bug#58937: text-property-search-backward misses one-character regions

[Eli Zaretskii <eliz@gnu.org>]

> From: Nicolas Graner <nicolas@graner.name>
> Cc: larsi@gnus.org, 58937@debbugs.gnu.org
> Date: Tue, 01 Nov 2022 17:48:37 +0100
> 
> > Date: Tue, 01 Nov 2022 12:24:49 +0200
> > From: Eli Zaretskii <eliz@gnu.org>
> > 
> > Please try the patch below and see if it gives good results.
> > 
> > diff --git a/lisp/emacs-lisp/text-property-search.el b/lisp/emacs-lisp/text-property-search.el
> > index d11980f..69bbbd2 100644
> > --- a/lisp/emacs-lisp/text-property-search.el
> > +++ b/lisp/emacs-lisp/text-property-search.el
> > @@ -208,8 +208,12 @@ text-property--find-end-backward
> >                  (goto-char end)
> >                  (setq ended t)))))
> >        ;; End this at the first place the property changes value.
> > -      (setq end (previous-single-property-change
> > -                 (point) property nil (point-min)))
> > +      (setq end
> > +            (if (text-property--match-p
> > +                 value (get-text-property (1- (point)) property) predicate)
> > +                (previous-single-property-change (point)
> > +                                                 property nil (point-min))
> > +              (point)))
> >        (goto-char end))
> >      (make-prop-match :beginning end
> >                       :end (1+ start)
> 
> It fixes the bug in all the cases I've tested so far, but also
> introduces a new one:
> 
> (with-current-buffer (generate-new-buffer "test")
>   (insert "123456789")
>   (put-text-property 2 4 'foo 'bar)
>   (goto-char 3)
>   (text-property-search-backward 'foo nil t))
> 
> Debugger entered--Lisp error: (args-out-of-range 0 0)
>   get-text-property(0 foo)

Thanks for testing, how about the following patch instead?

diff --git a/lisp/emacs-lisp/text-property-search.el b/lisp/emacs-lisp/text-property-search.el
index d11980f..f7ef84a 100644
--- a/lisp/emacs-lisp/text-property-search.el
+++ b/lisp/emacs-lisp/text-property-search.el
@@ -208,8 +208,13 @@ text-property--find-end-backward
                 (goto-char end)
                 (setq ended t)))))
       ;; End this at the first place the property changes value.
-      (setq end (previous-single-property-change
-                 (point) property nil (point-min)))
+      (setq end
+            (if (or (= (point) (point-min))
+                    (text-property--match-p
+                     value (get-text-property (1- (point)) property) predicate)
+                    (previous-single-property-change (point)
+                                                     property nil (point-min)))
+              (point)))
       (goto-char end))
     (make-prop-match :beginning end
                      :end (1+ start)