bug#62990: 30.0.50; UDP server closes connection upon receiving an empty packet

[Vasilij Schneidermann <mail@vasilij.de>]

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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

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Memory information:
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[-- Attachment #1.2: bug-client.el --]
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;; -*- 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))

[-- Attachment #1.3: bug-server.el --]
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;; -*- 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))

[-- Attachment #1.4: fixed-client.scm --]
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(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)

[-- Attachment #1.5: fixed-server.scm --]
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(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)))

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