os: add kernel_version

Library/Homebrew/extend/os/linux/diagnostic.rb

@@ -100,7 +100,7 @@ module Homebrew
         return unless OS::Linux::Kernel.below_minimum_version?
 
         <<~EOS
-          Your Linux kernel #{OS::Linux::Kernel.version} is too old.
+          Your Linux kernel #{OS.kernel_version} is too old.
           We only support kernel #{OS::Linux::Kernel.minimum_version} or later.
           You will be unable to use binary packages (bottles).
           #{please_create_pull_requests}

Library/Homebrew/os.rb

@@ -13,6 +13,10 @@ module OS
     RbConfig::CONFIG["host_os"].include? "linux"
   end
 
+  def self.kernel_version
+    @kernel_version ||= Version.new(Utils.safe_popen_read("uname", "-r").chomp)
+  end
+
   ::OS_VERSION = ENV["HOMEBREW_OS_VERSION"]
 
   if OS.mac?

Library/Homebrew/os/linux/kernel.rb

@@ -5,21 +5,12 @@ module OS
     module Kernel
       module_function
 
-      def version
-        return @version if @version
-
-        version = Utils.popen_read("uname", "-r").chomp
-        return Version::NULL unless version
-
-        @version = Version.new version
-      end
-
       def minimum_version
         Version.new "2.6.32"
       end
 
       def below_minimum_version?
-        version < minimum_version
+        OS.kernel_version < minimum_version
       end
     end
   end

Library/Homebrew/test/os/os_spec.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+describe OS do
+  describe ".kernel_version" do
+    it "is not empty" do
+      expect(described_class.kernel_version).not_to be_empty
+    end
+  end
+end