Fix runlevel fact on distros without /sbin/runlevel

lib/facter/runlevel.rb

@@ -6,6 +6,24 @@
 Facter.add('runlevel') do
   confine kernel: 'Linux'
   setcode do
-    `"/sbin/runlevel"`.split.last
+    runlevel = nil
+
+    if (runlevel_path = Facter::Core::Execution.which('runlevel'))
+      result = Facter::Core::Execution.exec(runlevel_path)
+      runlevel = result.split.last if result
+    elsif Facter::Core::Execution.which('systemctl')
+      # Map systemd targets to traditional SysV runlevels
+      target_to_runlevel = {
+        'poweroff.target'   => '0',
+        'rescue.target'     => '1',
+        'multi-user.target' => '3',
+        'graphical.target'  => '5',
+        'reboot.target'     => '6',
+      }
+      result = Facter::Core::Execution.exec('systemctl get-default')
+      runlevel = target_to_runlevel[result&.strip]
+    end
+
+    runlevel
   end
 end

spec/unit/facter/runlevel_spec.rb

@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'runlevel' do
+  before :each do
+    Facter.clear
+
+    # mock out Facter method called when evaluating confine for :kernel
+    allow(Facter::Core::Execution).to receive(:exec).with('uname -s').and_return('Linux')
+  end
+
+  context 'when runlevel binary is available' do
+    it 'returns the current runlevel' do
+      expect(Facter::Core::Execution).to receive(:which).with('runlevel').and_return('/sbin/runlevel')
+      expect(Facter::Core::Execution).to receive(:exec).with('/sbin/runlevel').and_return('N 3')
+
+      expect(Facter.fact('runlevel').value).to eq('3')
+    end
+  end
+
+  context 'when runlevel binary is not available' do
+    before :each do
+      expect(Facter::Core::Execution).to receive(:which).with('runlevel').and_return(nil)
+    end
+
+    context 'when systemctl is available' do
+      before :each do
+        expect(Facter::Core::Execution).to receive(:which).with('systemctl').and_return('/usr/bin/systemctl')
+      end
+
+      {
+        'poweroff.target'   => '0',
+        'rescue.target'     => '1',
+        'multi-user.target' => '3',
+        'graphical.target'  => '5',
+        'reboot.target'     => '6',
+      }.each do |target, expected_runlevel|
+        context "when the default target is #{target}" do
+          it "returns runlevel #{expected_runlevel}" do
+            expect(Facter::Core::Execution).to receive(:exec).with('systemctl get-default').and_return("#{target}\n")
+
+            expect(Facter.fact('runlevel').value).to eq(expected_runlevel)
+          end
+        end
+      end
+
+      context 'when the default target is an unmapped target' do
+        it 'returns nil' do
+          expect(Facter::Core::Execution).to receive(:exec).with('systemctl get-default').and_return("emergency.target\n")
+
+          expect(Facter.fact('runlevel').value).to be nil
+        end
+      end
+    end
+
+    context 'when systemctl is not available' do
+      it 'returns nil' do
+        expect(Facter::Core::Execution).to receive(:which).with('systemctl').and_return(nil)
+
+        expect(Facter.fact('runlevel').value).to be nil
+      end
+    end
+  end
+end