Facter module not available in at_exit

A fact with an at_exit callback that uses the Facter module will fail with an uninitialized constant error.

We're using this code to enable some fact caching:
https://github.com/google/macops/blob/master/facter/cache.rb

In it, we have an at_exit handler to ensure updated cached values are written on exit. Since the handler tries to use Facter.debug and other methods, a NameError is raised.

A simpler code snippet to replicate this:

require 'facter'

at_exit { Facter.warn('Facter.warn at exit') }

at_exit { puts 'puts at exit' }

Facter.add('uninitialized_constant') do

  setcode do

    'foo'

  end

end

This produces this output using facter 3:

$ sudo facter -p uninitialized_constant facterversion

facterversion => 3.1.4

uninitialized_constant => foo

puts at exit

/private/var/puppet/lib/facter/unint.rb:3:in `block in <top (required)>': uninitialized constant Facter (NameError)