bug#33697: 26.1; file-truename messes with match data

bug#33697: 26.1; file-truename messes with match data

[Kévin Le Gouguec]

Hi,

I set frame-title-format so that it displays the basename of the current
project's root folder for the active buffer.  However, my $HOME is under
version control; I do not want this particular "project" to show up in
the frame's title.

Here is the code that I use:

    (require 'subr-x)

    (defun my/project-name ()
      (when-let* ((project (project-current))
                  (root (car (project-roots project))))
        (when (not (file-equal-p root "~"))
          (file-name-nondirectory (string-trim-right root "/")))))

    (setq frame-title-format
          '(:eval
            (let ((project (my/project-name)))
              (concat (when project (format "[%s] " project)) "%b"))))

After adding the above to my .emacs, I started experiencing weird
failures with query-replace.  I managed to reproduce this with emacs -Q:

- evaluate the above
- move to the beginning of *scratch*
- M-% a RET b RET
⇒ perform-replace: Args out of range: #<buffer *scratch*>, 0, 2
  (point has moved after the first match)

I dug a bit and file-equal-p calls file-truename, whose first (cond …)
ends up calling string-match.  Although I did not delve much longer and
could be wrong, I imagine that query-replace finds the first match,
moves point to it, leaves the minibuffer; this causes frame-title-format
to be evaluated; file-equal-p calls file-truename, the global match data
is updated, then perform-replace gets confused about what to do.

Wrapping the (cond …) with (save-match-data …) solves the issue for me
(as does wrapping my/project-name).  I do not know if this is the
preferred way to address this; I do not even know if this is a "bug" (as
in, nobody said file-equal-p does *not* modify match data).


Thank you for your time.



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bug#33697: 26.1; file-truename messes with match data

[Eli Zaretskii]

merge 33697 31586
thanks

> From: kevin.legouguec@gmail.com (Kévin Le Gouguec)
> Date: Mon, 10 Dec 2018 22:02:09 +0100
> 
> I set frame-title-format so that it displays the basename of the current
> project's root folder for the active buffer.  However, my $HOME is under
> version control; I do not want this particular "project" to show up in
> the frame's title.
> 
> Here is the code that I use:
> 
>     (require 'subr-x)
> 
>     (defun my/project-name ()
>       (when-let* ((project (project-current))
>                   (root (car (project-roots project))))
>         (when (not (file-equal-p root "~"))
>           (file-name-nondirectory (string-trim-right root "/")))))
> 
>     (setq frame-title-format
>           '(:eval
>             (let ((project (my/project-name)))
>               (concat (when project (format "[%s] " project)) "%b"))))
> 
> After adding the above to my .emacs, I started experiencing weird
> failures with query-replace.  I managed to reproduce this with emacs -Q:
> 
> - evaluate the above
> - move to the beginning of *scratch*
> - M-% a RET b RET
> ⇒ perform-replace: Args out of range: #<buffer *scratch*>, 0, 2
>   (point has moved after the first match)

This is bug#31586, please see the discussions there.

bug#33697: 26.1; file-truename messes with match data

[Kévin Le Gouguec]

My bad, I assumed this would be a no-brainer and did not bother to
search the bug database.  I will be more diligent next time.

I would argue that this is different, since my code does not explicitly
call string-match; however bug#31586 seems to consider the bigger
picture, so I guess it makes more sense to focus on that one.

Thank you for pointing me towards this discussion!

bug#31586: 27.0.50; `frame-title-format' doesn't save match data

[Philipp]

emacs -Q -eval "(setq frame-title-format '(\"\" (:eval (string-match \".\" \"a\"))))"

Then go to the first character in the scratch buffer (M-<), and run

M-% a RET b RET

The first time you attempt to replace something, Emacs will signal an
error:

perform-replace: Args out of range: #<buffer *scratch*>, 0, 1

Backtrace:

Debugger entered--Lisp error: (args-out-of-range #<buffer *scratch*> 0 1)
  buffer-substring-no-properties(0 1)
  perform-replace("a" "b" t nil nil nil nil nil nil nil nil)
  query-replace("a" "b" nil nil nil nil nil)
  funcall-interactively(query-replace "a" "b" nil nil nil nil nil)
  call-interactively(query-replace nil nil)
  command-execute(query-replace)

Apparently the `string-match' in `frame-title-format' has overwritten
the match data.  This worked in Emacs 24.5.  It breaks the informal
contract for the match data, namely that any code is free to change it,
and code that relies on the match data staying intact needs to protect
it.


In GNU Emacs 27.0.50 (build 69, x86_64-apple-darwin17.4.0, NS appkit-1561.20 Version 10.13.3 (Build 17D102))
 of 2018-05-22 built on p
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bug#33697: 26.1; file-truename messes with match data

[Lars Ingebrigtsen]

Philipp <p.stephani2@gmail.com> writes:

> emacs -Q -eval "(setq frame-title-format '(\"\" (:eval (string-match \".\" \"a\"))))"
>
> Then go to the first character in the scratch buffer (M-<), and run
>
> M-% a RET b RET
>
> The first time you attempt to replace something, Emacs will signal an
> error:
>
> perform-replace: Args out of range: #<buffer *scratch*>, 0, 1
>
> Backtrace:
>
> Debugger entered--Lisp error: (args-out-of-range #<buffer *scratch*> 0 1)
>   buffer-substring-no-properties(0 1)

(I'm going through old bug reports that unfortunately weren't resolved
at the time.)

I can reproduce this in Emacs 26.1, but not in Emacs 28.1, so I guess
this has been fixed in the years since it was reported, and I'm closing
this bug report.  If you're still seeing the problem in recent Emacs
versions, please respond to the debbugs address and we'll reopen.

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