[racket] Possible bug involving sync on tcp ports

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Sun Sep 14 08:56:52 EDT 2014

It's possible that something is wrong with `sync` and TCP ports ---
although the test suite at least includes that combination --- but can
you provide a complete program?

For example, I'm interested in what `handle-rdy-to-send` does. In the
part that you show, the function will be triggered when one byte is
available on stdin. If `handle-rdy-to-send` tries to read more bytes
than are immediately available, then it can block; i.e., the blocking
part could be in `handle-rdy-to-send`, not in `sync`. That would also
explain why using `sync/timeout` doesn't change anything.

Again, that's just an example of what might be wrong other than `sync`.
A complete program would let us sort out the possibilities.

At Sun, 14 Sep 2014 06:57:34 -0500, Brett Stahlman wrote:
> Hello,
> I have the following loop in a tcp client program that sends forms typed by
> user at the terminal to a listening server, and receives such forms typed by
> other clients (broadcast to all clients by the server).
> (define server-msg-evt (wrap-evt in handle-server-msg))
> (define rdy-to-send-evt (wrap-evt (standard-input-port) handle-rdy-to-send))
> (let loop ()
>   (sync rdy-to-send-evt server-msg-evt)
>   (loop))
> The problem is that the server-msg-evt isn't reliably ready when the server
> flushes a message. I know the messages are being sent and flushed properly
> by
> the server because if I type a new form on stdin and hit enter, I see the
> following sequence:
> 1. Client processes new stdin form in handle-rdy-to-send
> 2. Client loops and calls sync
> 3. The server-msg-evt is *immediately* ready, and client handles the
>    message in handle-server-msg.
> Note: The message handled in step 3 may have been sent long before, but
> the client remained blocked in sync until the (unrelated) stdin event
> unblocked it.
> The really weird part is that the client blocks in this manner even if I
> change the sync to a sync/timeout!
> Any clue as to what could be going on here? I'm running both client and
> server
> in a DOS box on a Windows 7 machine.
> Thanks,
> Brett Stahlman
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