bug#35497: [PATCH v4] Don't rewrite buffer contents after saving by rename

[Jonathan Tomer]

When `file-precious-flag' is non-nil, files are saved by renaming a
temporary file to the new name; this is an atomic operation on POSIX
so other programs will not see the file in an intermediate state.
Unfortunately, due to a paren-matching error introduced in change
574c05e219476912db3105fa164accd9ba12b35f, we would then write the
contents again in the usual way after this rename.  In addition to
being wasteful, this is a serious bug: the whole point of
`file-precious-flag' is to prevent race conditions with other programs
that might otherwise see an empty file, but with this bug the race is
actually much *more* likely to be visible: the rename will alert any
inotify watchers of a change, and then the subsequent write is very
likely to truncate the file just as those programs start to read it!
* lisp/files.el (basic-save-buffer-2): Don't rewrite file contents
  after saving-by-renaming.
* 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.
---
 lisp/files.el                |  4 ++--
 test/lisp/files-tests.el     | 13 +++++++++++++
 test/lisp/net/tramp-tests.el | 26 ++++++++++++++++++++++++++
 3 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/lisp/files.el b/lisp/files.el
index c05d70a00e..72518e8127 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -5256,7 +5256,7 @@ basic-save-buffer-2
                     (set-file-extended-attributes buffer-file-name
                                                   (nth 1 setmodes)))
                 (set-file-modes buffer-file-name
-                                (logior (car setmodes) 128))))))
+                                (logior (car setmodes) 128)))))
        (let (success)
          (unwind-protect
              (progn
@@ -5272,7 +5272,7 @@ basic-save-buffer-2
            (and setmodes (not success)
                 (progn
                   (rename-file (nth 2 setmodes) buffer-file-name t)
-                  (setq buffer-backed-up nil))))))
+                  (setq buffer-backed-up nil)))))))
     setmodes))
 
 (declare-function diff-no-select "diff"
diff --git a/test/lisp/files-tests.el b/test/lisp/files-tests.el
index ae8ea41a79..0becde4184 100644
--- a/test/lisp/files-tests.el
+++ b/test/lisp/files-tests.el
@@ -1244,5 +1244,18 @@ files-tests-file-attributes-equal
                     (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."
+  (files-tests--with-temp-file temp-file-name
+    (files-tests--with-advice
+        write-region :before
+        (lambda (_start _end filename &rest r)
+          (should (not (string= filename temp-file-name))))
+      (with-current-buffer (find-file-noselect temp-file-name)
+        (setq-local file-precious-flag t)
+        (insert "foobar")
+        (should (null (save-buffer)))))))
+
 (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 cba697da18..dee5e5e0f9 100644
--- a/test/lisp/net/tramp-tests.el
+++ b/test/lisp/net/tramp-tests.el
@@ -2270,6 +2270,32 @@ tramp--test-print-duration
        ;; 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))
+        (advice (lambda (_start _end filename &rest r)
+                  (should-not (string= filename tmp-name)))))
+
+    (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))))
+
+      ;; 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))
-- 
2.21.0.593.g511ec345e18-goog