default.mspec: defer job tokens

* spec/default.mspec (MSpecScript::JobServer#cores): defer getting
  job tokens until required.  when running only one spec file,
  `-j` option has no effect.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58282 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/default.mspec

@@ -25,27 +25,40 @@ class MSpecScript
   ]
 end
 
-class MSpecScript
+module MSpecScript::JobServer
+  def cores
     if /(?:\A|\s)--jobserver-(?:auth|fds)=(\d+),(\d+)/ =~ ENV["MAKEFLAGS"]
+      cores = 0
       begin
         r = IO.for_fd($1.to_i(10), "rb", autoclose: false)
         w = IO.for_fd($2.to_i(10), "wb", autoclose: false)
-    rescue
-      r.close if r
-    else
         jobtokens = r.read_nonblock(1024)
+      else
         cores = jobtokens.size
         if cores > 0
           jobserver = w
+          w = nil
           at_exit {
             jobserver.print(jobtokens)
             jobserver.close
           }
-      end
+          MSpecScript::JobServer.module_eval do
             remove_method :cores
             define_method(:cores) do
               cores
             end
           end
+          return cores
+        end
+      ensure
+        r&.close
+        w&.close
+      end
+    end
+    super
   end
 end
+
+class MSpecScript
+  prepend JobServer
+end