bug#41423: 27.0.91; eshell file completion in tramp dir is slow (3 minutes) [regression on pretest]

[Gregory Heytings via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>
> Would it help to profile the completion process in this use case using 
> the built-in Lisp profiler?
>

No, because the bug is not about cpu or mem usage, but about doing 
something useless that just takes time (without eating much resources).

A much better way to profile this is to use:

(defvar pcomplete-completions-at-point-time 0)
(defun around-pcomplete-completions-at-point (fun)
   (message "calling pcomplete-completions-at-point")
   (setq pcomplete-completions-at-point-time (float-time))
   (let ((ret (funcall fun)))
     (message "returning from pcomplete-completions-at-point, call took %.2f seconds" (- (float-time) pcomplete-completions-at-point-time))
     ret))
(advice-add 'pcomplete-completions-at-point :around #'around-pcomplete-completions-at-point)

(let ((default-directory "/ssh:user@host:~/")) (eshell))

This will print:

calling pcomplete-completions-at-point
returning from pcomplete-completions-at-point, call took 0.00 seconds
calling pcomplete-completions-at-point
returning from pcomplete-completions-at-point, call took 0.00 seconds
calling pcomplete-completions-at-point
returning from pcomplete-completions-at-point, call took N seconds

The value of N depends on the speed of your connection.  On a fast 
connection it will be something around 50, on a slower one something 
around 100.  With a local directory (that is, without let-binding 
default-directory before entering eshell) it depends on your machine.  On 
a fast one it will be something around 0.10, on a slower one something 
around 0.50.

Note again that this third call to pcomplete-completions-at-point does 
nothing useful.  It just build a list of all possible commands, and throws 
it away.