A way to better support service_provider_fact

service_provider_func.rb

module Puppet::Parser::Functions
  newfunction(:service_provider_func, :type => :rvalue, :doc => <<-EOS
Returns the default service provider for the system overridden by any parent class
that has Service { provider => $my_service_provider } specified to cause an override.
This is useful when we actually need to know the clients service provider and allow them
to also override it in case they decide they need a different one due to puppet not picking
the right one for their new or old Operating System.

Example:
Centos 7(systemd)
  {
    $var = service_provider_func()
  }
  >>> $var prints 'systemd'
  {
    Service { 
      provider => 'upstart'
    }
    $var = service_provider_func()
  }
  >>> $var prints 'upstart'
    EOS
  ) do |args|
    override = self.lookupdefaults('Service')[:provider].value
    override = lookupvar('service_provider') if override.nil? || override.empty?

    return override
  end
end

include postgresql::server
Service <| name == 'postgresql' |> {
  provider => 'systemd',
}

File <| name == 'my init script' |> {
  content/source => something
}

If it's a file resource of the type file { /etc/init.d/bla: } then you don't really need an override at all, just drop your own with file { '/etc/systemd/...: }