GNU bug report logs - #30205
27.0.50; Minor mode commands enable the minor mode even if the body fails

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

From: Philipp Stephani <p.stephani2 <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 27.0.50;
 Minor mode commands enable the minor mode even if the body fails
Date: Mon, 22 Jan 2018 10:54:56 +0100

In *scratch*:

(define-minor-mode foo-mode nil nil nil nil (error "what"))

C-h v foo-mode shows that foo-mode is nil, as it should be.
But after M-x foo-mode, it is t, even though the mode command failed.
This can be confusing for mode commands that can conditionally fail,
e.g. depending on some external property.


In GNU Emacs 27.0.50 (build 3, x86_64-pc-linux-gnu, GTK+ Version 3.22.24)
 of 2018-01-22 built on localhost
Repository revision: d0fd9809d7574c67a181225fcc1c59afdbb1295c
Windowing system distributor 'The X.Org Foundation', version 11.0.11903000
System Description: Debian GNU/Linux

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --without-threads --enable-gcc-warnings=warn-only
 --enable-gtk-deprecation-warnings --without-pop --with-mailutils
 --enable-checking --enable-check-lisp-object-type --with-modules
 'CFLAGS=-O0 -ggdb3''

Configured features:
XPM JPEG TIFF GIF PNG SOUND DBUS GSETTINGS NOTIFY GNUTLS FREETYPE XFT
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 MODULES JSON

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

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 95245 8904)
 (symbols 48 20322 1)
 (miscs 40 41 121)
 (strings 32 28332 1955)
 (string-bytes 1 756515)
 (vectors 16 14131)
 (vector-slots 8 499256 13524)
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Message #8 received at 30205 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Philipp Stephani <p.stephani2 <at> gmail.com>
Cc: 30205 <at> debbugs.gnu.org
Subject: Re: bug#30205: 27.0.50; Minor mode commands enable the minor mode
 even if the body fails
Date: Sun, 14 Jul 2019 19:40:59 +0200

Philipp Stephani <p.stephani2 <at> gmail.com> writes:

> In *scratch*:
>
> (define-minor-mode foo-mode nil nil nil nil (error "what"))
>
> C-h v foo-mode shows that foo-mode is nil, as it should be.
> But after M-x foo-mode, it is t, even though the mode command failed.
> This can be confusing for mode commands that can conditionally fail,
> e.g. depending on some external property.

Hm...  OK, this is the function run when saying M-x foo-mode:

       (defun ,modefun (&optional arg ,@extra-args)
         ,(easy-mmode--mode-docstring doc pretty-name keymap-sym)
	 ;; Use `toggle' rather than (if ,mode 0 1) so that using
	 ;; repeat-command still does the toggling correctly.
	 (interactive (list (or current-prefix-arg 'toggle)))
	 (let ((,last-message (current-message)))
           (,@setter
            (if (eq arg 'toggle)
                (not ,getter)
              ;; A nil argument also means ON now.
              (> (prefix-numeric-value arg) 0)))
           ,@body
           ;; The on/off hooks are here for backward compatibility only.
           (run-hooks ',hook (if ,getter ',hook-on ',hook-off))

So `setter' is the thing that sets the mode variable, and then body is
run.

I agree with you that it would be better that the mode variable remains
unchanged if `body' fails.  But I `body' is likely to need to have that
set to work.  We could roll back the value to the previous value on
errors?

Hm.  On the other hand, if `body' has done most of the stuff it needs to
do and fails "late" in the process, then the mode will be in effect even
if it failed, and in that case it would be wrong to roll back.

So I don't know.  Does anybody have an opinion?

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

Message #11 received at 30205 <at> debbugs.gnu.org (full text, mbox):

From: "Basil L. Contovounesios" <contovob <at> tcd.ie>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: Philipp Stephani <p.stephani2 <at> gmail.com>, 30205 <at> debbugs.gnu.org
Subject: Re: bug#30205: 27.0.50; Minor mode commands enable the minor mode
 even if the body fails
Date: Thu, 18 Jul 2019 22:40:11 +0100

Lars Ingebrigtsen <larsi <at> gnus.org> writes:

> Philipp Stephani <p.stephani2 <at> gmail.com> writes:
>
>> In *scratch*:
>>
>> (define-minor-mode foo-mode nil nil nil nil (error "what"))
>>
>> C-h v foo-mode shows that foo-mode is nil, as it should be.
>> But after M-x foo-mode, it is t, even though the mode command failed.
>> This can be confusing for mode commands that can conditionally fail,
>> e.g. depending on some external property.
>
> Hm...  OK, this is the function run when saying M-x foo-mode:
>
>        (defun ,modefun (&optional arg ,@extra-args)
>          ,(easy-mmode--mode-docstring doc pretty-name keymap-sym)
> 	 ;; Use `toggle' rather than (if ,mode 0 1) so that using
> 	 ;; repeat-command still does the toggling correctly.
> 	 (interactive (list (or current-prefix-arg 'toggle)))
> 	 (let ((,last-message (current-message)))
>            (,@setter
>             (if (eq arg 'toggle)
>                 (not ,getter)
>               ;; A nil argument also means ON now.
>               (> (prefix-numeric-value arg) 0)))
>            ,@body
>            ;; The on/off hooks are here for backward compatibility only.
>            (run-hooks ',hook (if ,getter ',hook-on ',hook-off))
>
> So `setter' is the thing that sets the mode variable, and then body is
> run.
>
> I agree with you that it would be better that the mode variable remains
> unchanged if `body' fails.  But I `body' is likely to need to have that
> set to work.  We could roll back the value to the previous value on
> errors?
>
> Hm.  On the other hand, if `body' has done most of the stuff it needs to
> do and fails "late" in the process, then the mode will be in effect even
> if it failed, and in that case it would be wrong to roll back.
>
> So I don't know.  Does anybody have an opinion?

Unless we can provide a way to roll back the entire mode function
"transaction", I would regard errors in mode functions as UB and the
real bug here.  In other words, I don't see any benefit to using the
mode variable as an indicator of success, when mode functions shouldn't
fail to begin with.

-- 
Basil

Message #14 received at 30205 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: "Basil L. Contovounesios" <contovob <at> tcd.ie>
Cc: Philipp Stephani <p.stephani2 <at> gmail.com>, 30205 <at> debbugs.gnu.org
Subject: Re: bug#30205: 27.0.50; Minor mode commands enable the minor mode
 even if the body fails
Date: Fri, 19 Jul 2019 15:25:16 +0200

"Basil L. Contovounesios" <contovob <at> tcd.ie> writes:

> Unless we can provide a way to roll back the entire mode function
> "transaction", I would regard errors in mode functions as UB and the
> real bug here.  In other words, I don't see any benefit to using the
> mode variable as an indicator of success, when mode functions shouldn't
> fail to begin with.

I agree, and am closing this bug report.

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

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