Add a function to compare the OS version (#972)

* Add a function to compare the OS version

This function aims to reduce the boiler plate that a lot of modules have
to compare versions:

    if $facts['operatingsystem'] == 'Ubuntu' && versioncmp(facts['operatingsystemmajrelease'], '16.04') >= 0 {

Can now be reduced to:

    if os_version_gte('Ubuntu', '16.04') {

* Readme Updated

* Fix typo in README

* Readme format update

Author: ekohl

README.md

@@ -1997,6 +1997,20 @@ function in Puppet for the many available type conversions.
 
 *Type*: rvalue.
 
+#### `os_version_gte`
+
+Checks to see if the OS version is at least a certain version. Note that only the major version is taken into account.
+
+Example usage:
+```
+  if os_version_gte('Debian', '9') { }
+  if os_version_gte('Ubuntu', '18.04') { }
+```
+
+Returns:
+  - Boolean(0) # When OS is below the given version.
+  - Boolean(1) # When OS is equal to or greater than the given version.
+
 #### `parsejson`
 
 Converts a string of JSON into the correct Puppet structure (as a hash, array, string, integer, or a combination of such).

lib/puppet/functions/os_version_gte.rb

@@ -0,0 +1,20 @@
+# Checks if the OS version is at least a certain version. Note that only the
+# major version is taken into account.
+#
+# Example usage:
+#
+#     if os_version_gte('Debian', '9') { }
+#     if os_version_gte('Ubuntu', '18.04') { }
+Puppet::Functions.create_function(:os_version_gte) do
+  dispatch :os_version_gte do
+    param 'String[1]', :os
+    param 'String[1]', :version
+    return_type 'Boolean'
+  end
+
+  def os_version_gte(os, version)
+    facts = closure_scope['facts']
+    (facts['operatingsystem'] == os &&
+     Puppet::Util::Package.versioncmp(version, facts['operatingsystemmajrelease']) >= 0)
+  end
+end

spec/functions/os_version_gte_spec.rb

@@ -0,0 +1,33 @@
+require 'spec_helper'
+
+describe 'os_version_gte' do
+  context 'on Debian 9' do
+    let(:facts) do
+      {
+        :operatingsystem => 'Debian',
+        :operatingsystemmajrelease => '9',
+      }
+    end
+
+    it { is_expected.to run.with_params('Debian', '9').and_return(true) }
+    it { is_expected.to run.with_params('Debian', '8').and_return(false) }
+    it { is_expected.to run.with_params('Debian', '8.0').and_return(false) }
+    it { is_expected.to run.with_params('Ubuntu', '16.04').and_return(false) }
+    it { is_expected.to run.with_params('Fedora', '29').and_return(false) }
+  end
+
+  context 'on Ubuntu 16.04' do
+    let(:facts) do
+      {
+        :operatingsystem => 'Ubuntu',
+        :operatingsystemmajrelease => '16.04',
+      }
+    end
+
+    it { is_expected.to run.with_params('Debian', '9').and_return(false) }
+    it { is_expected.to run.with_params('Ubuntu', '16').and_return(false) }
+    it { is_expected.to run.with_params('Ubuntu', '16.04').and_return(true) }
+    it { is_expected.to run.with_params('Ubuntu', '18.04').and_return(true) }
+    it { is_expected.to run.with_params('Fedora', '29').and_return(false) }
+  end
+end