Add override_facts helper method

README.md

@@ -20,3 +20,79 @@ In your `spec/spec_helper.rb`
 ```ruby
 require 'voxpupuli/test/spec_helper'
 ```
+
+# Fact handling
+
+The recommended method is using [rspec-puppet-facts](https://github.com/mcanevet/rspec-puppet-facts) and is set up by default. This means the tests are writting as follows:
+
+```ruby
+require 'spec_helper'
+
+describe 'myclass' do
+  on_supported_os.each do |os, os_facts|
+    context "on #{os}" do
+      let(:facts) { os_facts }
+
+      it { is_expected.to compile.with_all_deps }
+    end
+  end
+end
+```
+
+Now a common case is to override facts in tests. Let's take the example of SELinux with legacy facts.
+
+```ruby
+require 'spec_helper'
+
+describe 'mytool' do
+  on_supported_os.each do |os, os_facts|
+    context "on #{os}" do
+      let(:facts) { os_facts }
+
+      it { is_expected.to compile.with_all_deps }
+
+      describe 'with SELinux enabled' do
+        let(:facts) { super().merge(selinux: true) }
+
+        it { is_expected.to contain_package('mytool-selinux') }
+      end
+
+      describe 'with SELinux disabled' do
+        let(:facts) { super().merge(selinux: false) }
+
+        it { is_expected.not_to contain_package('mytool-selinux') }
+      end
+    end
+  end
+end
+```
+
+This is all fairly straight forward, but it gets more complex when using modern facts. Modern facts are nested which means you need to do deep merging. There is [deep_merge](https://rubygems.org/gems/deep_merge) but its results are not at all useful for spec testing. That's why voxpupuli-test has an `override_facts` helper.
+
+```ruby
+require 'spec_helper'
+
+describe 'mytool' do
+  on_supported_os.each do |os, os_facts|
+    context "on #{os}" do
+      let(:facts) { os_facts }
+
+      it { is_expected.to compile.with_all_deps }
+
+      describe 'with SELinux enabled' do
+        let(:facts) { override_facts(super(), os: {selinux: {enabled: true}}) }
+
+        it { is_expected.to contain_package('mytool-selinux') }
+      end
+
+      describe 'with SELinux disabled' do
+        let(:facts) { override_facts(super(), os: {selinux: {enabled: false}}) }
+
+        it { is_expected.not_to contain_package('mytool-selinux') }
+      end
+    end
+  end
+end
+```
+
+Note that this helper deals with symbols/strings for you as well.

lib/voxpupuli/test/facts.rb

@@ -0,0 +1,34 @@
+# Override facts
+#
+# This doesn't use deep_merge because that's highly unpredictable. It can merge
+# nested hashes in place, modifying the original. It's also unable to override
+# true to false.
+#
+# A deep copy is obtained by using Marshal so it can be modified in place. Then
+# it recursively overrides values. If the result is a hash, it's recursed into.
+#
+# A typical example:
+#
+# let(:facts) do
+#   override_facts(super(), os: {'selinux' => {'enabled' => false}})
+# end
+def override_facts(base_facts, **overrides)
+  facts = Marshal.load(Marshal.dump(base_facts))
+  apply_overrides!(facts, overrides, false)
+  facts
+end
+
+# A private helper to override_facts
+def apply_overrides!(facts, overrides, enforce_strings)
+  overrides.each do |key, value|
+    # Nested facts are strings
+    key = key.to_s if enforce_strings
+
+    if value.is_a?(Hash)
+      facts[key] = {} unless facts.key?(key)
+      apply_overrides!(facts[key], value, true)
+    else
+      facts[key] = value
+    end
+  end
+end

lib/voxpupuli/test/spec_helper.rb

@@ -24,6 +24,7 @@ def suggest_facter_version
   end
 end
 
+require 'voxpupuli/test/facts'
 require 'puppetlabs_spec_helper/module_spec_helper'
 require 'rspec-puppet-facts'
 include RspecPuppetFacts

spec/facts_spec.rb

@@ -0,0 +1,93 @@
+require 'spec_helper'
+require 'voxpupuli/test/facts'
+
+describe 'override_facts' do
+  let(:base_facts) do
+    {
+      os: {
+        'family' => 'RedHat',
+        'name' => 'CentOS',
+        'release' => {
+          'full' => '7.7.1908',
+          'major' => '7',
+          'minor' => '7'
+        },
+      }
+    }
+  end
+
+  describe 'no overrides' do
+    let(:expected) do
+      {
+        os: {
+          'family' => 'RedHat',
+          'name' => 'CentOS',
+          'release' => {
+            'full' => '7.7.1908',
+            'major' => '7',
+            'minor' => '7'
+          },
+        }
+      }
+    end
+
+    it { expect(override_facts(base_facts)).to eq(expected) }
+  end
+
+  describe 'with addition at the top level' do
+    let(:expected) do
+      {
+        os: {
+          'family' => 'RedHat',
+          'name' => 'CentOS',
+          'release' => {
+            'full' => '7.7.1908',
+            'major' => '7',
+            'minor' => '7'
+          },
+        },
+        ruby: {
+          'sitedir' => '/usr/local/share/ruby/site_ruby',
+        }
+      }
+    end
+
+    it { expect(override_facts(base_facts, ruby: {sitedir: '/usr/local/share/ruby/site_ruby'})).to eq(expected) }
+  end
+
+  describe 'with deep merging' do
+    let(:expected) do
+      {
+        os: {
+          'family' => 'RedHat',
+          'name' => 'CentOS',
+          'release' => {
+            'full' => '7.7.1908',
+            'major' => '7',
+            'minor' => '8'
+          },
+        }
+      }
+    end
+
+    it { expect(override_facts(base_facts, os: {release: {minor: '8'}})).to eq(expected) }
+  end
+
+  describe 'with strings' do
+    let(:expected) do
+      {
+        os: {
+          'family' => 'RedHat',
+          'name' => 'CentOS',
+          'release' => {
+            'full' => '7.7.1908',
+            'major' => '7',
+            'minor' => '8'
+          },
+        }
+      }
+    end
+
+    it { expect(override_facts(base_facts, os: {'release' => {minor: '8'}})).to eq(expected) }
+  end
+end