Add stdlib::has_function

Add a new function to test for function availability in the
Puppet runtime, replacing the deprecated and removed
is_function_available with support for legacy and modern
functions.

Author: seanmil

lib/puppet/functions/stdlib/has_function.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+# @summary
+#   Returns whether the Puppet runtime has access to a given function.
+#
+# @example Using stdlib::has_function()
+#   stdlib::has_function('stdlib::has_function') # true
+#   stdlib::has_function('not_a_function') # false
+#
+# Determines whether the Puppet runtime has access to a function by the
+# name provided.
+#
+# @return
+#   Returns true if the provided function name is available, false otherwise.
+#
+Puppet::Functions.create_function(:'stdlib::has_function', Puppet::Functions::InternalFunction) do
+  dispatch :has_function do
+    scope_param
+    param 'String[1]', :function_name
+    return_type 'Boolean'
+  end
+
+  def has_function(scope, function_name) # rubocop:disable Naming/PredicateName
+    loaders = scope.compiler.loaders
+    loader = loaders.private_environment_loader
+    return true unless loader&.load(:function, function_name).nil?
+
+    # If the loader cannot find the function it might be
+    # a 3x-style function stubbed in on-the-fly for testing.
+    func_3x = Puppet::Parser::Functions.function(function_name.to_sym)
+    func_3x.is_a?(String) && !func_3x.empty?
+  end
+end

spec/functions/has_function_spec.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'stdlib::has_function' do
+  let(:pre_condition) { 'function puppet_func {}' }
+
+  before(:each) do
+    Puppet::Parser::Functions.newfunction(:test_3x_func) do |_args|
+      true
+    end
+  end
+
+  it { is_expected.not_to be_nil }
+
+  # Itself, a namespaced function:
+  it { is_expected.to run.with_params('stdlib::has_function').and_return(true) }
+
+  # A namespaced function which does not exist:
+  it { is_expected.to run.with_params('stdlib::not_a_function').and_return(false) }
+
+  # A top-function which does not exist:
+  it { is_expected.to run.with_params('not_a_function').and_return(false) }
+
+  # A Puppet core function:
+  it { is_expected.to run.with_params('assert_type').and_return(true) }
+
+  # A Puppet function stubbed during testing:
+  it { is_expected.to run.with_params('puppet_func').and_return(true) }
+
+  # A file-loaded 3x style function in stdlib:
+  it { is_expected.to run.with_params('validate_augeas').and_return(true) }
+
+  # A stubbed 3x-style function:
+  it { is_expected.to run.with_params('test_3x_func').and_return(true) }
+end