Implement file modification check in Tramp

* lisp/net/tramp.el (tramp-handle-lock-file): Check, whether FILE
is modified.

* test/lisp/net/tramp-tests.el (tramp-test39-detect-external-change):
New test.

1 files changed, 72 insertions, 0 deletions

diff --git a/test/lisp/net/tramp-tests.el b/test/lisp/net/tramp-tests.el
index 01fe7355f45..b41824a6cf3 100644
--- a/test/lisp/net/tramp-tests.el
+++ b/test/lisp/net/tramp-tests.el
@@ -6018,6 +6018,78 @@ Use direct async.")
         (ignore-errors (delete-file tmp-name1))
         (tramp-cleanup-connection tramp-test-vec 'keep-debug 'keep-password)))))
 
+;; The function was introduced in Emacs 28.1.
+(ert-deftest tramp-test39-detect-external-change ()
+  "Check that an external file modification is reported."
+  (skip-unless (tramp--test-enabled))
+  (skip-unless (not (tramp--test-ange-ftp-p)))
+  ;; Since Emacs 28.1.
+  (skip-unless (fboundp 'file-locked-p))
+
+  (dolist (quoted (if (tramp--test-expensive-test-p) '(nil t) '(nil)))
+    (dolist (create-lockfiles '(nil t))
+      (let ((tmp-name (tramp--test-make-temp-name nil quoted))
+            (remote-file-name-inhibit-cache t)
+            (remote-file-name-inhibit-locks nil)
+            tramp-allow-unsafe-temporary-files
+            (inhibit-message t)
+            ;; tramp-rclone.el and tramp-sshfs.el cache the mounted files.
+            (tramp-fuse-unmount-on-cleanup t)
+            auto-save-default
+            (backup-inhibited t)
+            noninteractive)
+        (with-temp-buffer
+          (unwind-protect
+              (progn
+                (setq buffer-file-name tmp-name
+                      buffer-file-truename tmp-name)
+                (insert "foo")
+                ;; Bug#53207: with `create-lockfiles' nil, saving the
+                ;; buffer results in a prompt.
+                (cl-letf (((symbol-function 'yes-or-no-p)
+                           (lambda (_) (ert-fail "Test failed unexpectedly"))))
+                  (save-buffer))
+                (should-not (file-locked-p tmp-name))
+
+                ;; For local files, just changing the file
+                ;; modification on disk doesn't hurt, because file
+                ;; contents in buffer and on disk are equal.  For
+                ;; remote files, file contents is not compared.  We
+                ;; mock an older modification time in buffer, because
+                ;; Tramp regards modification times equal if they
+                ;; differ for less than 2 seconds.
+                (set-visited-file-modtime (time-add (current-time) -60))
+                ;; Some Tramp methods cannot check the file
+                ;; modification time properly, for them it doesn't
+                ;; make sense to test.
+                (when (not (verify-visited-file-modtime))
+                  (cl-letf (((symbol-function 'read-char-choice)
+                             (lambda (prompt &rest _) (message "%s" prompt) ?y)))
+                    (ert-with-message-capture captured-messages
+                      (insert "bar")
+                      (when create-lockfiles
+                        (should (string-match-p
+                                 (format
+                                  "^%s changed on disk; really edit the buffer\\?"
+                                  (if (tramp--test-crypt-p)
+                                      ".+" (file-name-nondirectory tmp-name)))
+                                 captured-messages))
+                        (should (file-locked-p tmp-name)))))
+
+                  ;; `save-buffer' removes the file lock.
+                  (cl-letf (((symbol-function 'yes-or-no-p) #'tramp--test-always)
+                            ((symbol-function 'read-char-choice)
+                             (lambda (&rest _) ?y)))
+                    (save-buffer))
+                  (should-not (file-locked-p tmp-name))))
+
+            ;; Cleanup.
+            (set-buffer-modified-p nil)
+            (ignore-errors (delete-file tmp-name))
+            (tramp-cleanup-connection
+             tramp-test-vec 'keep-debug 'keep-password)))))))
+
+;; The functions were introduced in Emacs 26.1.
 (ert-deftest tramp-test40-make-nearby-temp-file ()
   "Check `make-nearby-temp-file' and `temporary-file-directory'."
   (skip-unless (tramp--test-enabled))