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

[Eli Zaretskii]

> Date: Tue, 01 Nov 2022 19:00:28 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: larsi@gnus.org, 58937@debbugs.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)
> 
> Sorry, please disregard the previous message and use the patch below
> instead:

Not my best day, I guess.  Please use this patch instead of the
previous two:

diff --git a/lisp/emacs-lisp/text-property-search.el 
b/lisp/emacs-lisp/text-property-search.el
index d11980f..d41222b 100644
--- a/lisp/emacs-lisp/text-property-search.el
+++ b/lisp/emacs-lisp/text-property-search.el
@@ -208,8 +208,14 @@ 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 (and (> (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)