[Emacs-diffs] master 1943220: Don't use file notification on non-file buffers

[Mattias Engdegård]

branch: master
commit 1943220d1b7ed28b2708c86acfe5493d5c6e3089
Author: Mattias Engdegård <address@hidden>
Commit: Mattias Engdegård <address@hidden>

    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 |  8 ++++++++
 etc/NEWS                    |  7 +++++++
 lisp/autorevert.el          |  6 +++++-
 lisp/dired.el               |  1 +
 lisp/files.el               | 10 ++++++++++
 5 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/doc/emacs/arevert-xtra.texi b/doc/emacs/arevert-xtra.texi
index 9e01a10..37e2f9e 100644
--- a/doc/emacs/arevert-xtra.texi
+++ b/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
 address@hidden
 
address@hidden 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 address@hidden 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.
 
diff --git a/etc/NEWS b/etc/NEWS
index 8c05915..b4aa8d9 100644
--- a/etc/NEWS
+++ b/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
 
diff --git a/lisp/autorevert.el b/lisp/autorevert.el
index 7cd5e7e..197a2bf 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)))
diff --git a/lisp/dired.el b/lisp/dired.el
index 3851265..ea1943d 100644
--- a/lisp/dired.el
+++ b/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.
diff --git a/lisp/files.el b/lisp/files.el
index 8fa7f16..1dec0ed 100644
--- a/lisp/files.el
+++ b/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.