Method sendCancelToQuery

src/Interpreters/ProcessList.cpp:710–743

CancellationCode ProcessList::sendCancelToQuery(const String & current_query_id, const String & current_user)
{
    QueryStatusPtr elem;

    /// Cancelling the query should be done without the lock.
    ///
    /// Since it may be not that trivial, for example in case of distributed
    /// queries it tries to cancel the query gracefully on shards and this can
    /// take a while, so acquiring a lock during this time will lead to wait
    /// all new queries for this cancellation.
    ///
    /// Another problem is that it can lead to a deadlock, because of
    /// OvercommitTracker.
    ///
    /// So here we first set is_cancelling, and later reset it.
    /// The ProcessListEntry cannot be destroy if is_cancelling is true.
    {
        LockAndBlocker lock(mutex);
        elem = tryGetProcessListElement(current_query_id, current_user);
        if (!elem)
            return CancellationCode::NotFound;
        elem->is_cancelling = true;
    }

    SCOPE_EXIT({
        DENY_ALLOCATIONS_IN_SCOPE;

        Lock lock(mutex);
        elem->is_cancelling = false;
        cancelled_cv.notify_all();
    });

    return elem->cancelQuery(CancelReason::CANCELLED_BY_USER);
}


CancellationCode ProcessList::sendCancelToQuery(QueryStatusPtr elem)