Do not raise an exception on a closed DRb socket

This rescues some exceptions that could happen with a closed or
shutdown DRb socket. This can prevent the server from
exiting if an client socket is closed directly after it is
accepted.

Fixes [Bug #8039]

lib/drb/drb.rb

@@ -1024,6 +1024,8 @@ module DRb
 
     def set_sockopt(soc) # :nodoc:
       soc.setsockopt(Socket::IPPROTO_TCP, Socket::TCP_NODELAY, 1)
+    rescue IOError, Errno::ECONNRESET, Errno::EINVAL
+      # closed/shutdown socket, ignore error
     end
   end
 

test/drb/test_drb.rb

@@ -327,4 +327,19 @@ class TestBug4409 < Test::Unit::TestCase
   end
 end
 
+class TestDRbTCP < Test::Unit::TestCase
+  def test_immediate_close
+    server = DRb::DRbServer.new('druby://:0')
+    host, port, = DRb::DRbTCPSocket.send(:parse_uri, server.uri)
+    socket = TCPSocket.open host, port
+    socket.shutdown
+    socket.close
+    client = DRb::DRbTCPSocket.new(server.uri, socket)
+    assert client
+    client.close
+    server.stop_service
+    server.thread.join
+  end
+end
+
 end