From 4c2f76cc7e6916ed35b79b22ffe9ef4e57414d26 Mon Sep 17 00:00:00 2001
From: Mauro Aranda <maurooaranda@gmail.com>
Date: Sun, 9 Mar 2025 08:10:17 -0300
Subject: [PATCH] Guard against user saving ediff-meta-diff-buffer

* lisp/vc/ediff-mult.el (ediff-collect-custom-diffs): If the
diff buffer is visiting a file, create a new buffer.  (Bug#3348)

* test/lisp/vc/ediff-mult-tests.el: New test file.

(cherry picked from commit d01b7c85fea3e177df112513af150cead4bbb3e5)
---
 lisp/vc/ediff-mult.el            |  9 +++++-
 test/lisp/vc/ediff-mult-tests.el | 54 ++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 test/lisp/vc/ediff-mult-tests.el

diff --git a/lisp/vc/ediff-mult.el b/lisp/vc/ediff-mult.el
index 493e98f76b7..ade20a4a299 100644
--- a/lisp/vc/ediff-mult.el
+++ b/lisp/vc/ediff-mult.el
@@ -1719,7 +1719,14 @@ multifile patches.  For `ediff-directory-revisions', we insist that
 all marked sessions must be active."
   (interactive)
   (let ((coding-system-for-read ediff-coding-system-for-read))
-    (or (ediff-buffer-live-p ediff-meta-diff-buffer)
+    (unless (and (ediff-buffer-live-p ediff-meta-diff-buffer)
+                 ;; We assume `ediff-meta-diff-buffer' doesn't
+                 ;; visit any file.  But if the user saves the
+                 ;; `ediff-meta-diff-buffer' to a file, that
+                 ;; assumption isn't right anymore.  (Bug#3348)
+                 ;; So, if `ediff-meta-diff-buffer' is visiting some
+                 ;; file, create a new buffer rather than reusing it.
+                 (not (buffer-file-name ediff-meta-diff-buffer)))
 	(setq ediff-meta-diff-buffer
 	      (get-buffer-create
 	       (ediff-unique-buffer-name "*Ediff Multifile Diffs" "*"))))
diff --git a/test/lisp/vc/ediff-mult-tests.el b/test/lisp/vc/ediff-mult-tests.el
new file mode 100644
index 00000000000..525088bf856
--- /dev/null
+++ b/test/lisp/vc/ediff-mult-tests.el
@@ -0,0 +1,54 @@
+;;; ediff-mult-tests.el --- Tests for ediff-mult.el  -*- lexical-binding:t -*-
+
+;; Copyright (C) 2025 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+;;
+;; GNU Emacs is free software: you can redistribute it and/or
+;; modify it under the terms of the GNU General Public License as
+;; published by the Free Software Foundation, either version 3 of the
+;; License, or (at your option) any later version.
+;;
+;; GNU Emacs is distributed in the hope that it will be useful, but
+;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;; General Public License for more details.
+;;
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+(require 'ediff-mult)
+
+(ert-deftest ediff-test-bug3348 ()
+  "After saving `ediff-meta-diff-buffer' to a file, we should not reuse it."
+  (let ((test-dir
+         (expand-file-name "bug-3348-testdir" temporary-file-directory)))
+    (make-directory test-dir t)
+    (cd test-dir)
+
+    (make-directory "dir-a" t)
+    (make-directory "dir-b" t)
+
+    (with-temp-file "dir-a/file"
+      (insert "aaa\n"))
+    (with-temp-file "dir-b/file"
+      (insert "bbb\n"))
+
+    (ediff-directories "dir-a" "dir-b" nil)
+    (switch-to-buffer "*Ediff Session Group Panel*")
+
+    (ediff-next-meta-item 1)
+    (ediff-mark-for-operation-at-pos nil)
+    (ediff-collect-custom-diffs)
+
+    (with-current-buffer "*Ediff Multifile Diffs*"
+      (write-file "foo.patch"))
+
+    (with-temp-file "dir-b/file"
+      (insert "BBB\n"))
+    (ediff-collect-custom-diffs)
+
+    (should-not (equal ediff-meta-diff-buffer (get-buffer "foo.patch")))))
-- 
2.39.5