> (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();
    shared->dictionaries_lib_path = path;
}
void Context::setUserScriptsPath(const String & path)
{
    auto lock = getLock();
    shared->user_scripts_path = path;
}
void Context::addWarningMessage(const String & msg) const
{
    auto lock = getLock();
    auto suppress_re = getConfigRef().getString("warning_supress_regexp", "");
    bool is_supressed = !suppress_re.empty() && re2::RE2::PartialMatch(msg, suppress_re);
    if (!is_supressed)
        shared->addWarningMessage(msg);
}
void Context::setConfig(const ConfigurationPtr & config)
{
    auto lock = getLock();
    shared->config = config;
    shared->access_control->setExternalAuthenticatorsConfig(*shared->config);
}
const Poco::Util::AbstractConfiguration & Context::getConfigRef() const
{
    auto lock = getLock();
    return shared->config ? *shared->config : Poco::Util::Application::instance().config();
}
AccessControl & Context::getAccessControl()