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.

doc/emacs/arevert-xtra.texi

@@ -35,6 +35,14 @@ the Buffer Menu.)  In this case, Auto Revert does not print any
 messages while reverting, even when @code{auto-revert-verbose} is
 non-@code{nil}.
 
+@vindex buffer-auto-revert-by-notification
+Some non-file buffers can be updated reliably by file notification on
+their default directory; Dired buffers is an example.  The major mode
+can indicate this by setting @code{buffer-auto-revert-by-notification}
+to a non-@code{nil} value in that buffer, allowing Auto Revert to
+avoid periodic polling.  Such notification does not include changes to
+files in that directory, only to the directory itself.
+
 The details depend on the particular types of buffers and are
 explained in the corresponding sections.
 

etc/NEWS

@@ -1446,6 +1446,13 @@ of an idle Emacs, but may fail on some network file systems; set
 notification is not supported.  The new variable currently has no
 effect in 'global-auto-revert-mode'.  The default value is nil.
 
+*** New variable 'buffer-auto-revert-by-notification'
+A major mode can declare that notification on the buffer's default
+directory is sufficient to know when updates are required, by setting
+the new variable 'buffer-auto-revert-by-notification' to a non-nil
+value.  Auto Revert mode can use this information to avoid polling the
+buffer periodically when 'auto-revert-avoid-polling' is non-nil.
+
 
 * New Modes and Packages in Emacs 27.1
 

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)))

lisp/dired.el

@@ -2148,6 +2148,7 @@ Keybindings:
     (setq buffer-invisibility-spec (list t)))
   (setq-local revert-buffer-function #'dired-revert)
   (setq-local buffer-stale-function #'dired-buffer-stale-p)
+  (setq-local buffer-auto-revert-by-notification t)
   (setq-local page-delimiter "\n\n")
   (setq-local dired-directory (or dirname default-directory))
   ;; list-buffers uses this to display the dir being edited in this buffer.

lisp/files.el

@@ -5890,6 +5890,16 @@ This should not be relied upon.
 For more information on how this variable is used by Auto Revert mode,
 see Info node `(emacs)Supporting additional buffers'.")
 
+(defvar-local buffer-auto-revert-by-notification nil
+  "Whether a buffer can rely on notification in Auto-Revert mode.
+If non-nil, monitoring changes to the directory of the current
+buffer is sufficient for knowing when that buffer needs to be
+updated in Auto Revert Mode.  Such notification does not include
+changes to files in that directory, only to the directory itself.
+
+This variable only applies to buffers where `buffer-file-name' is
+nil; other buffers are tracked by their files.")
+
 (defvar before-revert-hook nil
   "Normal hook for `revert-buffer' to run before reverting.
 The function `revert-buffer--default' runs this.