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Subject: bug#62990: 30.0.50; UDP server closes connection upon receiving an empty packet
Resent-From: Vasilij Schneidermann <mail@HIDDEN>
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Date: Fri, 21 Apr 2023 12:36:52 +0200
From: Vasilij Schneidermann <mail@HIDDEN>
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--u2JkDqnouQE3X9SG
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I'm doing network programming with Emacs Lisp and one of the tasks was
implementing a UDP server reading individual packets. I've found that
when a client sends an empty packet, the server hangs up with a
connection error and no longer accepts subsequent packets. This seems to
be reasonable behavior for TCP (from what I understand, empty TCP
messages are a sign of the connection being closed), but not for
UDP. I've attached test programs written in Emacs Lisp and Guile to
reproduce both correctly and incorrectly functioning servers and clients
respectively. One minor nitpick is that sending an empty UDP packet from
Emacs Lisp doesn't work either, but I'm unsure whether this is a bug
(socat behaves similarly).


In GNU Emacs 30.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version
 3.24.37, cairo version 1.17.8) of 2023-04-15 built on odonien
Repository revision: c60b59e04c3776a90adaa8c8fe53af3075a833b8
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101007
System Description: Arch Linux

Configured using:
 'configure 'CFLAGS=3D-g -ggdb -O0''

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY INOTIFY
PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS
TREE_SITTER WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

Important settings:
  value of $LC_MESSAGES:=20
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
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  global-font-lock-mode: t
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Load-path shadows:
None found.

Features:
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emacs)

Memory information:
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--u2JkDqnouQE3X9SG
Content-Type: text/plain; charset=utf-8
Content-Disposition: attachment; filename="bug-client.el"

;; -*- lexical-binding: t; -*-

(defun make-client ()
  (make-network-process
   :name "bug-client"
   :type 'datagram
   :host "127.0.0.1"
   :service 10000
   :family 'ipv4
   :coding 'binary))

(defvar client (make-client))

(process-send-string client "hello")
(process-send-string client "")
(process-send-string client "world")

(while t
  (accept-process-output nil 0.01))

--u2JkDqnouQE3X9SG
Content-Type: text/plain; charset=utf-8
Content-Disposition: attachment; filename="bug-server.el"

;; -*- lexical-binding: t; -*-

(defun server-filter (process string)
  (message "Received string (%s): %S" (length string) string))

(defun server-sentinel (process event)
  (message "Received event: %S" event)
  (when (eq (process-status process) 'closed)
    (delete-process process)
    (kill-buffer (process-buffer process))))

(defun make-server ()
  (make-network-process
   :name "bug-server"
   :type 'datagram
   :server t
   :host "127.0.0.1"
   :service 10000
   :family 'ipv4
   :coding 'binary
   :filter #'server-filter
   :sentinel #'server-sentinel))

(defvar server (make-server))

(while t
  (accept-process-output nil 0.01))

--u2JkDqnouQE3X9SG
Content-Type: text/plain; charset=utf-8
Content-Disposition: attachment; filename="fixed-client.scm"

(import (rnrs bytevectors))

(define client (socket PF_INET SOCK_DGRAM 0))
(connect client AF_INET (inet-pton AF_INET "127.0.0.1") 10000)

(send client (string->utf8 "hello"))
(send client (string->utf8 ""))
(send client (string->utf8 "world"))
(close client)

--u2JkDqnouQE3X9SG
Content-Type: text/plain; charset=utf-8
Content-Disposition: attachment; filename="fixed-server.scm"

(use-modules (ice-9 format))

(import (rnrs bytevectors))
(import (rnrs bytevectors gnu))
(import (srfi 18))

(define server (socket PF_INET SOCK_DGRAM 0))
(bind server AF_INET (inet-pton AF_INET "127.0.0.1") 10000)

(define buffer (make-u8vector 1024))

(let loop ()
  (let* ((len (car (recvfrom! server buffer)))
         (message (utf8->string (bytevector-slice buffer 0 len))))
    (format #t "Received string (~a): ~s~%" len message)
    (loop)))

--u2JkDqnouQE3X9SG--

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Vasilij Schneidermann <mail@HIDDEN>
Subject: bug#62990: Acknowledgement (30.0.50; UDP server closes connection
 upon receiving an empty packet)
Message-ID: <handler.62990.B.168207343618579.ack <at> debbugs.gnu.org>
References: <ZEJnRHE7R4hWSwWl@odonien>
X-Gnu-PR-Message: ack 62990
X-Gnu-PR-Package: emacs
Reply-To: 62990 <at> debbugs.gnu.org
Date: Fri, 21 Apr 2023 10:38:02 +0000

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--=20
62990: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=3D62990
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Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#62990: 30.0.50; UDP server closes connection upon receiving an empty packet
Resent-From: Robert Pluim <rpluim@HIDDEN>
Original-Sender: "Debbugs-submit" <debbugs-submit-bounces <at> debbugs.gnu.org>
Resent-CC: bug-gnu-emacs@HIDDEN
Resent-Date: Mon, 24 Apr 2023 08:52:01 +0000
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To: Vasilij Schneidermann <mail@HIDDEN>
Cc: 62990 <at> debbugs.gnu.org
From: Robert Pluim <rpluim@HIDDEN>
In-Reply-To: <ZEJnRHE7R4hWSwWl@odonien> (Vasilij Schneidermann's message of
 "Fri, 21 Apr 2023 12:36:52 +0200")
References: <ZEJnRHE7R4hWSwWl@odonien>
Date: Mon, 24 Apr 2023 10:51:06 +0200
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>>>>> On Fri, 21 Apr 2023 12:36:52 +0200, Vasilij Schneidermann <mail@vasil=
ij.de> said:

    Vasilij> I'm doing network programming with Emacs Lisp and one of the t=
asks was
    Vasilij> implementing a UDP server reading individual packets. I've fou=
nd that
    Vasilij> when a client sends an empty packet, the server hangs up with a
    Vasilij> connection error and no longer accepts subsequent packets. Thi=
s seems to
    Vasilij> be reasonable behavior for TCP (from what I understand, empty =
TCP
    Vasilij> messages are a sign of the connection being closed), but
    Vasilij> not for UDP.

Empty TCP messages are perfectly valid, but they should be hidden from
you. recvfrom returning 0 means the connection has been closed, but
that=CA=BCs a separate thing.

Empty UDP messages are also valid, and are indicated by recvfrom returning
0.

Could you show how you=CA=BCre generating the empty packets?

    Vasilij> I've attached test programs written in Emacs Lisp and Guile to
    Vasilij> reproduce both correctly and incorrectly functioning servers a=
nd clients
    Vasilij> respectively. One minor nitpick is that sending an empty UDP p=
acket from
    Vasilij> Emacs Lisp doesn't work either, but I'm unsure whether this is=
 a bug
    Vasilij> (socat behaves similarly).

It=CA=BCs allowed by the protocol. I guess it could be useful for people
wanting to implement their own keep-alive protocol over UDP.

Robert
--=20

Last modified: Mon, 24 Apr 2023 09:00:02 UTC

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