(#13610) Add is_function_available to stdlib

This function provides a simple wrapper around
Puppet::Parser::Functions.function for access within Puppet manifests.
This will allow users to check whether or not a plugin or functionality
such as hiera is installed on the server.

lib/puppet/parser/functions/is_function_available.rb

@@ -0,0 +1,23 @@
+#
+# is_function_available.rb
+#
+
+module Puppet::Parser::Functions
+  newfunction(:is_function_available, :type => :rvalue, :doc => <<-EOS
+This function accepts a string as an argument, determines whether the
+Puppet runtime has access to a function by that name.  It returns a
+true if the function exists, false if not.
+    EOS
+  ) do |arguments|
+
+    if (arguments.size != 1) then
+      raise(Puppet::ParseError, "is_function_available?(): Wrong number of arguments "+
+        "given #{arguments.size} for 1")
+    end
+
+    function = Puppet::Parser::Functions.function(arguments[0].to_sym)
+    function.is_a?(String) and not function.empty?
+  end
+end
+
+# vim: set ts=2 sw=2 et :

spec/unit/puppet/parser/functions/is_function_available.rb

@@ -0,0 +1,31 @@
+#!/usr/bin/env rspec
+require 'spec_helper'
+
+describe "the is_function_available function" do
+  before :all do
+    Puppet::Parser::Functions.autoloader.loadall
+  end
+
+  before :each do
+    @scope = Puppet::Parser::Scope.new
+  end
+
+  it "should exist" do
+    Puppet::Parser::Functions.function("is_function_available").should == "function_is_function_available"
+  end
+
+  it "should raise a ParseError if there is less than 1 arguments" do
+    lambda { @scope.function_is_function_available([]) }.should( raise_error(Puppet::ParseError))
+  end
+
+  it "should return false if a nonexistent function is passed" do
+    result = @scope.function_is_function_available(['jeff_mccunes_left_sock'])
+    result.should(eq(false))
+  end
+
+  it "should return true if an available function is passed" do
+    result = @scope.function_is_function_available(['require'])
+    result.should(eq(true))
+  end
+
+end