drb: close graceful shutdown pipe before socket

Closing a listen socket while entering select(2) may
trigger IOError or even deadlock because another thread
may give the file descriptor to another file description;
meaning the kernel can wait on the wrong description.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64532 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/drb/drb.rb

@@ -953,6 +953,7 @@ module DRb
     # returned by #open or by #accept, then it closes this particular
     # client-server session.
     def close
+      shutdown
       if @socket
         @socket.close
         @socket = nil

lib/drb/unix.rb

@@ -95,6 +95,7 @@ module DRb
     public
     def close
       return unless @socket
+      shutdown # DRbProtocol#shutdown
       path = @socket.path if @server_mode
       @socket.close
       File.unlink(path) if @server_mode