Add infrastructure for testing custom facts

This adds testing of custom facts by placing them in spec/facts, similar
to how classes have spec/classes and defines have spec/defines. The
subject is also set in the same way. It is possible to stub other facts
using a facts block and facts are properly cleared before and after a
run.

lib/rspec-puppet/example.rb

@@ -3,6 +3,7 @@
 require 'rspec-puppet/support'
 require 'rspec-puppet/example/define_example_group'
 require 'rspec-puppet/example/class_example_group'
+require 'rspec-puppet/example/fact_example_group'
 require 'rspec-puppet/example/function_example_group'
 require 'rspec-puppet/example/host_example_group'
 require 'rspec-puppet/example/type_example_group'
@@ -19,6 +20,7 @@ def c.rspec_puppet_include(group, type, file_path)
 
   c.rspec_puppet_include RSpec::Puppet::DefineExampleGroup, :define, %w[spec defines]
   c.rspec_puppet_include RSpec::Puppet::ClassExampleGroup, :class, %w[spec classes]
+  c.rspec_puppet_include RSpec::Puppet::FactExampleGroup, :define, %w[spec facts]
   c.rspec_puppet_include RSpec::Puppet::FunctionExampleGroup, :puppet_function, %w[spec functions]
   c.rspec_puppet_include RSpec::Puppet::HostExampleGroup, :host, %w[spec hosts]
   c.rspec_puppet_include RSpec::Puppet::TypeExampleGroup, :type, %w[spec types]

lib/rspec-puppet/example/fact_example_group.rb

@@ -0,0 +1,56 @@
+module RSpec::Puppet
+  # This module provides support for custom facts
+  module FactExampleGroup
+    def subject
+      setup_facter do
+        Facter.fact(self.class.top_level_description)
+      end
+    end
+
+    # TODO: proper matcher with a description
+    def have_value(value)
+      have_attributes(value: value)
+    end
+
+    def rspec_puppet_cleanup
+      Facter.clear
+      # TODO: clean LOAD_PATH again?
+    end
+
+    private
+
+    # TODO: duplicates adapter
+    def modulepath
+      if rspec_modulepath = RSpec.configuration.module_path
+        rspec_modulepath.split(File::PATH_SEPARATOR)
+      else
+        Puppet[:environmentpath].split(File::PATH_SEPARATOR).map do |path|
+          File.join(path, 'fixtures', 'modules')
+        end
+      end
+    end
+
+    def setup_facter
+      Dir.mktmpdir do |dir|
+        modulepath.map do |d|
+          Dir["#{d}/*/lib/facter"].entries.each do |entry|
+            $LOAD_PATH << File.expand_path(File.dirname(entry))
+          end
+        end
+
+        Facter.clear
+
+        if respond_to?(:facts)
+          allow(Facter).to receive(:value).and_call_original
+
+          facts.each do |fact, value|
+            # TODO: Facter.fact(fact).value?
+            allow(Facter).to receive(:value).with(fact.to_sym).and_return(value)
+          end
+        end
+
+        yield
+      end
+    end
+  end
+end

spec/facts/custom_spec.rb

@@ -0,0 +1,26 @@
+require 'spec_helper'
+
+describe 'custom' do
+  it { is_expected.not_to be_nil }
+  it { is_expected.to have_value('bar') }
+
+  context 'with overridden' do
+    let(:facts) do
+      {
+        myfact: 'set',
+      }
+    end
+
+    it { is_expected.to have_value('foo') }
+  end
+
+  context 'with unrelated fact overridden' do
+    let(:facts) do
+      {
+        kernel: 'unix',
+      }
+    end
+
+    it { is_expected.to have_value('bar') }
+  end
+end

spec/fixtures/modules/custom_fact/lib/facter/custom.rb

@@ -0,0 +1,3 @@
+Facter.add(:custom) do
+  setcode { Facter.value(:myfact) ? 'foo' : 'bar' }
+end