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

From: Eli Zaretskii <[email-masked]>
Date: Tue, 01 Nov 2022 12:24:49 +0200

> From: Nicolas Graner <[email-masked]>
> Date: Tue, 01 Nov 2022 00:20:58 +0100
> 
> (with-current-buffer (generate-new-buffer "test")
>   (insert "123456789")
>   (put-text-property 3 4 'foo 'bar)
>   (goto-char 6)
>   (text-property-search-backward 'foo))
> 
> The returned value is
>   #s(prop-match 1 4 nil)
> instead of
>   #s(prop-match 3 4 bar)
> and the point in the test buffer is moved to position 1 instead of 3.
> 
> This incorrect behavior is the same if you replace (goto-char 6) with
> (goto-char 5) or any other value greater than 4. However, the result is
> correct with (goto-char 4), i.e. when the backward search starts one
> position after the target character. This suggests an off-by-one error
> in the code.

It isn't due to an off-by-one error, AFAICT, it's because search for
changes in text properties starts from character _before_ point.

Please try the patch below and see if it gives good results.

Lars, any comments?

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)