covering_virtual = virtual_parts.getContainingPart(part_info);
    if (!covering_virtual.empty())
    {
        auto covering_virtual_info = MergeTreePartInfo::fromPartName(covering_virtual, format_version);
        if (covering_virtual_info.isFakeDropRangePart())
        {
            if (out_drop_range_info)
                *out_drop_range_info = covering_virtual_info;
            return true;
        }
    }
    return drop_parts.hasDropPart(part_info, out_drop_range_info);
}

bool ReplicatedMergeTreeQueue::checkPartInQueueAndGetSourceParts(const String & part_name, Strings & source_parts) const
{
    std::lock_guard lock(state_mutex);

    bool found = false;
    for (const auto & entry : queue)
    {
        if (entry->new_part_name == part_name && entry->source_parts.size() > source_parts.size())
        {
            source_parts.clear();
            source_parts.insert(source_parts.end(), entry->source_parts.begin(), entry->source_parts.end());
            found = true;
        }
    }

    return found;
}


bool ReplicatedMergeTreeQueue::load(zkutil::ZooKeeperPtr zookeeper)
{
    String queue_path = fs::path(replica_path) / "queue";
    LOG_DEBUG(log, "Loading queue from {}", queue_path);

    bool updated = false;
    std::optional<time_t> min_unprocessed_insert_time_changed;

    {
        std::lock_guard pull_logs_lock(pull_logs_to_queue_mutex);

        /// Reset batch size on initialization to recover from possible errors of too large batch size.
        current_multi_batch_size = 1;

        std::unordered_set<String> already_loaded_paths;
        {
            std::lock_guard lock(state_mutex);
            for (const LogEntryPtr & log_entry : queue)
                already_loaded_paths.insert(log_entry->znode_name);
        }

        Strings children = zookeeper->getChildren(queue_path);

        auto to_remove_it = std::remove_if(
            children.begin(), children.end(), [&](const String & path)
            {
                return already_loaded_paths.count(path);
            });

        LOG_DEBUG(log, "Having {} queue entries to load, {} entries already loaded.", (to_remove_it - children.begin()), (children.end() - to_remove_it));
        children.erase(to_remove_it, children.end());

        ::sort(children.begin(), children.end());

        auto children_num = children.size();
        std::vector<std::string> paths;
        paths.reserve(children_num);

        for (const String & child : children)
            paths.emplace_back(fs::path(queue_path) / child);

        auto results = zookeeper->get(paths);
        for (size_t i = 0; i < children_num; ++i)
        {
            auto res = results[i];
            LogEntryPtr entry = LogEntry::parse(res.data, res.stat, format_version);
            entry->znode_name = children[i];

            std::lock_guard lock(state_mutex);

            insertUnlocked(entry, min_unprocessed_insert_time_changed, lock);

            updated = true;
        }

        {  /// Mutation pointer is a part of "state" and must be updated with state mutex
            std::lock_guard lock(state_mutex);
            zookeeper->tryGet(fs::path(replica_path) / "mutation_pointer", mutation_pointer);
        }
    }

    updateTimesInZooKeeper(zookeeper, min_unprocessed_insert_time_changed, {});

    merge_strategy_picker.refreshState();

    LOG_TRACE(log, "Loaded queue");
    return updated;
}


void ReplicatedMergeTreeQueue::createLogEntriesToFetchBrokenParts()
{
    Strings broken_parts;
    {
        std::lock_guard lock(state_mutex);
        broken_parts = broken_parts_to_enqueue_fetches_on_loading;
    }

    /// It will lock state_mutex
    for (const auto & broken_part_name : broken_parts)
        storage.removePartAndEnqueueFetch(broken_part_name, /* storage_init = */true);

    std::lock_guard lock(state_mutex);
    /// broken_parts_to_enqueue_fetches_on_loading can be assigned only once on table startup,
    /// so actually no race conditions are possible
    assert(broken_parts == broken_parts_to_enqueue_fetches_on_loading);
    broken_parts_to_enqueue_fetches_on_loading.clear();
}


void ReplicatedMergeTreeQueue::insertUnlocked(
    const LogEntryPtr & entry, std::optional<time_t> & min_unprocessed_insert_time_changed,
    std::lock_guard<std::mutex> & state_lock)
{
    auto entry_virtual_parts = entry->getVirtualPartNames(format_version);

    LOG_TRACE(log, "Insert entry {} to queue with type {}", entry->znode_name, entry->getDescriptionForLogs(format_version));

    for (const String & virtual_part_name : entry_virtual_parts)
    {
        virtual_parts.add(virtual_part_name, nullptr);

        /// Don't add drop range parts to mutations
        /// they don't produce any useful parts
        /// Note: DROP_PART does not have virtual parts
        auto part_info = MergeTreePartInfo::fromPartName(virtual_part_name, format_version);
        if (part_info.isFakeDropRangePart())
            continue;

        addPartToMutations(virtual_part_name, part_info);
    }

    if (entry->type == LogEntry::DROP_PART)
    {
        /// DROP PART remove parts, so we remove it from virtual parts to
        /// preserve invariant virtual_parts = current_parts + queue.
        /// Also remove it from parts_to_do to avoid intersecting parts in parts_to_do
        /// if fast replica will execute DROP PART and assign a merge that contains dropped blocks.
        drop_parts.addDropPart(entry);
        String drop_part_name = *entry->getDropRange(format_version);
        virtual_parts.removePartAndCoveredParts(drop_part_name);
        removeCoveredPartsFromMutations(drop_part_name, /*remove_part = */ true, /*remove_covered_parts = */ true);
    }

    /// Put 'DROP PARTITION' entries at the beginning of the queue not to make superfluous fetches of parts that will be eventually deleted
    if (entry->getDropRange(format_version))
        queue.push_front(entry);
    else
        queue.push_back(entry);

    if (entry->type == LogEntry::GET_PART || entry->type == LogEntry::ATTACH_PART)
    {
        inserts_by_time.insert(entry);

        if (entry->create_time && (!min_unprocessed_insert_time || entry->create_time < min_unprocessed_insert_time))
        {
            min_unprocessed_insert_time.store(entry->create_time, std::memory_order_relaxed);
            min_unprocessed_insert_time_changed = min_unprocessed_insert_time;
        }
    }
    if (entry->type == LogEntry::ALTER_METADATA)
    {
        LOG_TRACE(log, "Adding alter metadata version {} to the queue", entry->alter_version);
        alter_sequence.addMetadataAlter(entry->alter_version, state_lock);
    }
}


void ReplicatedMergeTreeQueue::insert(zkutil::ZooKeeperPtr zookeeper, LogEntryPtr & entry)
{
    std::optional<time_t> min_unprocessed_insert_time_changed;

    {
        std::lock_guard lock(state_mutex);
        insertUnlocked(entry, min_unprocessed_insert_time_changed, lock);
    }

    updateTimesInZooKeeper(zookeeper, min_unprocessed_insert_time_changed, {});
}


void ReplicatedMergeTreeQueue::updateStateOnQueueEntryRemoval(
    const LogEntryPtr & entry,
    bool is_successful,
    std::optional<time_t> & min_unprocessed_insert_time_changed,
    std::optional<time_t> & max_processed_insert_time_changed,
    std::unique_lock<std::mutex> & state_lock)
{

    auto entry_virtual_parts = entry->getVirtualPartNames(format_version);
    LOG_TEST(log, "Removing {} entry {} from queue with type {}",
             is_successful ? "successful" : "unsuccessful",
             entry->znode_name, entry->getDescriptionForLogs(format_version));
    /// Update insert times.
    if (entry->type == LogEntry::GET_PART || entry->type == LogEntry::ATTACH_PART)
    {
        inserts_by_time.erase(entry);

        if (inserts_by_time.empty())
        {
            min_unprocessed_insert_time.store(0, std::memory_order_relaxed);
            min_unprocessed_insert_time_changed = min_unprocessed_insert_time;
        }
        else if ((*inserts_by_time.begin())->create_time > min_unprocessed_insert_time)
        {
            min_unprocessed_insert_time.store((*inserts_by_time.begin())->create_time, std::memory_order_relaxed);
            min_unprocessed_insert_time_changed = min_unprocessed_insert_time;
        }

        if (entry->create_time > max_processed_insert_time)
        {
            max_processed_insert_time.store(entry->create_time, std::memory_order_relaxed);
            max_processed_insert_time_changed = max_processed_insert_time;
        }
    }

    if (is_successful)
    {
        if (!entry->actual_new_part_name.empty())
        {
            LOG_TEST(log, "Entry {} has actual new part name {}, removing it from mutations", entry->znode_name, entry->actual_new_part_name);
            /// We don't add bigger fetched part to current_parts because we
            /// have an invariant `virtual_parts` = `current_parts` + `queue`.
            ///
            /// But we remove covered parts from mutations, because we actually
            /// have replacing part.
            ///
            /// NOTE actual_new_part_name is very confusing and error-prone. This approach must be fixed.
            removeCoveredPartsFromMutations(entry->actual_new_part_name, /*remove_part = */ false, /*remove_covered_parts = */ true);
        }

        LOG_TEST(log, "Adding parts [{}] to current parts", fmt::join(entry_virtual_parts, ", "));

        for (const String & virtual_part_name : entry_virtual_parts)
        {
            current_parts.add(virtual_part_name, nullptr);

            /// These parts are already covered by newer part, we don't have to
            /// mutate it.
            removeCoveredPartsFromMutations(virtual_part_name, /*remove_part = */ false, /*remove_covered_parts = */ true);
        }

        if (auto drop_range_part_name = entry->getDropRange(format_version))
        {
            if (entry->type == LogEntry::DROP_PART)
            {
                /// DROP PART doesn't have virtual parts so remove from current
                /// parts all covered parts.
                LOG_TEST(log, "Removing DROP_PART from current parts {}", *drop_range_part_name);
                current_parts.removePartAndCoveredParts(*drop_range_part_name);
                drop_parts.removeDropPart(entry);
            }
            else
            {
                LOG_TEST(log, "Removing DROP_RANGE from current and virtual parts {}", *drop_range_part_name);
                current_parts.remove(*drop_range_part_name);
                virtual_parts.remove(*drop_range_part_name);
            }

            /// NOTE: we don't need to remove part/covered parts from mutations (removeCoveredPartsFromMutations()) here because:
            /// - for DROP PART we have this during inserting to queue (see insertUnlocked())
            /// - for DROP PARTITION we have this in the loop above (when we adding parts to current_parts)
        }

        if (entry->type == LogEntry::ALTER_METADATA)
        {
            LOG_TRACE(log, "Finishing metadata alter with version {}", entry->alter_version);
            alter_sequence.finishMetadataAlter(entry->alter_version, state_lock);
        }
    }
    else
    {
        if (entry->type == LogEntry::DROP_PART)
        {
            drop_parts.removeDropPart(entry);
        }

        LOG_TEST(log, "Removing unsuccessful entry {} virtual parts [{}]", entry->znode_name, fmt::join(entry_virtual_parts, ", "));

        for (const String & virtual_part_name : entry_virtual_parts)
        {
            /// This part will never appear, so remove it from virtual parts
            virtual_parts.remove(virtual_part_name);

            /// Because execution of the entry is unsuccessful,
            /// `virtual_part_name` will never appear so we won't need to mutate
            /// it.
            removeCoveredPartsFromMutations(virtual_part_name, /*remove_part = */ true, /*remove_covered_parts = */ false);
        }
    }
}


void ReplicatedMergeTreeQueue::removeCoveredPartsFromMutations(const String & part_name, bool remove_part, bool remove_covered_parts)
{
    auto part_info = MergeTreePartInfo::fromPartName(part_name, format_version);

    LOG_TEST(log, "Removing part {} from mutations (remove_part: {}, remove_covered_parts: {})", part_name, remove_part, remove_covered_parts);

    auto in_partition = mutations_by_partition.find(part_info.partition_id);
    if (in_partition == mutations_by_partition.end())
        return;

    bool some_mutations_are_probably_done = false;

    for (auto & it : in_partition->second)
    {
        MutationStatus & status = *it.second;

        if (remove_part && remove_covered_parts)
            status.parts_to_do.removePartAndCoveredParts(part_name);
        else if (remove_covered_parts)
            status.parts_to_do.removePartsCoveredBy(part_name);
        else if (remove_part)
            status.parts_to_do.remove(part_name);
        else
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Called remove part from mutations, but nothing removed");

        if (status.parts_to_do.size() == 0)
            some_mutations_are_probably_done = true;

        if (!status.latest_failed_part.empty() && part_info.contains(status.latest_failed_part_info))
        {
            status.latest_failed_part.clear();
            status.latest_failed_part_info = MergeTreePartInfo();
            status.latest_fail_time = 0;
            status.latest_fail_reason.clear();
        }
    }

    if (some_mutations_are_probably_done)
        storage.mutations_finalizing_task->schedule();
}

void ReplicatedMergeTreeQueue::addPartToMutations(const String & part_name, const MergeTreePartInfo & part_info)
{
    LOG_TEST(log, "Adding part {} to mutations", part_name);
    assert(!part_info.isFakeDropRangePart());

    auto in_partition = mutations_by_partition.find(part_info.partition_id);
    if (in_partition == mutations_by_partition.end())
        return;

    auto from_it = in_partition->second.upper_bound(part_info.getDataVersion());
    for (auto it = from_it; it != in_partition->second.end(); ++it)
    {
        MutationStatus & status = *it->second;
        status.parts_to_do.add(part_name);
    }
}

void ReplicatedMergeTreeQueue::updateTimesInZooKeeper(
    zkutil::ZooKeeperPtr zookeeper,
    std::optional<time_t> min_unprocessed_insert_time_changed,
    std::optional<time_t> max_processed_insert_time_changed) const
{
    /// Here there can be a race condition (with different remove at the same time)
    ///  because we update times in ZooKeeper with unlocked mutex, while these times may change.
    /// Consider it unimportant (for a short time, ZK will have a slightly different time value).

    Coordination::Requests ops;

    if (min_unprocessed_insert_time_changed)
        ops.emplace_back(zkutil::makeSetRequest(
            replica_path + "/min_unprocessed_insert_time", toString(*min_unprocessed_insert_time_changed), -1));

    if (max_processed_insert_time_changed)
        ops.emplace_back(zkutil::makeSetRequest(
            replica_path + "/max_processed_insert_time", toString(*max_processed_insert_time_changed), -1));

    if (!ops.empty())
    {
        Coordination::Responses responses;
        auto code = zookeeper->tryMulti(ops, responses);

        if (code != Coordination::Error::ZOK)
            LOG_ERROR(log, "Couldn't set value of nodes for insert times "
                           "({}/min_unprocessed_insert_time, max_processed_insert_time): {}. "
                           "This shouldn't happen often.", replica_path, Coordination::errorMessage(code));
    }
}


void ReplicatedMergeTreeQueue::removeProcessedEntry(zkutil::ZooKeeperPtr zookeeper, LogEntryPtr & entry)
{
    std::optional<time_t> min_unprocessed_insert_time_changed;
    std::optional<time_t> max_processed_insert_time_changed;

    bool found = false;
    bool need_remove_from_zk = true;
    size_t queue_size = 0;

    /// First remove from memory then from ZooKeeper
    {
        std::unique_lock lock(state_mutex);
        if (entry->removed_by_other_entry)
        {
            need_remove_from_zk = false;
            queue_size = queue.size();
        }
        else
        {
            /// Remove the job from the queue in the RAM.
            /// You can not just refer to a pre-saved iterator, because someone else might be able to delete the task.
            /// Why do we view the queue from the end?
            ///  - because the task for execution first is moved to the end of the queue, so that in case of failure it remains at the end.
            for (Queue::iterator it = queue.end(); it != queue.begin();)
            {
                --it;

                if (*it == entry)
                {
                    found = true;
                    updateStateOnQueueEntryRemoval(
                            entry, /* is_successful = */ true,
                            min_unprocessed_insert_time_changed, max_processed_insert_time_changed, lock);

                    queue.erase(it);
                    queue_size = queue.size();
                    break;
                }
            }
        }
    }

    if (!found && need_remove_from_zk)
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Can't find {} in the memory queue. It is a bug. Entry: {}",
                                                      entry->znode_name, entry->toString());
    notifySubscribers(queue_size, &(entry->znode_name));

    if (!need_remove_from_zk)
        return;

    auto code = zookeeper->tryRemove(fs::path(replica_path) / "queue" / entry->znode_name);
    if (code != Coordination::Error::ZOK)
        LOG_ERROR(log, "Couldn't remove {}/queue/{}: {}. This shouldn't happen often.", replica_path, entry->znode_name, Coordination::errorMessage(code));

    updateTimesInZooKeeper(zookeeper, min_unprocessed_insert_time_changed, max_processed_insert_time_changed);
}

bool ReplicatedMergeTreeQueue::removeFailedQuorumPart(const MergeTreePartInfo & part_info)
{
    assert(part_info.level == 0);
    std::lock_guard lock(state_mutex);
    return virtual_parts.remove(part_info);
}

int32_t ReplicatedMergeTreeQueue::pullLogsToQueue(zkutil::ZooKeeperPtr zookeeper, Coordination::WatchCallback watch_callback, PullLogsReason reason)
{
    std::lock_guard lock(pull_logs_to_queue_mutex);

    if (reason != LOAD)
    {
        /// It's totally ok to load queue on readonly replica (that's what RestartingThread does on initialization).
        /// It's ok if replica became readonly due to connection loss after we got current zookeeper (in this case zookeeper must be expired).
        /// And it's ok if replica became readonly after shutdown.
        /// In other cases it's likely that someone called pullLogsToQueue(...) when queue is not initialized yet by RestartingThread.
        bool not_completely_initialized = storage.is_readonly && !zookeeper->expired() && !storage.shutdown_called;
        if (not_completely_initialized)
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Tried to pull logs to queue (reason: {}) on readonly replica {}, it's a bug",
                            reason, storage.getStorageID().getNameForLogs());
    }

    if (pull_log_blocker.isCancelled())
        throw Exception(ErrorCodes::ABORTED, "Log pulling is cancelled");

    String index_str = zookeeper->get(fs::path(replica_path) / "log_pointer");
    UInt64 index;

    /// The version of "/log" is modified when new entries to merge/mutate/drop appear.
    Coordination::Stat stat;
    zookeeper->get(fs::path(zookeeper_path) / "log", &stat);

    Strings log_entries = zookeeper->getChildrenWatch(fs::path(zookeeper_path) / "log", nullptr, watch_callback);

    /// We update mutations after we have loaded the list of log entries, but before we insert them
    /// in the queue.
    /// With this we ensure that if you read the log state L1 and then the state of mutations M1,
    /// then L1 "happened-before" M1.
    updateMutations(zookeeper);

    if (index_str.empty())
    {
        /// If we do not already have a pointer to the log, put a pointer to the first entry in it.
        index = log_entries.empty() ? 0 : parse<UInt64>(std::min_element(log_entries.begin(), log_entries.end())->substr(strlen("log-")));

        zookeeper->set(fs::path(replica_path) / "log_pointer", toString(index));
    }
    else
    {
        index = parse<UInt64>(index_str);
    }

    String min_log_entry = "log-" + padIndex(index);

    /// Multiple log entries that must be copied to the queue.

    std::erase_if(log_entries, [&min_log_entry](const String & entry) { return entry < min_log_entry; });

    if (!log_entries.empty())
    {
        ::sort(log_entries.begin(), log_entries.end());

        for (size_t entry_idx = 0, num_entries = log_entries.size(); entry_idx < num_entries;)
        {
            auto begin = log_entries.begin() + entry_idx;
            auto end = entry_idx + current_multi_batch_size >= log_entries.size()
                ? log_entries.end()
                : (begin + current_multi_batch_size);
            auto last = end - 1;

            /// Increment entry_idx before batch size increase (we copied at most current_multi_batch_size entries)
            entry_idx += current_multi_batch_size;

            /// Increase the batch size exponentially, so it will saturate to MAX_MULTI_OPS.
            if (current_multi_batch_size < MAX_MULTI_OPS)
                current_multi_batch_size = std::min<size_t>(MAX_MULTI_OPS, current_multi_batch_size * 2);

            String last_entry = *last;
            if (!startsWith(last_entry, "log-"))
                throw Exception(ErrorCodes::UNEXPECTED_NODE_IN_ZOOKEEPER, "Error in zookeeper data: unexpected node {} in {}/log",
                    last_entry, zookeeper_path);

            UInt64 last_entry_index = parse<UInt64>(last_entry.substr(strlen("log-")));

            LOG_DEBUG(log, "Pulling {} entries to queue: {} - {}", (end - begin), *begin, *last);

            Strings get_paths;
            get_paths.reserve(end - begin);

            for (auto it = begin; it != end; ++it)
                get_paths.emplace_back(fs::path(zookeeper_path) / "log" / *it);

            /// Simultaneously add all new entries to the queue and move the pointer to the log.

            Coordination::Requests ops;
            std::vector<LogEntryPtr> copied_entries;
            copied_entries.reserve(end - begin);

            std::optional<time_t> min_unprocessed_insert_time_changed;

            auto get_results = zookeeper->get(get_paths);
            auto get_num = get_results.size();
            for (size_t i = 0; i < get_num; ++i)
            {
                auto res = get_results[i];

                copied_entries.emplace_back(LogEntry::parse(res.data, res.stat, format_version));

                ops.emplace_back(zkutil::makeCreateRequest(
                    fs::path(replica_path) / "queue/queue-", res.data, zkutil::CreateMode::PersistentSequential));

                const auto & entry = *copied_entries.back();
                if (entry.type == LogEntry::GET_PART || entry.type == LogEntry::ATTACH_PART)
                {
                    std::lock_guard state_lock(state_mutex);
                    if (entry.create_time && (!min_unprocessed_insert_time || entry.create_time < min_unprocessed_insert_time))
                    {
                        min_unprocessed_insert_time.store(entry.create_time, std::memory_order_relaxed);
                        min_unprocessed_insert_time_changed = min_unprocessed_insert_time;
                    }
                }
            }

            ops.emplace_back(zkutil::makeSetRequest(
                fs::path(replica_path) / "log_pointer", toString(last_entry_index + 1), -1));

            if (min_unprocessed_insert_time_changed)
                ops.emplace_back(zkutil::makeSetRequest(
                    fs::path(replica_path) / "min_unprocessed_insert_time", toString(*min_unprocessed_insert_time_changed), -1));

            auto responses = zookeeper->multi(ops);

            /// Now we have successfully updated the queue in ZooKeeper. Update it in RAM.

            try
            {
                std::lock_guard state_lock(state_mutex);

                for (size_t copied_entry_idx = 0, num_copied_entries = copied_entries.size(); copied_entry_idx < num_copied_entries; ++copied_entry_idx)
                {
                    String path_created = dynamic_cast<const Coordination::CreateResponse &>(*responses[copied_entry_idx]).path_created;
                    copied_entries[copied_entry_idx]->znode_name = path_created.substr(path_created.find_last_of('/') + 1);

                    std::optional<time_t> unused = false;
                    insertUnlocked(copied_entries[copied_entry_idx], unused, state_lock);
                }

                last_queue_update = time(nullptr);
            }
            catch (...)
            {
                tryLogCurrentException(log);
                /// If it fails, the data in RAM is incorrect. In order to avoid possible further corruption of data in ZK, we will kill ourselves.
                /// This is possible only if there is an unknown logical error.
                std::terminate();
            }

            if (!copied_entries.empty())
            {
                LOG_DEBUG(log, "Pulled {} entries to queue.", copied_entries.size());
                merge_strategy_picker.refreshState();
            }
        }

        storage.background_operations_assignee.trigger();
    }

    return stat.version;
}


namespace
{


/// Simplified representation of queue entry. Contain two sets
/// 1) Which parts we will receive after entry execution
/// 2) Which parts we will drop/remove after entry execution
///
/// We use this representation to understand which parts mutation actually have to mutate.
struct QueueEntryRepresentation
{
    std::vector<std::string> produced_parts;
    std::vector<std::string> dropped_parts;
};

using QueueRepresentation = std::map<std::string, QueueEntryRepresentation>;

/// Produce a map from queue znode name to simplified entry representation.
QueueRepresentation getQueueRepresentation(const std::list<ReplicatedMergeTreeLogEntryPtr> & entries, MergeTreeDataFormatVersion format_version)
{
    using LogEntryType = ReplicatedMergeTreeLogEntryData::Type;
    QueueRepresentation result;
    for (const auto & entry : entries)
    {
        const auto & key = entry->znode_name;
        switch (entry->type)
        {
            /// explicetely specify all types of entries without default, so if
            /// someone decide to add new type it will produce a compiler warning (error in our case)
            case LogEntryType::GET_PART:
            case LogEntryType::ATTACH_PART:
            case LogEntryType::MERGE_PARTS:
            case LogEntryType::MUTATE_PART:
            {
                result[key].produced_parts.push_back(entry->new_part_name);
                break;
            }
            case LogEntryType::REPLACE_RANGE:
            {
                /// Quite tricky entry, it both produce and drop parts (in some cases)
                const auto & new_parts = entry->replace_range_entry->new_part_names;
                auto & produced_parts = result[key].produced_parts;
                produced_parts.insert(
                    produced_parts.end(), new_parts.begin(), new_parts.end());

                if (auto drop_range = entry->getDropRange(format_version))
                {
                    auto & dropped_parts = result[key].dropped_parts;
                    dropped_parts.push_back(*drop_range);
                }
                break;
            }
            case LogEntryType::DROP_RANGE:
            case LogEntryType::DROP_PART:
            {
                result[key].dropped_parts.push_back(entry->new_part_name);
                break;
            }
            /// These entries don't produce/drop any parts
            case LogEntryType::EMPTY:
            case LogEntryType::ALTER_METADATA:
            case LogEntryType::CLEAR_INDEX:
            case LogEntryType::CLEAR_COLUMN:
            case LogEntryType::SYNC_PINNED_PART_UUIDS:
            case LogEntryType::CLONE_PART_FROM_SHARD:
            {
                break;
            }
        }
    }
    return result;
}

/// Try to understand which part we need to mutate to finish mutation. In ReplicatedQueue we have two sets of parts:
/// current parts -- set of parts which we actually have (on disk)
/// virtual parts -- set of parts which we will have after we will execute our queue
///
/// From the first glance it can sound that these two sets should be enough to understand which parts we have to mutate
/// to finish mutation but it's not true:
/// 1) Obviously we cannot rely on current_parts because we can have stale state (some parts are absent, some merges not finished). We also have to account parts which we will
///    get after queue execution.
/// 2) But we cannot rely on virtual_parts for this, because they contain parts which we will get after we have executed our queue. So if we need to execute mutation 0000000001 for part all_0_0_0
///    and we have already pulled entry to mutate this part into own queue our virtual parts will contain part all_0_0_0_1, not part all_0_0_0.
///
/// To avoid such issues we simply traverse all entries in queue in order and applying diff (add parts/remove parts) to current parts if they could be affected by mutation. Such approach is expensive
/// but we do it only once since we get the mutation. After that we just update parts_to_do for each mutation when pulling entries into our queue (addPartToMutations, removePartFromMutations).
ActiveDataPartSet getPartNamesToMutate(
    const ReplicatedMergeTreeMutationEntry & mutation, const ActiveDataPartSet & current_parts,
    const QueueRepresentation & queue_representation, MergeTreeDataFormatVersion format_version)
{
    ActiveDataPartSet result(format_version);
    /// Traverse mutation by partition
    for (const auto & [partition_id, block_num] : mutation.block_numbers)
    {
        /// Note that we cannot simply count all parts to mutate using getPartsCoveredBy(appropriate part_info)
        /// because they are not consecutive in `parts`.
        MergeTreePartInfo covering_part_info(
            partition_id, 0, block_num, MergeTreePartInfo::MAX_LEVEL, MergeTreePartInfo::MAX_BLOCK_NUMBER);

        /// First of all add all affected current_parts
        for (const String & covered_part_name : current_parts.getPartsCoveredBy(covering_part_info))
        {
            auto part_info = MergeTreePartInfo::fromPartName(covered_part_name, current_parts.getFormatVersion());
            if (part_info.getDataVersion() < block_num)
                result.add(covered_part_name);
        }

        /// Traverse queue and update affected current_parts
        for (const auto & [_, entry_representation] : queue_representation)
        {
            /// First we have to drop something if entry drop parts
            for (const auto & part_to_drop : entry_representation.dropped_parts)
            {
                auto part_to_drop_info = MergeTreePartInfo::fromPartName(part_to_drop, format_version);
                if (part_to_drop_info.partition_id == partition_id)
                    result.removePartAndCoveredParts(part_to_drop);
            }

            /// After we have to add parts if entry adds them
            for (const auto & part_to_add : entry_representation.produced_parts)
            {
                auto part_to_add_info = MergeTreePartInfo::fromPartName(part_to_add, format_version);
                if (part_to_add_info.partition_id == partition_id && part_to_add_info.getDataVersion() < block_num)
                    result.add(part_to_add);
            }
        }
    }

    return result;
}

}

void ReplicatedMergeTreeQueue::updateMutations(zkutil::ZooKeeperPtr zookeeper, Coordination::WatchCallback watch_callback)
{
    std::lock_guard lock(update_mutations_mutex);

    Strings entries_in_zk = zookeeper->getChildrenWatch(fs::path(zookeeper_path) / "mutations", nullptr, watch_callback);
    StringSet entries_in_zk_set(entries_in_zk.begin(), entries_in_zk.end());

    /// Compare with the local state, delete obsolete entries and determine which new entries to load.
    Strings entries_to_load;
    bool some_active_mutations_were_killed = false;
    {
        std::lock_guard state_lock(state_mutex);

        for (auto it = mutations_by_znode.begin(); it != mutations_by_znode.end();)
        {
            const ReplicatedMergeTreeMutationEntry & entry = *it->second.entry;
            if (!entries_in_zk_set.contains(entry.znode_name))
            {
                if (!it->second.is_done)
                {
                    LOG_DEBUG(log, "Removing killed mutation {} from local state.", entry.znode_name);
                    some_active_mutations_were_killed = true;
                    if (entry.isAlterMutation())
                    {
                        LOG_DEBUG(log, "Removed alter {} because mutation {} were killed.", entry.alter_version, entry.znode_name);
                        alter_sequence.finishDataAlter(entry.alter_version, state_lock);
                    }
                }
                else
                    LOG_DEBUG(log, "Removing obsolete mutation {} from local state.", entry.znode_name);

                for (const auto & partition_and_block_num : entry.block_numbers)
                {
                    auto & in_partition = mutations_by_partition[partition_and_block_num.first];
                    in_partition.erase(partition_and_block_num.second);
                    if (in_partition.empty())
                        mutations_by_partition.erase(partition_and_block_num.first);
                }

                it = mutations_by_znode.erase(it);
            }
            else
                ++it;
        }

        for (const String & znode : entries_in_zk_set)
        {
            if (!mutations_by_znode.contains(znode))
                entries_to_load.push_back(znode);
        }
    }

    if (some_active_mutations_were_killed)
        storage.background_operations_assignee.trigger();

    if (!entries_to_load.empty())
    {
        LOG_INFO(log, "Loading {} mutation entries: {} - {}", toString(entries_to_load.size()), entries_to_load.front(), entries_to_load.back());

        std::vector<std::future<Coordination::GetResponse>> futures;
        for (const String & entry : entries_to_load)
            futures.emplace_back(zookeeper->asyncTryGet(fs::path(zookeeper_path) / "mutations" / entry));

        std::vector<ReplicatedMergeTreeMutationEntryPtr> new_mutations;
        for (size_t i = 0; i < entries_to_load.size(); ++i)
        {
            auto maybe_response = futures[i].get();
            if (maybe_response.error != Coordination::Error::ZOK)
            {
                assert(maybe_response.error == Coordination::Error::ZNONODE);
                /// It's ok if it happened on server startup or table creation and replica loads all mutation entries.
                /// It's also ok if mutation was killed.
                LOG_WARNING(log, "Cannot get mutation node {} ({}), probably it was concurrently removed", entries_to_load[i], maybe_response.error);
                continue;
            }
            new_mutations.push_back(std::make_shared<ReplicatedMergeTreeMutationEntry>(
                ReplicatedMergeTreeMutationEntry::parse(maybe_response.data, entries_to_load[i])));
        }

        bool some_mutations_are_probably_done = false;
        {
            std::lock_guard state_lock(state_mutex);

            for (const ReplicatedMergeTreeMutationEntryPtr & entry : new_mutations)
            {
                auto & mutation = mutations_by_znode.emplace(entry->znode_name, MutationStatus(entry, format_version))
                    .first->second;

                for (const auto & pair : entry->block_numbers)
                {
                    const String & partition_id = pair.first;
                    Int64 block_num = pair.second;
                    mutations_by_partition[partition_id].emplace(block_num, &mutation);
                    LOG_TRACE(log, "Adding mutation {} for partition {} for all block numbers less than {}", entry->znode_name, partition_id, block_num);
                }

                /// Initialize `mutation.parts_to_do`. We cannot use only current_parts + virtual_parts here so we
                /// traverse all the queue and build correct state of parts_to_do.
                auto queue_representation = getQueueRepresentation(queue, format_version);
                mutation.parts_to_do = getPartNamesToMutate(*entry, virtual_parts, queue_representation, format_version);

                if (mutation.parts_to_do.size() == 0)
                    some_mutations_are_probably_done = true;

                /// otherwise it's already done
                if (entry->isAlterMutation() && entry->znode_name > mutation_pointer)
                {
                    LOG_TRACE(log, "Adding mutation {} with alter version {} to the queue", entry->znode_name, entry->alter_version);
                    alter_sequence.addMutationForAlter(entry->alter_version, state_lock);
                }
            }
        }

        storage.merge_selecting_task->schedule();

        if (some_mutations_are_probably_done)
            storage.mutations_finalizing_task->schedule();
    }
}


ReplicatedMergeTreeMutationEntryPtr ReplicatedMergeTreeQueue::removeMutation(
    zkutil::ZooKeeperPtr zookeeper, const String & mutation_id)
{
    std::lock_guard lock(update_mutations_mutex);

    auto rc = zookeeper->tryRemove(fs::path(zookeeper_path) / "mutations" / mutation_id);
    if (rc == Coordination::Error::ZOK)
        LOG_DEBUG(log, "Removed mutation {} from ZooKeeper.", mutation_id);

    ReplicatedMergeTreeMutationEntryPtr entry;
    bool mutation_was_active = false;
    {
        std::lock_guard state_lock(state_mutex);

        auto it = mutations_by_znode.find(mutation_id);
        if (it == mutations_by_znode.end())
            return nullptr;

        mutation_was_active = !it->second.is_done;

        entry = it->second.entry;
        for (const auto & partition_and_block_num : entry->block_numbers)
        {
            auto & in_partition = mutations_by_partition[partition_and_block_num.first];
            in_partition.erase(partition_and_block_num.second);
            if (in_partition.empty())
                mutations_by_partition.erase(partition_and_block_num.first);
        }

        if (entry->isAlterMutation())
        {
            LOG_DEBUG(log, "Removed alter {} because mutation {} were killed.", entry->alter_version, entry->znode_name);
            alter_sequence.finishDataAlter(entry->alter_version, state_lock);
        }

        mutations_by_znode.erase(it);
        LOG_DEBUG(log, "Removed mutation {} from local state.", entry->znode_name);
    }

    if (mutation_was_active)
        storage.background_operations_assignee.trigger();

    return entry;
}


ReplicatedMergeTreeQueue::StringSet ReplicatedMergeTreeQueue::moveSiblingPartsForMergeToEndOfQueue(const String & part_name)
{
    std::lock_guard lock(state_mutex);

    /// Let's find the action to merge this part with others. Let's remember others.
    StringSet parts_for_merge;
    Queue::iterator merge_entry = queue.end();
    for (Queue::iterator it = queue.begin(); it != queue.end(); ++it)
    {
        if ((*it)->type == LogEntry::MERGE_PARTS || (*it)->type == LogEntry::MUTATE_PART)
        {
            if (std::find((*it)->source_parts.begin(), (*it)->source_parts.end(), part_name)
                != (*it)->source_parts.end())
            {
                parts_for_merge = StringSet((*it)->source_parts.begin(), (*it)->source_parts.end());
                merge_entry = it;
                break;
            }
        }
    }

    if (!parts_for_merge.empty())
    {
        /// Move to the end of queue actions that result in one of the parts in `parts_for_merge`.
        for (Queue::iterator it = queue.begin(); it != queue.end();)
        {
            auto it0 = it;
            ++it;

            if (it0 == merge_entry)
                break;

            const auto t = (*it0)->type;

            if ((t == LogEntry::MERGE_PARTS ||
                 t == LogEntry::GET_PART  ||
                 t == LogEntry::ATTACH_PART ||
                 t == LogEntry::MUTATE_PART)
                && parts_for_merge.contains((*it0)->new_part_name))
            {
                queue.splice(queue.end(), queue, it0, it);
            }
        }
    }

    return parts_for_merge;
}

bool ReplicatedMergeTreeQueue::checkReplaceRangeCanBeRemoved(const MergeTreePartInfo & part_info, LogEntryPtr entry_ptr, const ReplicatedMergeTreeLogEntryData & current) const
{
    if (entry_ptr->type != LogEntry::REPLACE_RANGE)
        return false;
    assert(entry_ptr->replace_range_entry);

    if (current.type != LogEntry::REPLACE_RANGE && current.type != LogEntry::DROP_RANGE && current.type != LogEntry::DROP_PART)
        return false;

    if (entry_ptr->replace_range_entry == current.replace_range_entry) /// same partition, don't want to drop ourselves
        return false;

    if (!part_info.contains(MergeTreePartInfo::fromPartName(entry_ptr->replace_range_entry->drop_range_part_name, format_version)))
        return false;

    size_t number_of_covered_parts = 0;
    for (const String & new_part_name : entry_ptr->replace_range_entry->new_part_names)
    {
        if (part_info.contains(MergeTreePartInfo::fromPartName(new_part_name, format_version)))
            ++number_of_covered_parts;
    }

    /// It must either cover all new parts from REPLACE_RANGE or no one. Otherwise it's a bug in replication,
    /// which may lead to intersecting entries.
    assert(number_of_covered_parts == 0 || number_of_covered_parts == entry_ptr->replace_range_entry->new_part_names.size());
    return number_of_covered_parts == entry_ptr->replace_range_entry->new_part_names.size();
}

void ReplicatedMergeTreeQueue::removePartProducingOpsInRange(
    zkutil::ZooKeeperPtr zookeeper,
    const MergeTreePartInfo & part_info,
    const std::optional<ReplicatedMergeTreeLogEntryData> & covering_entry)
{
    /// TODO is it possible to simplify it?
    Queue to_wait;
    size_t removed_entries = 0;
    std::optional<time_t> min_unprocessed_insert_time_changed;
    std::optional<time_t> max_processed_insert_time_changed;

    /// Remove operations with parts, contained in the range to be deleted, from the queue.
    std::unique_lock lock(state_mutex);

    [[maybe_unused]] bool called_from_alter_query_directly = covering_entry && covering_entry->replace_range_entry
        && covering_entry->replace_range_entry->columns_version < 0;
    [[maybe_unused]] bool called_for_broken_part = !covering_entry;
    assert(currently_executing_drop_replace_ranges.contains(part_info) || called_from_alter_query_directly || called_for_broken_part);

    for (Queue::iterator it = queue.begin(); it != queue.end();)
    {
        auto type = (*it)->type;
        bool is_simple_producing_op = type == LogEntry::GET_PART ||
                                      type == LogEntry::ATTACH_PART ||
                                      type == LogEntry::MERGE_PARTS ||
                                      type == LogEntry::MUTATE_PART;
        bool simple_op_covered = is_simple_producing_op && part_info.contains(MergeTreePartInfo::fromPartName((*it)->new_part_name, format_version));
        bool replace_range_covered = covering_entry && checkReplaceRangeCanBeRemoved(part_info, *it, *covering_entry);
        if (simple_op_covered || replace_range_covered)
        {
            const String & znode_name = (*it)->znode_name;

            if ((*it)->currently_executing)
                to_wait.push_back(*it);

            auto code = zookeeper->tryRemove(fs::path(replica_path) / "queue" / znode_name);
            if (code != Coordination::Error::ZOK)
                LOG_INFO(log, "Couldn't remove {}: {}", (fs::path(replica_path) / "queue" / znode_name).string(), Coordination::errorMessage(code));

            updateStateOnQueueEntryRemoval(
                *it, /* is_successful = */ false,
                min_unprocessed_insert_time_changed, max_processed_insert_time_changed, lock);

            LogEntryPtr removing_entry = std::move(*it);   /// Make it live a bit longer
            removing_entry->removed_by_other_entry = true;
            it = queue.erase(it);
            notifySubscribers(queue.size(), &znode_name);
            ++removed_entries;
        }
        else
            ++it;
    }

    updateTimesInZooKeeper(zookeeper, min_unprocessed_insert_time_changed, max_processed_insert_time_changed);

    LOG_DEBUG(log, "Removed {} entries from queue. Waiting for {} entries that are currently executing.", removed_entries, to_wait.size());

    /// Let's wait for the operations with the parts contained in the range to be deleted.
    for (LogEntryPtr & entry : to_wait)
        entry->execution_complete.wait(lock, [&entry] { return !entry->currently_executing; });
}


bool ReplicatedMergeTreeQueue::isCoveredByFuturePartsImpl(const LogEntry & entry, const String & new_part_name,
                                                          String & out_reason, std::unique_lock<std::mutex> & /* queue_lock */,
                                                          std::vector<LogEntryPtr> * covered_entries_to_wait) const
{
    /// Let's check if the same part is now being created by another action.
    auto entry_for_same_part_it = future_parts.find(new_part_name);
    if (entry_for_same_part_it != future_parts.end())
    {
        const LogEntry & another_entry = *entry_for_same_part_it->second;
        constexpr auto fmt_string = "Not executing log entry {} of type {} for part {} "
                                    "because another log entry {} of type {} for the same part ({}) is being processed.";
        LOG_INFO(LogToStr(out_reason, log), fmt_string, entry.znode_name, entry.type, entry.new_part_name,
                 another_entry.znode_name, another_entry.type, another_entry.new_part_name);
        return true;

        /** When the corresponding action is completed, then `isNotCoveredByFuturePart` next time, will succeed,
            *  and queue element will be processed.
            * Immediately in the `executeLogEntry` function it will be found that we already have a part,
            *  and queue element will be immediately treated as processed.
            */
    }

    /// A more complex check is whether another part is currently created by other action that will cover this part.
    /// NOTE The above is redundant, but left for a more convenient message in the log.
    auto result_part = MergeTreePartInfo::fromPartName(new_part_name, format_version);

    /// It can slow down when the size of `future_parts` is large. But it can not be large, since background pool is limited.
    for (const auto & future_part_elem : future_parts)
    {
        auto future_part = MergeTreePartInfo::fromPartName(future_part_elem.first, format_version);

        if (future_part.isDisjoint(result_part))
            continue;

        /// Parts are not disjoint. They can be even intersecting and it's not a problem,
        /// because we may have two queue entries producing intersecting parts if there's DROP_RANGE between them (so virtual_parts are ok).

        /// Give priority to DROP_RANGEs and allow processing them even if covered entries are currently executing.
        /// DROP_RANGE will cancel covered operations and will wait for them in removePartProducingOpsInRange.
        if (result_part.isFakeDropRangePart() && result_part.contains(future_part))
            continue;

        /// In other cases we cannot execute `entry` (or upgrade its actual_part_name to `new_part_name`)
        /// while any covered or covering parts are processed.
        /// But we also cannot simply return true and postpone entry processing, because it may lead to kind of livelock.
        /// Since queue is processed in multiple threads, it's likely that there will be at least one thread
        /// executing faulty entry for some small part, so bigger covering part will never be processed.
        /// That's why it's better to wait for covered entry to be executed (does not matter successfully or not)
        /// instead of exiting and postponing covering entry.

        if (covered_entries_to_wait)
        {
            if (entry.znode_name < future_part_elem.second->znode_name)
            {
                constexpr auto fmt_string = "Not executing log entry {} for part {} "
                                            "because it is not disjoint with part {} that is currently executing and another entry {} is newer.";
                LOG_TRACE(LogToStr(out_reason, log), fmt_string, entry.znode_name, new_part_name, future_part_elem.first, future_part_elem.second->znode_name);
                return true;
            }

            covered_entries_to_wait->push_back(future_part_elem.second);
            continue;
        }

        constexpr auto fmt_string = "Not executing log entry {} for part {} "
                                    "because it is not disjoint with part {} that is currently executing.";

        /// This message can be too noisy, do not print it more than once per second
        LOG_TEST(LogToStr(out_reason, LogFrequencyLimiter(log, 5)), fmt_string, entry.znode_name, new_part_name, future_part_elem.first);
        return true;
    }

    return false;
}

bool ReplicatedMergeTreeQueue::addFuturePartIfNotCoveredByThem(const String & part_name, LogEntry & entry, String & reject_reason)
{
    /// We have found `part_name` on some replica and are going to fetch it instead of covered `entry->new_part_name`.
    std::unique_lock lock(state_mutex);

    String covering_part = virtual_parts.getContainingPart(part_name);
    if (covering_part.empty())
    {
        /// We should not fetch any parts that absent in our `virtual_parts` set,
        /// because we do not know about such parts according to our replication queue (we know about them from some side-channel).
        /// Otherwise, it may break invariants in replication queue reordering, for example:
        /// 1. Our queue contains GET_PART all_2_2_0, log contains DROP_RANGE all_2_2_0 and MERGE_PARTS all_1_3_1
        /// 2. We execute GET_PART all_2_2_0, but fetch all_1_3_1 instead
        ///    (drop_parts.isAffectedByDropPart(...) is false-negative, because DROP_RANGE all_2_2_0 is not pulled yet).
        ///    It actually means, that MERGE_PARTS all_1_3_1 is executed too, but it's not even pulled yet.
        /// 3. Then we pull log, trying to execute DROP_RANGE all_2_2_0
        ///    and reveal that it was incorrectly reordered with MERGE_PARTS all_1_3_1 (drop range intersects merged part).
        reject_reason = fmt::format("Log entry for part {} or covering part is not pulled from log to queue yet.", part_name);
        return false;
    }

    /// FIXME get rid of actual_part_name.
    /// If new covering part jumps over non-disjoint DROP_PART we should execute DROP_PART first to avoid intersection
    if (drop_parts.isAffectedByDropPart(part_name, reject_reason))
        return false;

    std::vector<LogEntryPtr> covered_entries_to_wait;
    if (isCoveredByFuturePartsImpl(entry, part_name, reject_reason, lock, &covered_entries_to_wait))
        return false;

    CurrentlyExecuting::setActualPartName(entry, part_name, *this, lock, covered_entries_to_wait);
    return true;

}


bool ReplicatedMergeTreeQueue::shouldExecuteLogEntry(
    const LogEntry & entry,
    String & out_postpone_reason,
    MergeTreeDataMergerMutator & merger_mutator,
    MergeTreeData & data,
    std::unique_lock<std::mutex> & state_lock) const
{
    /// If our entry produce part which is already covered by
    /// some other entry which is currently executing, then we can postpone this entry.
    for (const String & new_part_name : entry.getVirtualPartNames(format_version))
    {
        /// Do not wait for any entries here, because we have only one thread that scheduling queue entries.
        /// We can wait in worker threads, but not in scheduler.
        if (isCoveredByFuturePartsImpl(entry, new_part_name, out_postpone_reason, state_lock, /* covered_entries_to_wait */ nullptr))
            return false;
    }

    if (entry.type != LogEntry::DROP_RANGE && entry.type != LogEntry::DROP_PART)
    {
        /// Do not touch any entries that are not disjoint with some DROP_PART to avoid intersecting parts
        if (drop_parts.isAffectedByDropPart(entry, out_postpone_reason))
            return false;
    }

    /// Optimization: it does not really make sense to generate parts that are going to be dropped anyway
    if (!entry.new_part_name.empty())
    {
        auto new_part_info = MergeTreePartInfo::fromPartName(entry.new_part_name, format_version);
        MergeTreePartInfo drop_info;
        if (entry.type != LogEntry::DROP_PART && !new_part_info.isFakeDropRangePart() && isGoingToBeDroppedImpl(new_part_info, &drop_info))
        {
            out_postpone_reason = fmt::format(
                "Not executing {} because it produces part {} that is going to be dropped by {}",
                entry.znode_name, entry.new_part_name, drop_info.getPartNameForLogs());
            return false;
        }
    }

    /// Check that fetches pool is not overloaded
    if ((entry.type == LogEntry::GET_PART || entry.type == LogEntry::ATTACH_PART)
        && !storage.canExecuteFetch(entry, out_postpone_reason))
    {
        /// Don't print log message about this, because we can have a lot of fetches,
        /// for example during replica recovery.
        return false;
    }

    if (entry.type == LogEntry::MERGE_PARTS || entry.type == LogEntry::MUTATE_PART)
    {
        /** If any of the required parts are now fetched or in merge process, wait for the end of this operation.
          * Otherwise, even if all the necessary parts for the merge are not present, you should try to make a merge.
          * If any parts are missing, instead of merge, there will be an attempt to download a part.
          * Such a situation is possible if the receive of a part has failed, and it was moved to the end of the queue.
          */
        size_t sum_parts_size_in_bytes = 0;
        for (const auto & name : entry.source_parts)
        {
            if (future_parts.contains(name))
            {
                constexpr auto fmt_string = "Not executing log entry {} of type {} for part {} "
                      "because part {} is not ready yet (log entry for that part is being processed).";
                LOG_TRACE(LogToStr(out_postpone_reason, log), fmt_string, entry.znode_name, entry.typeToString(), entry.new_part_name, name);
                return false;
            }

            auto part = data.getPartIfExists(name, {MergeTreeDataPartState::PreActive, MergeTreeDataPartState::Active, MergeTreeDataPartState::Outdated});
            if (part)
            {
                if (auto part_in_memory = asInMemoryPart(part))
                    sum_parts_size_in_bytes += part_in_memory->block.bytes();
                else
                    sum_parts_size_in_bytes += part->getBytesOnDisk();
            }
        }

        if (merger_mutator.merges_blocker.isCancelled())
        {
            constexpr auto fmt_string = "Not executing log entry {} of type {} for part {} because merges and mutations are cancelled now.";
            LOG_DEBUG(LogToStr(out_postpone_reason, log), fmt_string, entry.znode_name, entry.typeToString(), entry.new_part_name);
            return false;
        }

        const auto data_settings = data.getSettings();
        if (data_settings->allow_remote_fs_zero_copy_replication)
        {
            auto disks = storage.getDisks();
            bool only_s3_storage = true;
            for (const auto & disk : disks)
                if (!disk->supportZeroCopyReplication())
                    only_s3_storage = false;

            String replica_to_execute_merge;
            if (!disks.empty() && only_s3_storage && storage.checkZeroCopyLockExists(entry.new_part_name, disks[0], replica_to_execute_merge))
            {
                constexpr auto fmt_string = "Not executing merge/mutation for the part {}, waiting for {} to execute it and will fetch after.";
                out_postpone_reason = fmt::format(fmt_string, entry.new_part_name, replica_to_execute_merge);
                LOG_TEST(log, fmt_string, entry.new_part_name, replica_to_execute_merge);
                return false;
            }
        }

        if (merge_strategy_picker.shouldMergeOnSingleReplica(entry))
        {
            auto replica_to_execute_merge = merge_strategy_picker.pickReplicaToExecuteMerge(entry);

            if (replica_to_execute_merge && !merge_strategy_picker.isMergeFinishedByReplica(replica_to_execute_merge.value(), entry))
            {
                constexpr auto fmt_string = "Not executing merge for the part {}, waiting for {} to execute merge.";
                out_postpone_reason = fmt::format(fmt_string, entry.new_part_name, replica_to_execute_merge.value());
                return false;
            }
        }

        UInt64 max_source_parts_size = entry.type == LogEntry::MERGE_PARTS ? merger_mutator.getMaxSourcePartsSizeForMerge()
                                                                           : merger_mutator.getMaxSourcePartSizeForMutation();
        /** If there are enough free threads in background pool to do large merges (maximal size of merge is allowed),
          * then ignore value returned by getMaxSourcePartsSizeForMerge() and execute merge of any size,
          * because it may be ordered by OPTIMIZE or early with different settings.
          * Setting max_bytes_to_merge_at_max_space_in_pool still working for regular merges,
          * because the leader replica does not assign merges of greater size (except OPTIMIZE PARTITION and OPTIMIZE FINAL).
          */
        bool ignore_max_size = false;
        if (entry.type == LogEntry::MERGE_PARTS)
        {
            ignore_max_size = max_source_parts_size == data_settings->max_bytes_to_merge_at_max_space_in_pool;

            if (isTTLMergeType(entry.merge_type))
            {
                if (merger_mutator.ttl_merges_blocker.isCancelled())
                {
                    constexpr auto fmt_string = "Not executing log entry {} for part {} because merges with TTL are cancelled now.";
                    LOG_DEBUG(LogToStr(out_postpone_reason, log), fmt_string, entry.znode_name, entry.new_part_name);
                    return false;
                }
                size_t total_merges_with_ttl = data.getTotalMergesWithTTLInMergeList();
                if (total_merges_with_ttl >= data_settings->max_number_of_merges_with_ttl_in_pool)
                {
                    constexpr auto fmt_string = "Not executing log entry {} for part {} because {} merges with TTL already executing, maximum {}.";
                    LOG_DEBUG(LogToStr(out_postpone_reason, log), fmt_string, entry.znode_name, entry.new_part_name, total_merges_with_ttl,
                              data_settings->max_number_of_merges_with_ttl_in_pool);
                    return false;
                }
            }
        }

        if (!ignore_max_size && sum_parts_size_in_bytes > max_source_parts_size)
        {
            constexpr auto fmt_string = "Not executing log entry {} of type {} for part {}"
                                        " because source parts size ({}) is greater than the current maximum ({}).";
            LOG_DEBUG(LogToStr(out_postpone_reason, LogFrequencyLimiter(log, 5)), fmt_string, entry.znode_name, entry.typeToString(), entry.new_part_name,
                      ReadableSize(sum_parts_size_in_bytes), ReadableSize(max_source_parts_size));

            return false;
        }
    }

    /// Alters must be executed one by one. First metadata change, and after that data alter (MUTATE_PART entries with).
    /// corresponding alter_version.
    if (entry.type == LogEntry::ALTER_METADATA)
    {
        if (!alter_sequence.canExecuteMetaAlter(entry.alter_version, state_lock))
        {
            int head_alter = alter_sequence.getHeadAlterVersion(state_lock);
            constexpr auto fmt_string = "Cannot execute alter metadata {} with version {} because another alter {} must be executed before";
            LOG_TRACE(LogToStr(out_postpone_reason, log), fmt_string, entry.znode_name, entry.alter_version, head_alter);
            return false;
        }
    }

    /// If this MUTATE_PART is part of alter modify/drop query, than we have to execute them one by one
    if (entry.isAlterMutation())
    {
        if (!alter_sequence.canExecuteDataAlter(entry.alter_version, state_lock))
        {
            int head_alter = alter_sequence.getHeadAlterVersion(state_lock);
            if (head_alter == entry.alter_version)
            {
                constexpr auto fmt_string = "Cannot execute alter data {} with version {} because metadata still not altered";
                LOG_TRACE(LogToStr(out_postpone_reason, log), fmt_string, entry.znode_name, entry.alter_version);
            }
            else
            {
                constexpr auto fmt_string = "Cannot execute alter data {} with version {} because another alter {} must be executed before";
                LOG_TRACE(LogToStr(out_postpone_reason, log), fmt_string, entry.znode_name, entry.alter_version, head_alter);
            }

            return false;
        }
    }

    /// DROP_RANGE, DROP_PART and REPLACE_RANGE entries remove other entries, which produce parts in the range.
    /// If such part producing operations are currently executing, then DROP/REPLACE RANGE wait them to finish.
    /// Deadlock is possible if multiple DROP/REPLACE RANGE entries are executing in parallel and wait each other.
    /// But it should not happen if ranges are disjoint.
    /// See also removePartProducingOpsInRange(...) and ReplicatedMergeTreeQueue::CurrentlyExecuting.

    if (auto drop_range = entry.getDropRange(format_version))
    {
        auto drop_range_info = MergeTreePartInfo::fromPartName(*drop_range, format_version);
        for (const auto & info : currently_executing_drop_replace_ranges)
        {
            if (drop_range_info.isDisjoint(info))
                continue;
            constexpr auto fmt_string = "Not executing log entry {} of type {} for part {} "
                "because another DROP_RANGE or REPLACE_RANGE entry with not disjoint range {} is currently executing.";
            LOG_TRACE(LogToStr(out_postpone_reason, log), fmt_string, entry.znode_name,
                      entry.typeToString(),
                      entry.new_part_name,
                      info.getPartNameForLogs());
            return false;
        }
    }

    if (entry.type == LogEntry::DROP_PART)
    {
        /// We should avoid reordering of REPLACE_RANGE and DROP_PART,
        /// because if replace_range_entry->new_part_names contains drop_range_entry->new_part_name
        /// and we execute DROP PART before REPLACE_RANGE, then DROP PART will be no-op
        /// (because part is not created yet, so there is nothing to drop;
        /// DROP_RANGE does not cover all parts of REPLACE_RANGE, so removePartProducingOpsInRange(...) will not remove anything too)
        /// and part will never be removed. Replicas may diverge due to such reordering.
        /// We don't need to do anything for other entry types, because removePartProducingOpsInRange(...) will remove them as expected.

        auto drop_part_info = MergeTreePartInfo::fromPartName(entry.new_part_name, format_version);
        for (const auto & replace_entry : queue)
        {
            if (replace_entry->type != LogEntry::REPLACE_RANGE)
                continue;

            for (const auto & new_part_name : replace_entry->replace_range_entry->new_part_names)
            {
                auto new_part_info = MergeTreePartInfo::fromPartName(new_part_name, format_version);
                if (!new_part_info.isDisjoint(drop_part_info))
                {
                    constexpr auto fmt_string = "Not executing log entry {} of type {} for part {} "
                        "because it probably depends on {} (REPLACE_RANGE).";
                    LOG_TRACE(LogToStr(out_postpone_reason, log), fmt_string, entry.znode_name, entry.typeToString(),
                              entry.new_part_name, replace_entry->znode_name);
                    return false;
                }
            }
        }
    }

    return true;
}


Int64 ReplicatedMergeTreeQueue::getCurrentMutationVersionImpl(
    const String & partition_id, Int64 data_version, std::lock_guard<std::mutex> & /* state_lock */) const
{
    auto in_partition = mutations_by_partition.find(partition_id);
    if (in_partition == mutations_by_partition.end())
        return 0;

    auto it = in_partition->second.upper_bound(data_version);
    if (it == in_partition->second.begin())
        return 0;

    --it;
    return it->first;
}


Int64 ReplicatedMergeTreeQueue::getCurrentMutationVersion(const String & partition_id, Int64 data_version) const
{
    std::lock_guard lock(state_mutex);
    return getCurrentMutationVersionImpl(partition_id, data_version, lock);
}


ReplicatedMergeTreeQueue::CurrentlyExecuting::CurrentlyExecuting(
    const ReplicatedMergeTreeQueue::LogEntryPtr & entry_, ReplicatedMergeTreeQueue & queue_, std::unique_lock<std::mutex> & /* state_lock */)
    : entry(entry_), queue(queue_)
{
    if (auto drop_range = entry->getDropRange(queue.format_version))
    {
        auto drop_range_info = MergeTreePartInfo::fromPartName(*drop_range, queue.format_version);
        [[maybe_unused]] bool inserted = queue.currently_executing_drop_replace_ranges.emplace(drop_range_info).second;
        assert(inserted);
    }
    entry->currently_executing = true;
    ++entry->num_tries;
    entry->last_attempt_time = time(nullptr);

    for (const String & new_part_name : entry->getVirtualPartNames(queue.format_version))
    {
        if (!queue.future_parts.emplace(new_part_name, entry).second)
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Tagging already tagged future part {}. This is a bug. "
                                                       "It happened on attempt to execute {}: {}",
                                                       new_part_name, entry->znode_name, entry->toString());
    }
}


void ReplicatedMergeTreeQueue::CurrentlyExecuting::setActualPartName(
    ReplicatedMergeTreeQueue::LogEntry & entry,
    const String & actual_part_name,
    ReplicatedMergeTreeQueue & queue,
    std::unique_lock<std::mutex> & state_lock,
    std::vector<LogEntryPtr> & covered_entries_to_wait)
{
    if (!entry.actual_new_part_name.empty())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Entry actual part isn't empty yet. This is a bug.");

    entry.actual_new_part_name = actual_part_name;

    /// Check if it is the same (and already added) part.
    if (entry.actual_new_part_name == entry.new_part_name)
        return;

    if (!queue.future_parts.emplace(entry.actual_new_part_name, entry.shared_from_this()).second)
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Attaching already existing future part {}. This is a bug. "
                                                   "It happened on attempt to execute {}: {}",
                                                   entry.actual_new_part_name, entry.znode_name, entry.toString());

    for (LogEntryPtr & covered_entry : covered_entries_to_wait)
    {
        if (&entry == covered_entry.get())
            continue;
        LOG_TRACE(queue.log, "Waiting for {} producing {} to finish before executing {} producing not disjoint part {}",
                  covered_entry->znode_name, covered_entry->new_part_name, entry.znode_name, entry.new_part_name);
        covered_entry->execution_complete.wait(state_lock, [&covered_entry] { return !covered_entry->currently_executing; });
    }
}


ReplicatedMergeTreeQueue::CurrentlyExecuting::~CurrentlyExecuting()
{
    std::lock_guard lock(queue.state_mutex);

    if (auto drop_range = entry->getDropRange(queue.format_version))
    {
        auto drop_range_info = MergeTreePartInfo::fromPartName(*drop_range, queue.format_version);
        [[maybe_unused]] bool removed = queue.currently_executing_drop_replace_ranges.erase(drop_range_info);
        assert(removed);
    }
    entry->currently_executing = false;
    entry->execution_complete.notify_all();

    for (const String & new_part_name : entry->getVirtualPartNames(queue.format_version))
    {
        if (!queue.future_parts.erase(new_part_name))
        {
            LOG_ERROR(queue.log, "Untagging already untagged future part {}. This is a bug.", new_part_name);
            assert(false);
        }
    }

    if (!entry->actual_new_part_name.empty())
    {
        if (entry->actual_new_part_name != entry->new_part_name && !queue.future_parts.erase(entry->actual_new_part_name))
        {
            LOG_ERROR(queue.log, "Untagging already untagged future part {}. This is a bug.", entry->actual_new_part_name);
            assert(false);
        }

        entry->actual_new_part_name.clear();
    }
}


ReplicatedMergeTreeQueue::SelectedEntryPtr ReplicatedMergeTreeQueue::selectEntryToProcess(MergeTreeDataMergerMutator & merger_mutator, MergeTreeData & data)
{
    LogEntryPtr entry;

    std::unique_lock lock(state_mutex);

    for (auto it = queue.begin(); it != queue.end(); ++it)
    {
        if ((*it)->currently_executing)
            continue;

        if (shouldExecuteLogEntry(**it, (*it)->postpone_reason, merger_mutator, data, lock))
        {
            entry = *it;
            /// We gave a chance for the entry, move it to the tail of the queue, after that
            /// we move it to the end of the queue.
            queue.splice(queue.end(), queue, it);
            break;
        }
        else
        {
            ++(*it)->num_postponed;
            (*it)->last_postpone_time = time(nullptr);
        }
    }

    if (entry)
        return std::make_shared<SelectedEntry>(entry, std::unique_ptr<CurrentlyExecuting>{new CurrentlyExecuting(entry, *this, lock)});
    else
        return {};
}


bool ReplicatedMergeTreeQueue::processEntry(
    std::function<zkutil::ZooKeeperPtr()> get_zookeeper,
    LogEntryPtr & entry,
    std::function<bool(LogEntryPtr &)> func)
{
    std::exception_ptr saved_exception;

    try
    {
        /// We don't have any backoff for failed entries
        /// we just count amount of tries for each of them.
        if (func(entry))
            removeProcessedEntry(get_zookeeper(), entry);
    }
    catch (...)
    {
        saved_exception = std::current_exception();
    }

    if (saved_exception)
    {
        std::lock_guard lock(state_mutex);
        entry->exception = saved_exception;
        entry->last_exception_time = time(nullptr);
        return false;
    }

    return true;
}


ReplicatedMergeTreeQueue::OperationsInQueue ReplicatedMergeTreeQueue::countMergesAndPartMutations() const
{
    std::lock_guard lock(state_mutex);

    size_t count_merges = 0;
    size_t count_mutations = 0;
    size_t count_merges_with_ttl = 0;
    for (const auto & entry : queue)
    {
        if (entry->type == ReplicatedMergeTreeLogEntry::MERGE_PARTS)
        {
            ++count_merges;
            if (isTTLMergeType(entry->merge_type))
                ++count_merges_with_ttl;
        }
        else if (entry->type == ReplicatedMergeTreeLogEntry::MUTATE_PART)
            ++count_mutations;
    }

    return OperationsInQueue{count_merges, count_mutations, count_merges_with_ttl};
}


size_t ReplicatedMergeTreeQueue::countMutations() const
{
    std::lock_guard lock(state_mutex);
    return mutations_by_znode.size();
}


size_t ReplicatedMergeTreeQueue::countFinishedMutations() const
{
    std::lock_guard lock(state_mutex);

    size_t count = 0;
    for (const auto & pair : mutations_by_znode)
    {
        const auto & mutation = pair.second;
        if (!mutation.is_done)
            break;

        ++count;
    }

    return count;
}


ReplicatedMergeTreeMergePredicate ReplicatedMergeTreeQueue::getMergePredicate(zkutil::ZooKeeperPtr & zookeeper, PartitionIdsHint && partition_ids_hint)
{
    return ReplicatedMergeTreeMergePredicate(*this, zookeeper, std::move(partition_ids_hint));
}


MutationCommands ReplicatedMergeTreeQueue::getFirstAlterMutationCommandsForPart(const MergeTreeData::DataPartPtr & part) const
{
    std::lock_guard lock(state_mutex);
    auto in_partition = mutations_by_partition.find(part->info.partition_id);
    if (in_partition == mutations_by_partition.end())
        return MutationCommands{};

    Int64 part_version = part->info.getDataVersion();
    for (auto [mutation_version, mutation_status] : in_partition->second)
        if (mutation_version > part_version && mutation_status->entry->alter_version != -1)
            return mutation_status->entry->commands;

    return MutationCommands{};
}

MutationCommands ReplicatedMergeTreeQueue::getMutationCommands(
    const MergeTreeData::DataPartPtr & part, Int64 desired_mutation_version) const
{
    /// NOTE: If the corresponding mutation is not found, the error is logged (and not thrown as an exception)
    /// to allow recovering from a mutation that cannot be executed. This way you can delete the mutation entry
    /// from /mutations in ZK and the replicas will simply skip the mutation.

    /// NOTE: However, it's quite dangerous to skip MUTATE_PART. Replicas may diverge if one of them have executed part mutation,
    /// and then mutation was killed before execution of MUTATE_PART on remaining replicas.

    if (part->info.getDataVersion() > desired_mutation_version)
    {
        LOG_WARNING(log, "Data version of part {} is already greater than desired mutation version {}", part->name, desired_mutation_version);
        return MutationCommands{};
    }

    std::lock_guard lock(state_mutex);

    auto in_partition = mutations_by_partition.find(part->info.partition_id);
    if (in_partition == mutations_by_partition.end())
    {
        LOG_WARNING(log, "There are no mutations for partition ID {} (trying to mutate part {} to {})", part->info.partition_id, part->name, toString(desired_mutation_version));
        return MutationCommands{};
    }

    auto begin = in_partition->second.upper_bound(part->info.getDataVersion());

    auto end = in_partition->second.lower_bound(desired_mutation_version);
    if (end == in_partition->second.end() || end->first != desired_mutation_version)
        LOG_WARNING(log,
            "Mutation with version {} not found in partition ID {} (trying to mutate part {})",
            desired_mutation_version,
            part->info.partition_id,
            part->name);
    else
        ++end;

    MutationCommands commands;
    for (auto it = begin; it != end; ++it)
        commands.insert(commands.end(), it->second->entry->commands.begin(), it->second->entry->commands.end());

    return commands;
}


bool ReplicatedMergeTreeQueue::tryFinalizeMutations(zkutil::ZooKeeperPtr zookeeper)
{
    std::vector<ReplicatedMergeTreeMutationEntryPtr> candidates;
    {
        std::lock_guard lock(state_mutex);

        for (auto & kv : mutations_by_znode)
        {
            const String & znode = kv.first;
            MutationStatus & mutation = kv.second;

            if (mutation.is_done)
                continue;

            if (znode <= mutation_pointer)
            {
                LOG_TRACE(log, "Marking mutation {} done because it is <= mutation_pointer ({})", znode, mutation_pointer);
                mutation.is_done = true;
                mutation.latest_fail_reason.clear();
                alter_sequence.finishDataAlter(mutation.entry->alter_version, lock);
                if (mutation.parts_to_do.size() != 0)
                {
                    LOG_INFO(log, "Seems like we jumped over mutation {} when downloaded part with bigger mutation number.{}", znode, " It's OK, tasks for rest parts will be skipped, but probably a lot of mutations were executed concurrently on different replicas.");
                    mutation.parts_to_do.clear();
                }
            }
            else if (mutation.parts_to_do.size() == 0)
            {
                /// Why it doesn't mean that mutation 100% finished? Because when we were creating part_to_do set
                /// some INSERT queries could be in progress. So we have to double-check that no affected committing block
                /// numbers exist and no new parts were surprisingly committed.
                LOG_TRACE(log, "Will check if mutation {} is done", mutation.entry->znode_name);
                candidates.emplace_back(mutation.entry);
            }
        }
    }

    if (candidates.empty())
        return false;
    else
        LOG_DEBUG(log, "Trying to finalize {} mutations", candidates.size());

    /// We need to check committing block numbers and new parts which could be committed.
    /// Actually we don't need most of predicate logic here but it all the code related to committing blocks
    /// and updatating queue state is implemented there.
    PartitionIdsHint partition_ids_hint;
    for (const auto & candidate : candidates)
        for (const auto & partitions : candidate->block_numbers)
            partition_ids_hint.insert(partitions.first);

    auto merge_pred = getMergePredicate(zookeeper, std::move(partition_ids_hint));

    std::vector<const ReplicatedMergeTreeMutationEntry *> finished;
    for (const auto & candidate : candidates)
    {
        if (merge_pred.isMutationFinished(candidate->znode_name, candidate->block_numbers))
            finished.push_back(candidate.get());
    }

    if (!finished.empty())
    {
        zookeeper->set(fs::path(replica_path) / "mutation_pointer", finished.back()->znode_name);

        std::lock_guard lock(state_mutex);

        mutation_pointer = finished.back()->znode_name;

        for (const ReplicatedMergeTreeMutationEntry * entry : finished)
        {
            auto it = mutations_by_znode.find(entry->znode_name);
            if (it != mutations_by_znode.end())
            {
                LOG_TRACE(log, "Mutation {} is done", entry->znode_name);
                it->second.is_done = true;
                it->second.latest_fail_reason.clear();
                if (entry->isAlterMutation())
                {
                    LOG_TRACE(log, "Finishing data alter with version {} for entry {}", entry->alter_version, entry->znode_name);
                    alter_sequence.finishDataAlter(entry->alter_version, lock);
                }
            }
        }
    }

    /// Mutations may finish in non sequential order because we may fetch
    /// already mutated parts from other replicas. So, because we updated
    /// mutation pointer we have to recheck all previous mutations, they may be
    /// also finished.
    return !finished.empty();
}


ReplicatedMergeTreeQueue::Status ReplicatedMergeTreeQueue::getStatus() const
{
    std::lock_guard lock(state_mutex);

    Status res;

    res.future_parts = static_cast<UInt32>(future_parts.size());
    res.queue_size = static_cast<UInt32>(queue.size());
    res.last_queue_update = static_cast<UInt32>(last_queue_update);

    res.inserts_in_queue = 0;
    res.merges_in_queue = 0;
    res.part_mutations_in_queue = 0;
    res.queue_oldest_time = 0;
    res.inserts_oldest_time = 0;
    res.merges_oldest_time = 0;
    res.part_mutations_oldest_time = 0;

    for (const LogEntryPtr & entry : queue)
    {
        if (entry->create_time && (!res.queue_oldest_time || entry->create_time < res.queue_oldest_time))
            res.queue_oldest_time = static_cast<UInt32>(entry->create_time);

        if (entry->type == LogEntry::GET_PART || entry->type == LogEntry::ATTACH_PART)
        {
            ++res.inserts_in_queue;

            if (entry->create_time && (!res.inserts_oldest_time || entry->create_time < res.inserts_oldest_time))
            {
                res.inserts_oldest_time = static_cast<UInt32>(entry->create_time);
                res.oldest_part_to_get = entry->new_part_name;
            }
        }

        if (entry->type == LogEntry::MERGE_PARTS)
        {
            ++res.merges_in_queue;

            if (entry->create_time && (!res.merges_oldest_time || entry->create_time < res.merges_oldest_time))
            {
                res.merges_oldest_time = static_cast<UInt32>(entry->create_time);
                res.oldest_part_to_merge_to = entry->new_part_name;
            }
        }

        if (entry->type == LogEntry::MUTATE_PART)
        {
            ++res.part_mutations_in_queue;

            if (entry->create_time && (!res.part_mutations_oldest_time || entry->create_time < res.part_mutations_oldest_time))
            {
                res.part_mutations_oldest_time = static_cast<UInt32>(entry->create_time);
                res.oldest_part_to_mutate_to = entry->new_part_name;
            }
        }
    }

    return res;
}


void ReplicatedMergeTreeQueue::getEntries(LogEntriesData & res) const
{
    res.clear();
    std::lock_guard lock(state_mutex);

    res.reserve(queue.size());
    for (const auto & entry : queue)
        res.emplace_back(*entry);
}


void ReplicatedMergeTreeQueue::getInsertTimes(time_t & out_min_unprocessed_insert_time, time_t & out_max_processed_insert_time) const
{
    out_min_unprocessed_insert_time = min_unprocessed_insert_time.load(std::memory_order_relaxed);
    out_max_processed_insert_time = max_processed_insert_time.load(std::memory_order_relaxed);
}


std::optional<MergeTreeMutationStatus> ReplicatedMergeTreeQueue::getIncompleteMutationsStatus(const String & znode_name, std::set<String> * mutation_ids) const
{

    std::lock_guard lock(state_mutex);
    auto current_mutation_it = mutations_by_znode.find(znode_name);
    /// killed
    if (current_mutation_it == mutations_by_znode.end())
        return {};

    const MutationStatus & status = current_mutation_it->second;
    MergeTreeMutationStatus result
    {
        .is_done = status.is_done,
        .latest_failed_part = status.latest_failed_part,
        .latest_fail_time = status.latest_fail_time,
        .latest_fail_reason = status.latest_fail_reason,
    };

    if (mutation_ids && !status.latest_fail_reason.empty())
    {
        const auto & latest_failed_part_info = status.latest_failed_part_info;
        auto in_partition = mutations_by_partition.find(latest_failed_part_info.partition_id);
        if (in_partition != mutations_by_partition.end())
        {
            const auto & version_to_status = in_partition->second;
            auto begin_it = version_to_status.upper_bound(latest_failed_part_info.getDataVersion());
            for (auto it = begin_it; it != version_to_status.end(); ++it)
            {
                /// All mutations with the same failure
                if (!it->second->is_done && it->second->latest_fail_reason == status.latest_fail_reason)
                    mutation_ids->insert(it->second->entry->znode_name);
            }
        }
    }
    return result;
}

std::vector<MergeTreeMutationStatus> ReplicatedMergeTreeQueue::getMutationsStatus() const
{
    std::lock_guard lock(state_mutex);

    std::vector<MergeTreeMutationStatus> result;
    for (const auto & pair : mutations_by_znode)
    {
        const MutationStatus & status = pair.second;
        const ReplicatedMergeTreeMutationEntry & entry = *status.entry;
        Names parts_to_mutate = status.parts_to_do.getParts();

        for (const MutationCommand & command : entry.commands)
        {
            WriteBufferFromOwnString buf;
            formatAST(*command.ast, buf, false, true);
            result.push_back(MergeTreeMutationStatus
            {
                entry.znode_name,
                buf.str(),
                entry.create_time,
                entry.block_numbers,
                parts_to_mutate,
                status.is_done,
                status.latest_failed_part,
                status.latest_fail_time,
                status.latest_fail_reason,
            });
        }
    }

    return result;
}

ReplicatedMergeTreeQueue::QueueLocks ReplicatedMergeTreeQueue::lockQueue()
{
    return QueueLocks(state_mutex, pull_logs_to_queue_mutex, update_mutations_mutex);
}

ReplicatedMergeTreeMergePredicate::ReplicatedMergeTreeMergePredicate(
    ReplicatedMergeTreeQueue & queue_, zkutil::ZooKeeperPtr & zookeeper, PartitionIdsHint && partition_ids_hint_)
    : queue(queue_)
    , partition_ids_hint(std::move(partition_ids_hint_))
    , prev_virtual_parts(queue.format_version)
{
    {
        std::lock_guard lock(queue.state_mutex);
        prev_virtual_parts = queue.virtual_parts;
    }

    /// Load current quorum status.
    auto quorum_status_future = zookeeper->asyncTryGet(fs::path(queue.zookeeper_path) / "quorum" / "status");

    /// Load current inserts
    /// Hint avoids listing partitions that we don't really need.
    /// Dropped (or cleaned up by TTL) partitions are never removed from ZK,
    /// so without hint it can do a few thousands requests (if not using MultiRead).
    Strings partitions;
    if (partition_ids_hint.empty())
        partitions = zookeeper->getChildren(fs::path(queue.zookeeper_path) / "block_numbers");
    else
        std::copy(partition_ids_hint.begin(), partition_ids_hint.end(), std::back_inserter(partitions));

    std::vector<std::string> paths;
    paths.reserve(partitions.size());
    for (const String & partition : partitions)
        paths.push_back(fs::path(queue.zookeeper_path) / "block_numbers" / partition);

    auto locks_children = zookeeper->getChildren(paths);

    for (size_t i = 0; i < partitions.size(); ++i)
    {
        Strings partition_block_numbers = locks_children[i].names;
        for (const String & entry : partition_block_numbers)
        {
            if (!startsWith(entry, "block-"))
                continue;
            Int64 block_number = parse<Int64>(entry.substr(strlen("block-")));
            String zk_path = fs::path(queue.zookeeper_path) / "block_numbers" / partitions[i] / entry;
            committing_blocks[partitions[i]].insert(block_number);
        }
    }

    merges_version = queue_.pullLogsToQueue(zookeeper, {}, ReplicatedMergeTreeQueue::MERGE_PREDICATE);

    {
        /// We avoid returning here a version to be used in a lightweight transaction.
        ///
        /// When pinned parts set is changed a log entry is added to the queue in the same transaction.
        /// The log entry serves as a synchronization point, and it also increments `merges_version`.
        ///
        /// If pinned parts are fetched after logs are pulled then we can safely say that it contains all locks up to `merges_version`.
        String s = zookeeper->get(queue.zookeeper_path + "/pinned_part_uuids");
        pinned_part_uuids.fromString(s);
    }

    Coordination::GetResponse quorum_status_response = quorum_status_future.get();
    if (quorum_status_response.error == Coordination::Error::ZOK)
    {
        ReplicatedMergeTreeQuorumEntry quorum_status;
        quorum_status.fromString(quorum_status_response.data);
        inprogress_quorum_part = quorum_status.part_name;
    }
    else
        inprogress_quorum_part.clear();
}

bool ReplicatedMergeTreeMergePredicate::operator()(
    const MergeTreeData::DataPartPtr & left,
    const MergeTreeData::DataPartPtr & right,
    const MergeTreeTransaction *,
    String * out_reason) const
{
    if (left)
        return canMergeTwoParts(left, right, out_reason);
    else
        return canMergeSinglePart(right, out_reason);
}


bool ReplicatedMergeTreeMergePredicate::canMergeTwoParts(
    const MergeTreeData::DataPartPtr & left,
    const MergeTreeData::DataPartPtr & right,
    String * out_reason) const
{
    /// A sketch of a proof of why this method actually works:
    ///
    /// The trickiest part is to ensure that no new parts will ever appear in the range of blocks between left and right.
    /// Inserted parts get their block numbers by acquiring an ephemeral lock (see EphemeralLockInZooKeeper.h).
    /// These block numbers are monotonically increasing in a partition.
    ///
    /// Because there is a window between the moment the inserted part gets its block number and
    /// the moment it is committed (appears in the replication log), we can't get the name of all parts up to the given
    /// block number just by looking at the replication log - some parts with smaller block numbers may be currently committing
    /// and will appear in the log later than the parts with bigger block numbers.
    ///
    /// We also can't take a consistent snapshot of parts that are already committed plus parts that are about to commit
    /// due to limitations of ZooKeeper transactions.
    ///
    /// So we do the following (see the constructor):
    /// * copy virtual_parts from queue to prev_virtual_parts
    ///   (a set of parts which corresponds to executing the replication log up to a certain point)
    /// * load committing_blocks (inserts and mutations that have already acquired a block number but haven't appeared in the log yet)
    /// * do pullLogsToQueue() again to load fresh queue.virtual_parts and mutations.
    ///
    /// Now we have an invariant: if some part is in prev_virtual_parts then:
    /// * all parts with smaller block numbers are either in committing_blocks or in queue.virtual_parts
    ///   (those that managed to commit before we loaded committing_blocks).
    /// * all mutations with smaller block numbers are either in committing_blocks or in queue.mutations_by_partition
    ///
    /// So to check that no new parts will ever appear in the range of blocks between left and right we first check that
    /// left and right are already present in prev_virtual_parts (we can't give a definite answer for parts that were committed later)
    /// and then check that there are no blocks between them in committing_blocks and no parts in queue.virtual_parts.
    ///
    /// Similarly, to check that there will be no mutation with a block number between two parts from prev_virtual_parts
    /// (only then we can merge them without mutating the left part), we first check committing_blocks
    /// and then check that these two parts have the same mutation version according to queue.mutations_by_partition.

    if (left->info.partition_id != right->info.partition_id)
    {
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Parts {} and {} belong to different partitions", left->name, right->name);
    }

    for (const MergeTreeData::DataPartPtr & part : {left, right})
    {
        if (pinned_part_uuids.part_uuids.contains(part->uuid))
        {
            if (out_reason)
                *out_reason = "Part " + part->name + " has uuid " + toString(part->uuid) + " which is currently pinned";
            return false;
        }

        if (part->name == inprogress_quorum_part)
        {
            if (out_reason)
                *out_reason = "Quorum insert for part " + part->name + " is currently in progress";
            return false;
        }

        if (prev_virtual_parts.getContainingPart(part->info).empty())
        {
            if (out_reason)
                *out_reason = "Entry for part " + part->name + " hasn't been read from the replication log yet";
            return false;
        }
    }

    Int64 left_max_block = left->info.max_block;
    Int64 right_min_block = right->info.min_block;
    if (left_max_block > right_min_block)
        std::swap(left_max_block, right_min_block);

    if (left_max_block + 1 < right_min_block)
    {
        if (!partition_ids_hint.empty() && !partition_ids_hint.contains(left->info.partition_id))
        {
            if (out_reason)
                *out_reason = fmt::format("Uncommitted block were not loaded for unexpected partition {}", left->info.partition_id);
            return false;
        }

        auto committing_blocks_in_partition = committing_blocks.find(left->info.partition_id);
        if (committing_blocks_in_partition != committing_blocks.end())
        {
            const std::set<Int64> & block_numbers = committing_blocks_in_partition->second;

            auto block_it = block_numbers.upper_bound(left_max_block);
            if (block_it != block_numbers.end() && *block_it < right_min_block)
            {
                if (out_reason)
                    *out_reason = "Block number " + toString(*block_it) + " is still being inserted between parts "
                        + left->name + " and " + right->name;

                return false;
            }
        }
    }

    std::lock_guard lock(queue.state_mutex);

    for (const MergeTreeData::DataPartPtr & part : {left, right})
    {
        /// We look for containing parts in queue.virtual_parts (and not in prev_virtual_parts) because queue.virtual_parts is newer
        /// and it is guaranteed that it will contain all merges assigned before this object is constructed.
        String containing_part = queue.virtual_parts.getContainingPart(part->info);
        if (containing_part != part->name)
        {
            if (out_reason)
                *out_reason = "Part " + part->name + " has already been assigned a merge into " + containing_part;
            return false;
        }
    }

    if (left_max_block + 1 < right_min_block)
    {
        /// Fake part which will appear as merge result
        MergeTreePartInfo gap_part_info(
            left->info.partition_id, left_max_block + 1, right_min_block - 1,
            MergeTreePartInfo::MAX_LEVEL, MergeTreePartInfo::MAX_BLOCK_NUMBER);

        /// We don't select parts if any smaller part covered by our merge must exist after
        /// processing replication log up to log_pointer.
        Strings covered = queue.virtual_parts.getPartsCoveredBy(gap_part_info);
        if (!covered.empty())
        {
            if (out_reason)
                *out_reason = "There are " + toString(covered.size()) + " parts (from " + covered.front()
                    + " to " + covered.back() + ") that are still not present or being processed by "
                    + " other background process on this replica between " + left->name + " and " + right->name;
            return false;
        }
    }

    Int64 left_mutation_ver = queue.getCurrentMutationVersionImpl(
        left->info.partition_id, left->info.getDataVersion(), lock);

    Int64 right_mutation_ver = queue.getCurrentMutationVersionImpl(
        left->info.partition_id, right->info.getDataVersion(), lock);

    if (left_mutation_ver != right_mutation_ver)
    {
        if (out_reason)
            *out_reason = "Current mutation versions of parts " + left->name + " and " + right->name + " differ: "
                + toString(left_mutation_ver) + " and " + toString(right_mutation_ver) + " respectively";
        return false;
    }

    return MergeTreeData::partsContainSameProjections(left, right);
}

bool ReplicatedMergeTreeMergePredicate::canMergeSinglePart(
    const MergeTreeData::DataPartPtr & part,
    String * out_reason) const
{
    if (pinned_part_uuids.part_uuids.contains(part->uuid))
    {
        if (out_reason)
            *out_reason = fmt::format("Part {} has uuid {} which is currently pinned", part->name, part->uuid);
        return false;
    }

    if (part->name == inprogress_quorum_part)
    {
        if (out_reason)
            *out_reason = fmt::format("Quorum insert for part {} is currently in progress", part->name);
        return false;
    }

    if (prev_virtual_parts.getContainingPart(part->info).empty())
    {
        if (out_reason)
            *out_reason = fmt::format("Entry for part {} hasn't been read from the replication log yet", part->name);
        return false;
    }

    std::lock_guard lock(queue.state_mutex);

    /// We look for containing parts in queue.virtual_parts (and not in prev_virtual_parts) because queue.virtual_parts is newer
    /// and it is guaranteed that it will contain all merges assigned before this object is constructed.
    String containing_part = queue.virtual_parts.getContainingPart(part->info);
    if (containing_part != part->name)
    {
        if (out_reason)
            *out_reason = fmt::format("Part {} has already been assigned a merge into {}", part->name, containing_part);
        return false;
    }

    return true;
}


bool ReplicatedMergeTreeMergePredicate::partParticipatesInReplaceRange(const MergeTreeData::DataPartPtr & part, String * out_reason) const
{
    std::lock_guard lock(queue.state_mutex);
    for (const auto & entry : queue.queue)
    {
        if (entry->type != ReplicatedMergeTreeLogEntry::REPLACE_RANGE)
            continue;

        for (const auto & part_name : entry->replace_range_entry->new_part_names)
        {
            if (part->info.isDisjoint(MergeTreePartInfo::fromPartName(part_name, queue.format_version)))
                continue;

            if (out_reason)
                *out_reason = fmt::format("Part {} participates in REPLACE_RANGE {} ({})", part_name, entry->new_part_name, entry->znode_name);

            return true;
        }
    }
    return false;
}


std::optional<std::pair<Int64, int>> ReplicatedMergeTreeMergePredicate::getDesiredMutationVersion(const MergeTreeData::DataPartPtr & part) const
{
    /// Assigning mutations is easier than assigning merges because mutations appear in the same order as
    /// the order of their version numbers (see StorageReplicatedMergeTree::mutate).
    /// This means that if we have loaded the mutation with version number X then all mutations with
    /// the version numbers less than X are also loaded and if there is no merge or mutation assigned to
    /// the part (checked by querying queue.virtual_parts), we can confidently assign a mutation to
    /// version X for this part.

    /// We cannot mutate part if it's being inserted with quorum and it's not
    /// already reached.
    if (part->name == inprogress_quorum_part)
        return {};

    std::lock_guard lock(queue.state_mutex);

    if (queue.virtual_parts.getContainingPart(part->info) != part->name)
        return {};

    auto in_partition = queue.mutations_by_partition.find(part->info.partition_id);
    if (in_partition == queue.mutations_by_partition.end())
        return {};

    Int64 current_version = queue.getCurrentMutationVersionImpl(part->info.partition_id, part->info.getDataVersion(), lock);
    Int64 max_version = in_partition->second.rbegin()->first;

    int alter_version = -1;
    for (auto [mutation_version, mutation_status] : in_partition->second)
    {
        max_version = mutation_version;
        if (mutation_status->entry->isAlterMutation())
        {
            /// We want to assign mutations for part which version is bigger
            /// than part current version. But it doesn't make sense to assign
            /// more fresh versions of alter-mutations if previous alter still
            /// not done because alters execute one by one in strict order.
            if (mutation_version > current_version || !mutation_status->is_done)
            {
                alter_version = mutation_status->entry->alter_version;
                break;
            }
        }
    }

    if (current_version >= max_version)
        return {};

    return std::make_pair(max_version, alter_version);
}


bool ReplicatedMergeTreeMergePredicate::isMutationFinished(const std::string & znode_name, const std::map<String, int64_t> & block_numbers) const
{
    /// Check committing block numbers, maybe some affected inserts
    /// still not written to disk and committed to ZK.
    for (const auto & kv : block_numbers)
    {
        const String & partition_id = kv.first;
        Int64 block_num = kv.second;

        if (!partition_ids_hint.empty() && !partition_ids_hint.contains(partition_id))
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Partition id {} was not provided as hint, it's a bug", partition_id);

        auto partition_it = committing_blocks.find(partition_id);
        if (partition_it != committing_blocks.end())
        {
            size_t blocks_count = std::distance(
                partition_it->second.begin(), partition_it->second.lower_bound(block_num));
            if (blocks_count)
            {
                LOG_TRACE(queue.log, "Mutation {} is not done yet because in partition ID {} there are still {} uncommitted blocks.", znode_name, partition_id, blocks_count);
                return false;
            }
        }
    }

    std::lock_guard lock(queue.state_mutex);
    /// When we creating predicate we have updated the queue. Some committing inserts can now be committed so
    /// we check parts_to_do one more time. Also this code is async so mutation actually could be deleted from memory.
    if (auto it = queue.mutations_by_znode.find(znode_name); it != queue.mutations_by_znode.end())
    {
        if (it->second.parts_to_do.size() == 0)
            return true;

        LOG_TRACE(queue.log, "Mutation {} is not done because some parts [{}] were just committed", znode_name, fmt::join(it->second.parts_to_do.getParts(), ", "));
        return false;
    }
    else
    {
        LOG_TRACE(queue.log, "Mutation {} is done because it doesn't exist anymore", znode_name);
        return true;
    }
}

bool ReplicatedMergeTreeMergePredicate::isGoingToBeDropped(const MergeTreePartInfo & new_drop_range_info,
                                                           MergeTreePartInfo * out_drop_range_info) const
{
    return queue.isGoingToBeDropped(new_drop_range_info, out_drop_range_info);
}

String ReplicatedMergeTreeMergePredicate::getCoveringVirtualPart(const String & part_name) const
{
    std::lock_guard lock(queue.state_mutex);
    return queue.virtual_parts.getContainingPart(MergeTreePartInfo::fromPartName(part_name, queue.format_version));
}


ReplicatedMergeTreeQueue::SubscriberHandler
ReplicatedMergeTreeQueue::addSubscriber(ReplicatedMergeTreeQueue::SubscriberCallBack && callback,
                                        std::unordered_set<String> & out_entry_names, SyncReplicaMode sync_mode)
{
    std::lock_guard<std::mutex> lock(state_mutex);
    std::lock_guard lock_subscribers(subscribers_mutex);

    if (sync_mode != SyncReplicaMode::PULL)
    {
        /// We must get the list of entries to wait atomically with adding the callback
        bool lightweight_entries_only = sync_mode == SyncReplicaMode::LIGHTWEIGHT;
        static constexpr std::array lightweight_entries =
        {
            LogEntry::GET_PART,
            LogEntry::ATTACH_PART,
            LogEntry::DROP_RANGE,
            LogEntry::REPLACE_RANGE,
            LogEntry::DROP_PART
        };
        out_entry_names.reserve(queue.size());
        for (const auto & entry : queue)
        {
            if (!lightweight_entries_only
                || std::find(lightweight_entries.begin(), lightweight_entries.end(), entry->type) != lightweight_entries.end())
                out_entry_names.insert(entry->znode_name);
        }
        LOG_TEST(log, "Waiting for {} entries to be processed: {}", out_entry_names.size(), fmt::join(out_entry_names, ", "));
    }

    auto it = subscribers.emplace(subscribers.end(), std::move(callback));

    /// Atomically notify about current size
    (*it)(queue.size(), nullptr);

    return SubscriberHandler(it, *this);
}

void ReplicatedMergeTreeQueue::notifySubscribersOnPartialShutdown()
{
    size_t queue_size;
    {
        std::lock_guard<std::mutex> lock(state_mutex);
        queue_size = queue.size();
    }
    std::lock_guard lock_subscribers(subscribers_mutex);
    for (auto & subscriber_callback : subscribers)
        subscriber_callback(queue_size, nullptr);
}

ReplicatedMergeTreeQueue::SubscriberHandler::~SubscriberHandler()
{
    std::lock_guard lock(queue.subscribers_mutex);
    queue.subscribers.erase(it);
}

void ReplicatedMergeTreeQueue::notifySubscribers(size_t new_queue_size, const String * removed_log_entry_id)
{
    std::lock_guard lock_subscribers(subscribers_mutex);
    for (auto & subscriber_callback : subscribers)
        subscriber_callback(new_queue_size, removed_log_entry_id);
}

String padIndex(Int64 index)
{
    String index_str = toString(index);
    return std::string(10 - index_str.size(), '0') + index_str;
}

void ReplicatedMergeTreeQueue::removeCurrentPartsFromMutations()
{
    std::lock_guard state_lock(state_mutex);
    for (const auto & part_name : current_parts.getParts())
        removeCoveredPartsFromMutations(part_name, /*remove_part = */ true, /*remove_covered_parts = */ true);
}

}