Add a function to write package names

This function writes out package names that have been
defined in a given puppet catalog.

In order to work this should be place last (or very late)
in a manifest to ensure it picks up packages.

Change-Id: Ie21b5bf7df71337da02ea43915dc4e70d3052bb7

lib/puppet/parser/functions/write_package_names.rb

@@ -0,0 +1,22 @@
+require 'fileutils'
+
+module Puppet::Parser::Functions
+  newfunction(:write_package_names, :doc => "Write package names which are managed via this puppet run to a file.") do |arg|
+    if arg[0].class == String
+      begin
+        output_file = arg[0]
+        packages = catalog.resources.collect { |r| r.title if r.type == 'Package' }.compact
+        FileUtils.mkdir_p(File.dirname(output_file))
+        File.open(output_file, 'w') do |f|
+            packages.each do |pkg_name|
+                f.write(pkg_name + "\n")
+            end
+        end
+      rescue JSON::ParserError
+        raise Puppet::ParseError, "Syntax error: #{arg[0]} is invalid"
+      end
+    else
+      raise Puppet::ParseError, "Syntax error: #{arg[0]} is not a String"
+    end
+  end
+end