Fall back to polling in autorevert when needed

* lisp/autorevert.el (auto-revert-notify-handler): When a
`stopped' event arrives from file notification, fall back to polling.

* test/automated/file-notify-tests.el
(file-notify-test03-autorevert): Extend test for polling when file
notification ceases to work.

2 files changed, 76 insertions, 32 deletions

diff --git a/lisp/autorevert.el b/lisp/autorevert.el
index 37ee8eedcfd..f0c12d2d97e 100644
--- a/lisp/autorevert.el
+++ b/lisp/autorevert.el
@@ -570,37 +570,54 @@ no more reverts are possible until the next call of
       ;; Since we watch a directory, a file name must be returned.
       (cl-assert (stringp file))
       (when (eq action 'renamed) (cl-assert (stringp file1)))
-      ;; Loop over all buffers, in order to find the intended one.
-      (cl-dolist (buffer buffers)
-        (when (buffer-live-p buffer)
-          (with-current-buffer buffer
-            (when (or
-                   ;; A buffer associated with a file.
-                   (and (stringp buffer-file-name)
-                        (or
-                         (and (memq action '(attribute-changed changed created))
-                              (string-equal
-                               (file-name-nondirectory file)
-                               (file-name-nondirectory buffer-file-name)))
-                         (and (eq action 'renamed)
-                              (string-equal
-                               (file-name-nondirectory file1)
-                               (file-name-nondirectory buffer-file-name)))))
-                   ;; A buffer w/o a file, like dired.
-                   (and (null buffer-file-name)
-                        (memq action '(created renamed deleted))))
-              ;; Mark buffer modified.
-              (setq auto-revert-notify-modified-p t)
-
-              ;; Revert the buffer now if we're not locked out.
-              (when (/= auto-revert-buffers-counter-lockedout
-                        auto-revert-buffers-counter)
-                (auto-revert-handler)
-                (setq auto-revert-buffers-counter-lockedout
-                      auto-revert-buffers-counter))
-
-              ;; No need to check other buffers.
-              (cl-return))))))))
+
+      (if (eq action 'stopped)
+          ;; File notification has stopped.  Continue with polling.
+          (cl-dolist (buffer buffers)
+            (with-current-buffer buffer
+              (when (or
+                     ;; A buffer associated with a file.
+                     (and (stringp buffer-file-name)
+                          (string-equal
+                           (file-name-nondirectory file)
+                           (file-name-nondirectory buffer-file-name)))
+                     ;; A buffer w/o a file, like dired.
+                     (null buffer-file-name))
+                (auto-revert-notify-rm-watch)
+                (setq-local auto-revert-use-notify nil))))
+
+        ;; Loop over all buffers, in order to find the intended one.
+        (cl-dolist (buffer buffers)
+          (when (buffer-live-p buffer)
+            (with-current-buffer buffer
+              (when (or
+                     ;; A buffer associated with a file.
+                     (and (stringp buffer-file-name)
+                          (or
+                           (and (memq
+                                 action '(attribute-changed changed created))
+                                (string-equal
+                                 (file-name-nondirectory file)
+                                 (file-name-nondirectory buffer-file-name)))
+                           (and (eq action 'renamed)
+                                (string-equal
+                                 (file-name-nondirectory file1)
+                                 (file-name-nondirectory buffer-file-name)))))
+                     ;; A buffer w/o a file, like dired.
+                     (and (null buffer-file-name)
+                          (memq action '(created renamed deleted))))
+                ;; Mark buffer modified.
+                (setq auto-revert-notify-modified-p t)
+
+                ;; Revert the buffer now if we're not locked out.
+                (when (/= auto-revert-buffers-counter-lockedout
+                          auto-revert-buffers-counter)
+                  (auto-revert-handler)
+                  (setq auto-revert-buffers-counter-lockedout
+                        auto-revert-buffers-counter))
+
+                ;; No need to check other buffers.
+                (cl-return)))))))))
 
 (defun auto-revert-active-p ()
   "Check if auto-revert is active (in current buffer or globally)."
diff --git a/test/automated/file-notify-tests.el b/test/automated/file-notify-tests.el
index 472c6927b87..f411c6b76b0 100644
--- a/test/automated/file-notify-tests.el
+++ b/test/automated/file-notify-tests.el
@@ -461,6 +461,8 @@ Don't wait longer than TIMEOUT seconds for the events to be delivered."
 
             ;; Modify file.  We wait for a second, in order to
             ;; have another timestamp.
+            (with-current-buffer (get-buffer-create "*Messages*")
+              (narrow-to-region (point-max) (point-max)))
             (sleep-for 1)
             (write-region
              "another text" nil file-notify--test-tmpfile nil 'no-message)
@@ -472,9 +474,34 @@ Don't wait longer than TIMEOUT seconds for the events to be delivered."
                (string-match
                 (format-message "Reverting buffer `%s'." (buffer-name buf))
                 (buffer-string))))
-            (should (string-match "another text" (buffer-string)))))
+            (should (string-match "another text" (buffer-string)))
+
+            ;; Stop file notification.  Autorevert shall still work via polling.
+            (file-notify-rm-watch auto-revert-notify-watch-descriptor)
+            (file-notify--wait-for-events
+             timeout (null auto-revert-use-notify))
+            (should-not auto-revert-use-notify)
+            (should-not auto-revert-notify-watch-descriptor)
+
+            ;; Modify file.  We wait for a second, in order to
+            ;; have another timestamp.
+            (with-current-buffer (get-buffer-create "*Messages*")
+              (narrow-to-region (point-max) (point-max)))
+            (sleep-for 2)
+            (write-region
+             "foo bla" nil file-notify--test-tmpfile nil 'no-message)
+
+            ;; Check, that the buffer has been reverted.
+            (with-current-buffer (get-buffer-create "*Messages*")
+              (file-notify--wait-for-events
+               timeout
+               (string-match
+                (format-message "Reverting buffer `%s'." (buffer-name buf))
+                (buffer-string))))
+            (should (string-match "foo bla" (buffer-string)))))
 
       ;; Cleanup.
+      (with-current-buffer "*Messages*" (widen))
       (ignore-errors (kill-buffer buf))
       (file-notify--test-cleanup))))