From 895562a8b24fc637fb422d38ea64e7d445b3480d Mon Sep 17 00:00:00 2001
From: Michael Albinus <michael.albinus@gmx.de>
Date: Thu, 3 Feb 2022 14:21:23 +0100
Subject: [PATCH] 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.
---
 lisp/net/tramp.el            |  8 ++++
 test/lisp/net/tramp-tests.el | 72 ++++++++++++++++++++++++++++++++++++
 2 files changed, 80 insertions(+)

diff --git a/lisp/net/tramp.el b/lisp/net/tramp.el
index c6e55ff6889..121ede42c43 100644
--- a/lisp/net/tramp.el
+++ b/lisp/net/tramp.el
@@ -4008,6 +4008,14 @@ Do not set it manually, it is used buffer-local in `tramp-get-lock-pid'.")
   ;; was visited.
   (catch 'dont-lock
     (unless (eq (file-locked-p file) t) ;; Locked by me.
+      (when (and buffer-file-truename
+		 (not (verify-visited-file-modtime))
+		 (file-exists-p file))
+	;; In filelock.c, `userlock--ask-user-about-supersession-threat'
+	;; is called, which also checks file contents.  This is unwise
+	;; for remote files.
+	(ask-user-about-supersession-threat file))
+
       (when-let ((info (tramp-get-lock-file file))
 		 (match (string-match tramp-lock-file-info-regexp info)))
 	(unless (ask-user-about-lock
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))
-- 
2.39.5