[ruby-core:27388] [Bug #2554] Net::FTP should not use MSG_OOB

Bug #2554: Net::FTP should not use MSG_OOB
http://redmine.ruby-lang.org/issues/show/2554

Author: Hongli Lai
Status: Open, Priority: Normal
ruby -v: ruby 1.8.7 (2009-12-24 patchlevel 248) [x86_64-linux]

Net::FTP#abort, #status etc currently send commands with MSG_OOB. This causes a freeze when running RubySpec on x86_64 Linux (Ubuntu 9.10):

  hongli@ubuntu:~/rubyspec$ ./mspec/bin/mspec library/net/ftp/abort_spec.rb -e "Net::FTP#abort sends the ABOR command to the server"
  (watch it freeze)

This particular test sets up a dummy FTP server, connects to it with Net::FTP, and then calls Net::FTP#abort. Net::FTP#abort freezes while waiting for the FTP server response.

The dummy FTP server has a very simple and naive implementation. Its command handling main loop looks like this:

  loop do
    command = @socket.recv(1024)
    if command is a QUIT command
      break
    else
      handle_command(command)
    end
  end

The problem is that MSG_OOB only sends the last byte of the data as urgent data; the rest is sent as regular data. From http://popcnt.org/2007/07/what-happened-to-tcp-flag-urgent-msgoob.html:

  Usually when socket receives tcp packet with URG flag it treats it as normal tcp data. recv() is going to read urgent data as it was normal tcp stream. The only difference is that the last byte of data is discarded. The last byte in urgent data was always a problem due to incoherent rfc.

Net::FTP#abort calls @sock.send("ABOR\r\n", Socket::MSG_OOB). This causes the server to receive "ABOR\r" (notice the lack of "\n"); the "\r" is discard from the normal TCP stream, and can only be read if the server reads the socket with MSG_OOB. However, after having read "ABOR\r", select() on x86_64 Linux still indicates that the socket is readable. This causes the server to call recv() without MSG_OOB on the socket again, and this recv() freezes because it cannot read the urgent data.

The FTP protocol description does not seem to make any mention that the ABOR command should be sent with MSG_OOB. This makes be believe that Net::FTP is bugged, not the server.

This freeze seems to be operating system specific and configure flag specific:
- The freeze occurs on Ruby 1.8.7-p248 on x86_64 Linux, when compiled from source with no extra configure flags.
- The freeze does not occur on the default Ruby as packaged by x86_64 Ubuntu 9.10. This is Ruby 1.8.7-p174, and I believe configured with --enable-pthreads --enable-shared.
- The freeze does not occur on OS X Snow Leopard.


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