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

From: [email-masked] (Kévin Le Gouguec)
Date: Mon, 10 Dec 2018 22:02:09 +0100

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.

In GNU Emacs 26.1 (build 1, x86_64-suse-linux-gnu, X toolkit, Xaw3d scroll bars)
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
Recent messages:
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Global key bindings for ‘rg’ enabled with prefix: C-c s
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For information about GNU Emacs and the GNU system, type C-h C-a.

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