Puppet: ensure a file is empty

define blah($ensure = present) {
    if $ensure == "present" {
        exec { do_something_to_make_it_be_here: }
    } else {
        exec { do_something_to_make_it_not_be_here: }
    }
}

For more than just ensure present/absent, you can switch to a case in the same vein. Don't go to subclasses, because as you say they lose dependency information.

So, to answer your question directly, I don't believe getting a list of exported resources directly from erb is possible. This is due to the nature of exported resources. To Puppet, they're just more resources that need to be created on the host.

But, there is a way to accomplish what you're looking to do. I do it in a few places in my environment.

Here we create a directory of files, one for each host we want to flag as a "bacula_client". We use the purge, force, and recurse options to remove files that are not managed by Puppet (i.e. if you want to remove a system from this "list").

class bacula::client {

  @@file { "/etc/bacula_clients/$fqdn":
    ensure => present,
    content => "",
    require => File['/etc/bacula_clients'],
    tag => "bacula_client",
  }

}

class bacula::server {

  #
  # .. include whatever else the server requires, like package {} file {} service {}
  #

  file { "/etc/bacula_clients":
    ensure => directory,
    purge => true,
    recurse => true,
    force => true,
  }

  # Populate directory of client files.
  File <<| tag == "bacula_client" |>>

}

Next, we use some Ruby code in the .erb to scan this directory for files, and act on them:

<% 
bacula_clients_dir = '/etc/bacula_clients'
d = Dir.open(bacula_clients_dir)

# Remove directories from the list of entries in the directory (specifically '.' and '..'):
backupclients = d.entries.delete_if { |e| File.directory? "#{bacula_clients_dir}/#{e}" }

backupclients.each do |backup_files| 
-%>
Job {
  Name = "Server"
  JobDefs = "DefaultJob"
  Client = <%= backup_files %>-fd
  Pool = TeraMonth
  Write Bootstrap = "/var/lib/bacula/<%= backup_files %>.bsr"
}
<% end -%>