GNU bug report logs - #19016
24.4.50; tq.el does the wrong thing when enqueuing inside a callback

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Andreas Politz <politza <at> hochschule-trier.de>
To: bug-gnu-emacs <at> gnu.org
Subject: 24.4.50; tq.el does the wrong thing when enqueuing inside a callback
Date: Tue, 11 Nov 2014 04:48:31 +0100

[Message part 1 (text/plain, inline)]

tq calls the wrong callback in the some situations.  The function
tq-process-buffer contains the following snippet.

(unwind-protect
    (condition-case nil
        (funcall (tq-queue-head-fn tq)
                 (tq-queue-head-closure tq)
                 answer)
      (error nil))
  (tq-queue-pop tq))

tq-queue-pop removes the top question and sends the next one.  But here
a problem occurs, if the callback function adds another question, sends
it and waits for it's result.  Because this invokes tq-process-buffer
recursively with the first question still at top.  Resulting in the
callback to the first question called a second time, third time ... .

Here is a lisp file which should demonstrate this.

[Message part 2 (application/emacs-lisp, attachment)]

[Message part 3 (text/plain, inline)]

..and a patch

[Message part 4 (text/x-diff, inline)]

=== modified file 'lisp/emacs-lisp/tq.el'
*** lisp/emacs-lisp/tq.el	2014-02-10 01:34:22 +0000
--- lisp/emacs-lisp/tq.el	2014-11-11 03:39:31 +0000
***************
*** 153,167 ****
  		     (buffer-name buf)))
  	  (goto-char (point-min))
  	  (if (re-search-forward (tq-queue-head-regexp tq) nil t)
! 	      (let ((answer (buffer-substring (point-min) (point))))
  		(delete-region (point-min) (point))
! 		(unwind-protect
! 		    (condition-case nil
! 			(funcall (tq-queue-head-fn tq)
! 				 (tq-queue-head-closure tq)
! 				 answer)
! 		      (error nil))
! 		  (tq-queue-pop tq))
  		(tq-process-buffer tq))))))))
  
  (provide 'tq)
--- 153,167 ----
  		     (buffer-name buf)))
  	  (goto-char (point-min))
  	  (if (re-search-forward (tq-queue-head-regexp tq) nil t)
! 	      (let ((answer (buffer-substring (point-min) (point)))
!                     (fn (tq-queue-head-fn tq))
!                     (closure (tq-queue-head-closure tq)))
  		(delete-region (point-min) (point))
! 		(tq-queue-pop tq)
!                 (condition-case err
!                     (funcall fn closure answer)
!                   (error (message "Error while processing tq callback: %s"
!                                   (error-message-string err))))
  		(tq-process-buffer tq))))))))
  
  (provide 'tq)

[Message part 5 (text/plain, inline)]

(Also adds messaging for errors in the callback, which should not get
unnoticed.)

A. Politz





In GNU Emacs 24.4.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.10)
 of 2014-11-01 on luca
Repository revision: 114441 politza <at> fh-trier.de-20140823093555-j5hxuaxih7d5vvxr
Windowing system distributor `The X.Org Foundation', version 11.0.11204000
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Recent messages:
Preloaded Page <39>
Preloaded Page <40>
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Quit
Type "q" in help window to restore its previous buffer.
Mark set
End of buffer [13 times]
Mark set
Starting new Ispell process aspell with en dictionary...
Checking spelling of ENQUEUING... [2 times]

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Message #10 received at 19016 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Andreas Politz <politza <at> hochschule-trier.de>
Cc: 19016 <at> debbugs.gnu.org
Subject: Re: bug#19016: 24.4.50; tq.el does the wrong thing when enqueuing
 inside a callback
Date: Sat, 03 Aug 2019 16:33:54 +0200

Andreas Politz <politza <at> hochschule-trier.de> writes:

> tq-queue-pop removes the top question and sends the next one.  But here
> a problem occurs, if the callback function adds another question, sends
> it and waits for it's result.  Because this invokes tq-process-buffer
> recursively with the first question still at top.  Resulting in the
> callback to the first question called a second time, third time ... .
>
> Here is a lisp file which should demonstrate this.
>
>
> ..and a patch

Makes sense to me.  I've now applied this to Emacs 27.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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