> (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.

{
    if (isGlobalContext())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Global context cannot have external tables");
    std::shared_ptr<TemporaryTableHolder> holder;
    {
        auto lock = getLock();
        auto iter = external_tables_mapping.find(table_name);
        if (iter == external_tables_mapping.end())
            return {};
        holder = iter->second;
        external_tables_mapping.erase(iter);
    }
    return holder;
}
void Context::addScalar(const String & name, const Block & block)
{
    if (isGlobalContext())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Global context cannot have scalars");
    scalars[name] = block;
}
void Context::addSpecialScalar(const String & name, const Block & block)
{
    if (isGlobalContext())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Global context cannot have local scalars");
    special_scalars[name] = block;
}
bool Context::hasScalar(const String & name) const