Don't rewrite buffer contents after saving by rename (Bug#35497)

* lisp/files.el (basic-save-buffer-2): Don't rewrite file contents
after saving-by-renaming.  (Bug#35497)

* test/lisp/files-tests.el (files-tests-dont-rewrite-precious-files):
* test/lisp/net/tramp-tests.el
(tramp-test10-write-region-file-precious-flag): Regression tests
for this change.

2 files changed, 44 insertions, 0 deletions

diff --git a/test/lisp/files-tests.el b/test/lisp/files-tests.el
index ae8ea41a797..fe2e958f1c3 100644
--- a/test/lisp/files-tests.el
+++ b/test/lisp/files-tests.el
@@ -1244,5 +1244,20 @@ See <https://debbugs.gnu.org/35241>."
                     (executable-find (file-name-nondirectory tmpfile))))))
       (delete-file tmpfile))))
 
+(ert-deftest files-tests-dont-rewrite-precious-files ()
+  "Test that `file-precious-flag' forces files to be saved by
+renaming only, rather than modified in-place."
+  (let* ((temp-file-name (make-temp-file "files-tests"))
+         (advice (lambda (_start _end filename &rest _r)
+                   (should-not (string= filename temp-file-name)))))
+    (unwind-protect
+        (with-current-buffer (find-file-noselect temp-file-name)
+          (advice-add #'write-region :before advice)
+          (setq-local file-precious-flag t)
+          (insert "foobar")
+          (should (null (save-buffer))))
+      (ignore-errors (advice-remove #'write-region advice))
+      (ignore-errors (delete-file temp-file-name)))))
+
 (provide 'files-tests)
 ;;; files-tests.el ends here
diff --git a/test/lisp/net/tramp-tests.el b/test/lisp/net/tramp-tests.el
index 7d3c43408d4..e35be0e3121 100644
--- a/test/lisp/net/tramp-tests.el
+++ b/test/lisp/net/tramp-tests.el
@@ -41,6 +41,7 @@
 
 ;;; Code:
 
+(require 'cl-seq)
 (require 'dired)
 (require 'ert)
 (require 'ert-x)
@@ -2270,6 +2271,34 @@ This checks also `file-name-as-directory', `file-name-directory',
 	;; Cleanup.
 	(ignore-errors (delete-file tmp-name))))))
 
+(ert-deftest tramp-test10-write-region-file-precious-flag ()
+  "Check that `file-precious-flag' is respected with Tramp in use."
+  (skip-unless (tramp--test-enabled))
+  (skip-unless (tramp--test-sh-p))
+
+  (let* ((tmp-name (tramp--test-make-temp-name))
+         written-files
+         (advice (lambda (_start _end filename &rest _r)
+                   (push filename written-files))))
+
+    (unwind-protect
+        (with-current-buffer (find-file-noselect tmp-name)
+          ;; Write initial contents.  Adapt `visited-file-modtime'
+          ;; in order to suppress confirmation.
+          (insert "foo")
+          (write-region nil nil tmp-name)
+          (set-visited-file-modtime)
+          ;; Run the test.
+          (advice-add 'write-region :before advice)
+          (setq-local file-precious-flag t)
+          (insert "bar")
+          (should (null (save-buffer)))
+          (should-not (cl-member tmp-name written-files :test #'string=)))
+
+      ;; Cleanup.
+      (ignore-errors (advice-remove 'write-region advice))
+      (ignore-errors (delete-file tmp-name)))))
+
 (ert-deftest tramp-test11-copy-file ()
   "Check `copy-file'."
   (skip-unless (tramp--test-enabled))