file_tree_hash.rb

require 'md5'

module Puppet::Parser::Functions
  newfunction(:file_tree_hash,  :type => :rvalue, :arity => 1, :doc => <<-EOS
    Returns MD5 has for given prefix. It travers file tree recursively. 
    EOS
  ) do |arguments|
    prefix = arguments[0] 

    content = Dir .glob("#{prefix}/**/*") \
                  .sort  \
                  .reject { |el| File.directory?(el) } \
                  .map { |el| content = File.read(el); MD5.md5(content).to_s } \
                  .join('')
    
    MD5.md5(content).to_s
  end
end

site.pp

node 'foo' {
  $update_time = file_tree_hash("/foo/bar")
  notice($update_time)
}