And waits for them. That's pointless.
{
    try
    {
        queue.updateMutations(getZooKeeper(), mutations_updating_task->getWatchCallback());
    }
    catch (const Coordination::Exception & e)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
        if (e.code == Coordination::Error::ZSESSIONEXPIRED)
            return;
        mutations_updating_task->scheduleAfter(QUEUE_UPDATE_ERROR_SLEEP_MS);
    }
    catch (...)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
        mutations_updating_task->scheduleAfter(QUEUE_UPDATE_ERROR_SLEEP_MS);
    }
}
ReplicatedMergeTreeQueue::SelectedEntryPtr StorageReplicatedMergeTree::selectQueueEntry()