GNU bug report logs - #61091
30.0.50; y-or-n-p clobbers match data

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

From: Richard Stallman <rms <at> gnu.org>
To: bug-gnu-emacs <at> gnu.org
Subject: 30.0.50; y-or-n-p clobbers match data
Date: Thu, 26 Jan 2023 23:05:10 -0500

I find in some of my personal code that calls to y-or-n-p
clobber the match data.  That code worked ok when I first wrote it,
and for several years after that.  Putting (save-match-data...)
around the call to y-or-n-p seems to fix it.

Did some recent change cause y-or-n-p to do searches
and not save the match data?

I will work up a test case for you if necessary, but that
will be a pain.  I hope with this info that work can be avoided.



In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 2.24.32, cairo version 1.16.0) of 2022-12-28 built on freetop
Repository revision: efc44727daaee4d3f9aeb19864074472e99b296a
Repository branch: master
System Description: Trisquel GNU/Linux Nabia (10.0)

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[[[ To any NSA and FBI agents reading my email: please consider    ]]]
[[[ whether defending the US Constitution against all enemies,     ]]]
[[[ foreign or domestic, requires you to follow Snowden's example. ]]]


-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)

Message #8 received at 61091 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: rms <at> gnu.org
Cc: 61091 <at> debbugs.gnu.org
Subject: Re: bug#61091: 30.0.50; y-or-n-p clobbers match data
Date: Fri, 27 Jan 2023 09:52:43 +0200

> From: Richard Stallman <rms <at> gnu.org>
> Date: Thu, 26 Jan 2023 23:05:10 -0500
> 
> I find in some of my personal code that calls to y-or-n-p
> clobber the match data.  That code worked ok when I first wrote it,
> and for several years after that.  Putting (save-match-data...)
> around the call to y-or-n-p seems to fix it.
> 
> Did some recent change cause y-or-n-p to do searches
> and not save the match data?

y-or-n-p originally was implemented in C and was relatively simple.
Since then it was moved to Lisp and became a monster, see subr.el.  It
is anyone's guess where in that code we clobber the match data.  Given
a test case, we could debug and find what does this, but is it really
worth our while?  In general, Lisp programs should not rely on any
function not to clobber match data, unless that function is documented
to preserve match data.

Message #11 received at 61091 <at> debbugs.gnu.org (full text, mbox):

From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 61091 <at> debbugs.gnu.org, rms <at> gnu.org
Subject: Re: bug#61091: 30.0.50; y-or-n-p clobbers match data
Date: Fri, 27 Jan 2023 15:38:14 +0100

Eli Zaretskii <eliz <at> gnu.org> writes:

> y-or-n-p originally was implemented in C and was relatively simple.
> Since then it was moved to Lisp and became a monster, see subr.el.  It
> is anyone's guess where in that code we clobber the match data.

I see that the code calls `substitute-command-keys' which potentially
changes match data.

Michael.

Message #14 received at 61091 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Cc: 61091 <at> debbugs.gnu.org, rms <at> gnu.org
Subject: Re: bug#61091: 30.0.50; y-or-n-p clobbers match data
Date: Fri, 27 Jan 2023 17:00:24 +0200

> From: Michael Heerdegen <michael_heerdegen <at> web.de>
> Cc: rms <at> gnu.org,  61091 <at> debbugs.gnu.org
> Date: Fri, 27 Jan 2023 15:38:14 +0100
> 
> Eli Zaretskii <eliz <at> gnu.org> writes:
> 
> > y-or-n-p originally was implemented in C and was relatively simple.
> > Since then it was moved to Lisp and became a monster, see subr.el.  It
> > is anyone's guess where in that code we clobber the match data.
> 
> I see that the code calls `substitute-command-keys' which potentially
> changes match data.

Yes, that's also my prime suspect.

Message #17 received at 61091 <at> debbugs.gnu.org (full text, mbox):

From: Richard Stallman <rms <at> gnu.org>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 61091 <at> debbugs.gnu.org
Subject: Re: bug#61091: 30.0.50; y-or-n-p clobbers match data
Date: Sun, 29 Jan 2023 00:18:24 -0500

[[[ To any NSA and FBI agents reading my email: please consider    ]]]
[[[ whether defending the US Constitution against all enemies,     ]]]
[[[ foreign or domestic, requires you to follow Snowden's example. ]]]

  >   In general, Lisp programs should not rely on any
  > function not to clobber match data, unless that function is documented
  > to preserve match data.

Allowance should be made for the fact that `y-or-n-p'
was safe for the match data for over 35 years.

-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)

Message #20 received at 61091 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: rms <at> gnu.org
Cc: 61091 <at> debbugs.gnu.org
Subject: Re: bug#61091: 30.0.50; y-or-n-p clobbers match data
Date: Sun, 29 Jan 2023 08:40:39 +0200

> From: Richard Stallman <rms <at> gnu.org>
> Cc: 61091 <at> debbugs.gnu.org
> Date: Sun, 29 Jan 2023 00:18:24 -0500
> 
>   >   In general, Lisp programs should not rely on any
>   > function not to clobber match data, unless that function is documented
>   > to preserve match data.
> 
> Allowance should be made for the fact that `y-or-n-p'
> was safe for the match data for over 35 years.

I don't object if it's easy to do so.  But we decided long ago that
significant efforts for that purpose are not worth our while, and even
documented this in the ELisp reference manual.

Message #23 received at 61091 <at> debbugs.gnu.org (full text, mbox):

From: Richard Stallman <rms <at> gnu.org>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 61091 <at> debbugs.gnu.org
Subject: Re: bug#61091: 30.0.50; y-or-n-p clobbers match data
Date: Sat, 04 Feb 2023 00:15:24 -0500

[[[ To any NSA and FBI agents reading my email: please consider    ]]]
[[[ whether defending the US Constitution against all enemies,     ]]]
[[[ foreign or domestic, requires you to follow Snowden's example. ]]]

  > > Allowance should be made for the fact that `y-or-n-p'
  > > was safe for the match data for over 35 years.

  > I don't object if it's easy to do so.

I think it should be easy.  How about trying it?

-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)

Message #28 received at 61091-done <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: rms <at> gnu.org
Cc: 61091-done <at> debbugs.gnu.org
Subject: Re: bug#61091: 30.0.50; y-or-n-p clobbers match data
Date: Sat, 04 Feb 2023 10:16:54 +0200

> From: Richard Stallman <rms <at> gnu.org>
> Cc: 61091 <at> debbugs.gnu.org
> Date: Sat, 04 Feb 2023 00:15:24 -0500
> 
>   > > Allowance should be made for the fact that `y-or-n-p'
>   > > was safe for the match data for over 35 years.
> 
>   > I don't object if it's easy to do so.
> 
> I think it should be easy.  How about trying it?

Done with the patch below.

diff --git a/lisp/subr.el b/lisp/subr.el
index 0f754fc..32c9974 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -3573,12 +3573,14 @@ y-or-n-p
 			    (if (or (zerop l) (eq ?\s (aref prompt (1- l))))
 				"" " ")
 			    (if dialog ""
-                              (substitute-command-keys
-                               (if help-form
-                                   (format "(\\`y', \\`n' or \\`%s') "
-                                           (key-description
-                                            (vector help-char)))
-                                 "(\\`y' or \\`n') ")))))))
+                              ;; Don't clobber caller's match data.
+                              (save-match-data
+                                (substitute-command-keys
+                                 (if help-form
+                                     (format "(\\`y', \\`n' or \\`%s') "
+                                             (key-description
+                                              (vector help-char)))
+                                   "(\\`y' or \\`n') "))))))))
         ;; Preserve the actual command that eventually called
         ;; `y-or-n-p' (otherwise `repeat' will be repeating
         ;; `exit-minibuffer').

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