Subject: 29.0.60; project-list-buffers is slow

From: Dmitry Gutov <[email-masked]>
Date: Sat, 10 Dec 2022 03:49:47 +0200

X-Debbugs-Cc: [email-masked]

When you invoke 'C-x p C-b', there's a noticeable pause before the
buffer list is displayed. It's ~400ms over here when there are 46
non-hidden buffers in the running session.

What I think is happening, the predicate is called 46, which in turn 
calls (project-buffers pr) 46 times, and that ends up being 46 times 
slower than one might have anticipated.

Not sure what's the best fix here (especially in time for the release),
but if the FILTER-PREDICATE arg to list-buffers-noselect turned into a
factory function (e.g. FILTER-PREDICATE-MAKER), that would be one
solution.

Another would be to only leave the legacy codepath: it's not affected,
project-buffers is only called once there.

Curiously, though, it shows a different list of buffers. It also
includes "hidden" buffers - diff-syntax, Echo Area, etc. We should look
into that either way.

And the patch is this:

diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 016dfdd5b4..ad49df0423 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -1342,16 +1342,13 @@ project-list-buffers
    (interactive "P")
    (let ((pr (project-current t)))
      (display-buffer
-     (if (version< emacs-version "29.0.50")
-         (let ((buf (list-buffers-noselect arg (project-buffers pr))))
-           (with-current-buffer buf
-             (setq-local revert-buffer-function
-                         (lambda (&rest _ignored)
-                           (list-buffers--refresh (project-buffers pr))
-                           (tabulated-list-print t))))
-           buf)
-       (list-buffers-noselect
-        arg nil (lambda (buf) (memq buf (project-buffers pr))))))))
+     (let ((buf (list-buffers-noselect arg (project-buffers pr))))
+       (with-current-buffer buf
+         (setq-local revert-buffer-function
+                     (lambda (&rest _ignored)
+                       (list-buffers--refresh (project-buffers pr))
+                       (tabulated-list-print t))))
+       buf))))
   (defcustom project-kill-buffer-conditions
    '(buffer-file-name    ; All file-visiting buffers are included.