CODEWITHSUNDEEP
pythontcp
vrfrom07
vrfrom07

Reputation: 59

Python - Read remaining data from socket after TCP RST

I'm implementing a file transfer protocol with the following use case:

What happened in that case on server side (Python running on Windows) is that I catch a ConnectionResetException (this is normal, the client has disconnected the socket) while sending the data to the client. I would want to read the latest data sent by the client (the message used to abort the call), but calling mysocket.recv() still raises a ConnectionResetException.

With a wireshark capture, I can clearly see that the message was properly sent by the client prior to TCP disonnection.

Any idea floks? Thanks!

VR

Upvotes: 1

Views: 633

Answers (1)

JimD.
JimD.

Reputation: 2403

In order to understand what to do about this situation, you need to understand how a TCP connection is closed (see, e.g. this) and how the socket API relates to a clean shutdown (without fail, see this).

Your client is most likely calling close to terminate the connection. The problem with this is that there may be unread data in the socket receive queue or data arriving shortly from the other end that you will no longer be able to read, which is basically an error condition. To signal to the other end that data sent cannot be delivered to the receiving application, a reset is sent (well, technically, "SHOULD be sent" as per the RFC) and the TCP connection is abnormally terminated.

You might think that enabling SO_LINGER will help (many, many bits have been spilt over this so I won't elaborate further), but it won't solve the problem of unread data by the client causing the reset.

The client needs to instead call shutdown(SHUT_WR) to indicate that it is done sending, and then continue to call recv() until it reads 0 bytes indicating the other side is done sending. You may then call close().

Note that the Python 2 socket documentation states that

Depending on the platform, shutting down one half of the connection can also close the opposite half (e.g. on Mac OS X, shutdown(SHUT_WR) does not allow further reads on the other end of the connection).

This sounds like a bug to me. To get around this, you would have to send your cancel message, then keep reading until you get 0 bytes so that you know the server received the cancel message. You may then close the socket.

The Python 3.8 docs make no such disclaimer.

Upvotes: 1

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