Fix purge of unwanted kernels on DNF based machines

Previously when `installonly_limit` of a node is adjusted then
`package-cleanup` was called to remove unwanted old kernels.
This is invalid for dnf based systems and a new command must be used.

This DNF command will remove kernel packages except in two cases:
* It is the running kernel as determined from `$facts['kernelrelease']`
* It is one of last N kernel versions where N is the `installonly_limit`

Fixes #295

manifests/init.pp

@@ -205,19 +205,22 @@
     default                  => '3',
   }
 
-  $_pc_cmd = [
-    '/usr/bin/package-cleanup',
-    '--oldkernels',
-    "--count=${_real_installonly_limit}",
-    '-y',
-    $keep_kernel_devel ? {
-      true    => '--keepdevel',
-      default => undef,
+  $_keep_kernel_devel = $keep_kernel_devel ? {
+    true    => $facts['package_provider'] ? {
+      'yum'   => '--keepdevel ',
+      'dnf'   => '--exclude kernel-release',
+      default => fail("Fact package_provider is not set to \'yum\' or \'dnf\' - giving up"),
     },
-  ].filter |$val| { $val =~ NotUndef }
+    default => '',
+  }
+
+  $_pc_cmd = $facts['package_provider'] ? {
+    'yum'   => "/usr/bin/package-cleanup --oldkernels --count=${_real_installonly_limit} -y${$_keep_kernel_devel}",
+    default => "/usr/bin/dnf -y remove $(/usr/bin/dnf repoquery --installonly --latest-limit=-${_real_installonly_limit}${_keep_kernel_devel} | /usr/bin/grep -v ${facts['kernelrelease']})"
+  }
 
   exec { 'package-cleanup_oldkernels':
-    command     => shellquote($_pc_cmd),
+    command     => $_pc_cmd,
     refreshonly => true,
     require     => Package[$utils_package_name],
     subscribe   => $_clean_old_kernels_subscribe,

spec/acceptance/yum_config_install_limit_2_spec.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require 'spec_helper_acceptance'
+
+describe 'yum::config installonly_limit 1' do
+  context 'simple parameters' do
+    # Using puppet_apply as a helper
+    it 'must work idempotently with no errors' do
+      pp = <<-EOS
+      yum::config{'installonly_limit':
+        ensure   => 2,   # yum (and not dnf) does not like the value 1
+      }
+      include yum
+      EOS
+      # Run it twice and test for idempotency
+      apply_manifest(pp, catch_failures: true)
+      apply_manifest(pp, catch_changes:  true)
+    end
+  end
+end

spec/classes/init_spec.rb

@@ -10,7 +10,7 @@
 
   it 'contains Exec[package-cleanup_oldkernels' do
     is_expected.to contain_exec('package-cleanup_oldkernels').with(
-      command: "/usr/bin/package-cleanup --oldkernels --count=#{value} -y",
+      command: %r{/usr/bin/dnf -y remove \$\(/usr/bin/dnf repoquery --installonly --latest-limit=-\$\{value\} | /usr/bin/grep -v \S+\)},
       refreshonly: true
     ).that_subscribes_to('Yum::Config[installonly_limit]')
   end
@@ -45,6 +45,20 @@
         it { is_expected.to have_yumrepo_resource_count(0) }
       end
 
+      context 'with package_provider yum' do
+        let(:facts) do
+          facts.merge({ package_provider: 'yum' })
+        end
+
+        it { is_expected.to compile.with_all_deps }
+
+        it {
+          is_expected.to contain_exec('package-cleanup_oldkernels').with(
+            command: '/usr/bin/package-cleanup --oldkernels --count=3 -y'
+          )
+        }
+      end
+
       context 'when `manage_os_default_repos` is enabled' do
         let(:params) { { 'manage_os_default_repos' => true } }