bug#2499: 23.0.91; syntax-ppss / beginning-of-defun not saving match data

bug#2499: 23.0.91; syntax-ppss / beginning-of-defun not saving match data

[tcr]

`beginning-of-defun' (and `syntax-ppss' as it invokes the former by
the variable `syntax-begin-function') does not save match data.

The issue also applies to 23.0.60.1, and 22.1.1.

Test case:

  (let ((buffer-sexpr "(defun *HERE*"))
  (with-temp-buffer
    (lisp-mode)
    (insert buffer-sexpr)
    (search-backward "*HERE*")
    (let ((old (match-data)))
      (progn (beginning-of-defun))
      (equal old (match-data)))))

               ==> nil

There are several possible options now:

  (i) `beginning-of-defun' is allowed to not save match data. If so,
      please document it.

  (ii) If (i) is true, still consider whether `syntax-ppss' is allowed
       to not save match data, too. Perhaps it should rather funcall
       `syntax-begin-function' wrapped in a `save-match-data'.
       Whatever the decision, please document it.

Thank you!


In GNU Emacs 23.0.91.1 (i686-pc-linux-gnu, GTK+ Version 2.12.9)
 of 2009-02-27 on thaleron
Windowing system distributor `The X.Org Foundation', version 11.0.10400090
configured using `configure  '--prefix=/home/tcr/software/emacs-cvs' '--with-x-toolkit=gtk' '--without-gpm' '--with-x' '--without-xpm' '--enable-asserts' 'CFLAGS=-g''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default-enable-multibyte-characters: t

Major mode: Lisp Interaction

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

Recent input:
<down> <return> C-x b * <backspace> <return> C-y C-M-x 
C-j <up> <up> <up> <right> <right> <right> <right> 
<right> <right> <right> M-d s a v e - m a t c h - d 
a t a C-e <down> C-j <up> <up> <right> <right> <right> 
<right> <right> <right> <right> <up> C-M-k r <backspace> 
p r o g n M-x r e p o r t - e m a c s - b u g <ret
urn>

Recent messages:
Loading /home/tcr/.emacs.d/config/lisp-config.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
nil

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bug#2499: 23.0.91; syntax-ppss / beginning-of-defun not saving match data

[Stefan Monnier]

>   (i) `beginning-of-defun' is allowed to not save match data. If so,
>       please document it.

99.9% of the functions are allowed to not save match data.  We do not
bother to document it.  We should document the functions that are not
allowed to do it, OTOH.


        Stefan

bug#2499: 23.0.91; syntax-ppss / beginning-of-defun not saving match data

[Tobias C. Rittweiler]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> >   (i) `beginning-of-defun' is allowed to not save match data. If so,
> >       please document it.
>
> 99.9% of the functions are allowed to not save match data.  

Oh, I didn't know that! I thought the policy was the other way
around. (Perhaps 34.6, The Match Data, should make this policy more
explicit. If so, and you say so, I'll prepare a patch.)


> We do not bother to document it.  We should document the functions
> that are not allowed to do it, OTOH.

I think it's still worth to think about making `syntax-ppss' saving the
match data as this function may easily be implicitly invoked by
something deep down.

Thank you for your prompt reply!,

  -T.

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bug#2499: 23.0.91; syntax-ppss / beginning-of-defun not saving match data

[Stefan Monnier]

>> >   (i) `beginning-of-defun' is allowed to not save match data. If so,
>> >       please document it.
>> 99.9% of the functions are allowed to not save match data.  
> Oh, I didn't know that! I thought the policy was the other way
> around. (Perhaps 34.6, The Match Data, should make this policy more
> explicit. If so, and you say so, I'll prepare a patch.)

Patch welcome.

>> We do not bother to document it.  We should document the functions
>> that are not allowed to do it, OTOH.
> I think it's still worth to think about making `syntax-ppss' saving the
> match data as this function may easily be implicitly invoked by
> something deep down.

Basically any function that does a non-trivial amount of work should be
expected to affect the match-data.  I.e. it's the responsability of the
match-data user to make sure she reads the match-data soon after the
regexp match took place, rather than responsability of all other
functions to preserve the match-data.


        Stefan

bug#2499: 23.0.91; syntax-ppss / beginning-of-defun not saving match data

[Tobias C. Rittweiler]

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Stefan Monnier <monnier@iro.umontreal.ca> writes:

> > > >   (i) `beginning-of-defun' is allowed to not save match data. If so,
> > > >       please document it.
> > >
> > > 99.9% of the functions are allowed to not save match data.  
> >
> > Oh, I didn't know that! I thought the policy was the other way
> > around. (Perhaps 34.6, The Match Data, should make this policy more
> > explicit. If so, and you say so, I'll prepare a patch.)
>
> Patch welcome.

Attached.

  -T.


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Index: doc/lispref/searching.texi
===================================================================
RCS file: /sources/emacs/emacs/doc/lispref/searching.texi,v
retrieving revision 1.11
diff -u -r1.11 searching.texi
--- doc/lispref/searching.texi	8 Jan 2009 05:29:01 -0000	1.11
+++ doc/lispref/searching.texi	3 Mar 2009 20:48:30 -0000
@@ -1204,6 +1204,12 @@
 can't avoid another intervening search, you must save and restore the
 match data around it, to prevent it from being overwritten.
 
+  Notice that all functions are allowed to overwrite the match data
+unless they're explicitly documented not to do so. A consequence is that
+functions that are run implictly in the background (@pxref{Timers}, and
+@ref{Idle Timers}) should likely save and restore the match data
+explicitly.
+
 @menu
 * Replacing Match::	  Replacing a substring that was matched.
 * Simple Match Data::     Accessing single items of match data,

bug#2499: 23.0.91; syntax-ppss / beginning-of-defun not saving match data

[Lars Magne Ingebrigtsen]

"Tobias C. Rittweiler" <tcr@freebits.de> writes:

>> Patch welcome.
>
> Attached.

I've now applied your patch to Emacs 24.

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  bloggy blog http://lars.ingebrigtsen.no/