bug#81395: 32.0.50; diff-apply-hunk should be case-sensitive

Stefan Monnier <[email protected]> writes:

>> Fair. Especially considering my reproducer. However, I ran into case
>> sensitivity issue when the diff was on a single file, without any
>> ambiguity.
>>
>> I can try to produce a cleaner reproducer, but I think it is clear that
>> the code does case-insensitive regexp search.
>
> Indeed the matching is case-insensitive.  I remember making this
> decision consciously.  🙂
> I'm OK if you make it case-sensitive, tho.

See the attached patch.

>> 2. Create test.diff
>> diff -u /tmp/test.txt /tmp/test2.txt
>> --- /tmp/test.txt	2026-07-26 11:42:55.175090850 +0200
>> +++ /tmp/test.txt	2026-07-26 11:43:11.705090601 +0200
>> @@ -6,4 +6,4 @@
>>  
>>  bar
>>  
>> -This is test.
>> +This is test2.
>>
>> Diff finished.  Sun Jul 26 11:43:23 2026
>>
>> 3. emacs -Q /tmp/test.diff
>> 4. C-c C-a
>> 5. Observe "hunk already applied".
>
> Hmm... so single-stepping through `diff-find-source-location` shows that
> it finds both the "old" text and the "new" text, so it sees that the
> patch can be applied *and* it can be reverse-applied.
> Then it uses the funny:
>
>     (and maybe-new maybe-old (null reverse) (setq switched t) maybe-new)
>
> to decide that it should say "hunk already applied" (which it does by
> setting SWITCHED to t).  I can't remember enough what REVERSE means
> here, but it looks like a plain bug: either we should test `reverse`
> rather than `(null reverse)` or maybe the caller should pass `(not
> reverse)` or something.
>
> I guess we could also check the "offset" between the diff's line-number
> and the place where we found the corresponding text to see which of OLD
> or NEW is closest to what the hunk describes (if it's exactly at the
> right place, it's more likely to be what was meant), but if we apply
> hunks one by one starting from the beginning, then the line numbers end
> up not matching exactly anyway (because of the changes made from previous
> hunks), so distrusting approximate matches is not a panacea.

May we keep track of the applied hunks in some way? For example,
applying hunks can be recorded, so that line offsets can be tracked.


-- 
Ihor Radchenko // yantar92,
Org mode maintainer,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
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From 15f478218ebc9245caffe86916b0f1c7543aaf86 Mon Sep 17 00:00:00 2001
Message-ID: <15f478218ebc9245caffe86916b0f1c7543aaf86.1785264195.git.yantar92@posteo.net>
From: Ihor Radchenko <[email protected]>
Date: Tue, 28 Jul 2026 20:42:30 +0200
Subject: [PATCH] lisp/vc/diff-mode.el (diff-find-text): Match case-sensitively
 (bug#81395)

---
 lisp/vc/diff-mode.el | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index c1227573351..89052dbc7cd 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -2069,6 +2069,7 @@ diff-find-text
   "Return the buffer position (BEG . END) of the nearest occurrence of TEXT.
 If TEXT isn't found, nil is returned."
   (let* ((orig (point))
+         (case-fold-search nil)
 	 (forw (and (search-forward text nil t)
 		    (cons (match-beginning 0) (match-end 0))))
 	 (back (and (goto-char (+ orig (length text)))
-- 
2.54.0