bug#59108: 28.2; rmail-get-new-mail sets unmodified if no new mail even if buffer modified before

[Eli Zaretskii <eliz@gnu.org>]

> From: Ken Olum <kdo@cosmos.phy.tufts.edu>
> Date: Mon, 07 Nov 2022 11:20:20 -0500
> 
> rmail-get-new-mail-1 sets buffer-modified-p at the beginning to deal
> with locking issues right away.  Then if there is no new mail it clears
> buffer-modified-p.  Thus if the buffer was modified already it gets
> marked unmodified without being saved.
> 
> I think it should restore buffer-modified-p to whatever it was to begin
> with when it doesn't find any new mail.  I'll send a patch if you like.

Thanks.  Does the patch below give good results?

diff --git a/lisp/mail/rmail.el b/lisp/mail/rmail.el
index f095d5e..e3372a6 100644
--- a/lisp/mail/rmail.el
+++ b/lisp/mail/rmail.el
@@ -1751,6 +1751,7 @@ rmail-get-new-mail-1
 	    (spam-filter-p (and (featurep 'rmail-spam-filter)
 				rmail-use-spam-filter))
 	    (blurb "")
+            (mod-p (buffer-modified-p))
 	    result success suffix)
 	(narrow-to-region (point) (point))
 	;; Read in the contents of the inbox files, renaming them as
@@ -1766,10 +1767,11 @@ rmail-get-new-mail-1
 		  (rmail-insert-inbox-text files nil)
 		(setq delete-files (rmail-insert-inbox-text files t))))
 	  ;; If there was no new mail, or we aborted before actually
-	  ;; trying to get any, mark buffer unmodified.  Otherwise the
-	  ;; buffer is correctly marked modified and the file locked
-	  ;; until we save out the new mail.
-	  (if (= (point-min) (point-max))
+	  ;; trying to get any, mark buffer unmodified, unless it was
+	  ;; modified originally.  Otherwise the buffer is correctly
+	  ;; marked modified and the file locked until we save out the
+	  ;; new mail.
+	  (if (and (null mod-p) (= (point-min) (point-max)))
 	      (set-buffer-modified-p nil)))
 	;; Scan the new text and convert each message to
 	;; Rmail/mbox format.