> (you don't have to strictly follow this form)

**Use case**

Sometimes, for really sophisticated scenarios it can be hard to control the amount of in-progress backups run externally. For the purpose of simpler control we can introduce server-level setting which disallows running concurrent backups of any type. 

Something like this in `config.xml` (enabled by default):
`<allow_concurrent_backups>true</allow_concurrent_backups>`

**Describe the solution you'd like**

It's important that backups can be executed in distributed way on multiple servers. Need to take it into account.

**Describe alternatives you've considered**
External check of `system.backups` table, but it can be complex.

{
    auto lock = getLock();
    return shared->user_scripts_path;
}
Strings Context::getWarnings() const
{
    Strings common_warnings;
    {
        auto lock = getLock();
        common_warnings = shared->warnings;
    }
    for (const auto & setting : settings)
    {
        if (setting.isValueChanged() && setting.isObsolete())
        {
            common_warnings.emplace_back("Some obsolete setting is changed. "
                                         "Check 'select * from system.settings where changed' and read the changelog.");
            break;
        }
    }
    return common_warnings;
}
VolumePtr Context::getTemporaryVolume() const
{
    auto lock = getLock();
    if (shared->temp_data_on_disk)
        return shared->temp_data_on_disk->getVolume();
    return nullptr;
}
TemporaryDataOnDiskScopePtr Context::getTempDataOnDisk() const