Don't use file notification on non-file buffers

Allow non-file buffers to declare that notification on their
default-directory is sufficient to know when auto-revert updates are
required by setting the new variable `buffer-auto-revert-by-notification'
to non-nil.  If nil, the default, then auto-revert will poll
those buffers instead. (bug#35418).
Currently, only Dired sets that variable.

* lisp/autorevert.el (auto-revert-buffers):
Modify condition for using notification.
* lisp/files.el (buffer-auto-revert-by-notification): New variable.
* lisp/dired.el (dired-mode): Set buffer-auto-revert-by-notification.
* doc/emacs/arevert-xtra.texi (Non-File Buffers): Document new variable.
* etc/NEWS (Changes in Specialized Modes and Packages): Describe new variable.

1 files changed, 5 insertions, 1 deletions

diff --git a/lisp/autorevert.el b/lisp/autorevert.el
index 7cd5e7ee8bf..197a2bf1578 100644
--- a/lisp/autorevert.el
+++ b/lisp/autorevert.el
@@ -858,8 +858,12 @@ the timer when no buffers need to be checked."
                   (auto-revert-remove-current-buffer))
               (when (auto-revert-active-p)
                 ;; Enable file notification.
+                ;; Don't bother creating a notifier for non-file buffers
+                ;; unless it explicitly indicates that this works.
                 (when (and auto-revert-use-notify
-                           (not auto-revert-notify-watch-descriptor))
+                           (not auto-revert-notify-watch-descriptor)
+                           (or buffer-file-name
+                               buffer-auto-revert-by-notification))
                   (auto-revert-notify-add-watch))
                 (auto-revert-handler)))))
 	(setq bufs (cdr bufs)))