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

[Gregory Heytings]

A last note: this bug exists because in Emacs 27 eshell uses 
`pcomplete-completions-at-point': TAB is bound to `completion-at-point' 
and `completion-at-point-functions' is `(pcomplete-completions-at-point 
t)'.

In Emacs 26 eshell used the (now obsolete) `pcomplete' function: TAB was 
bound to `eshell-pcomplete', which was defined as follows:

(defun eshell-pcomplete (&optional interactively)
   "Eshell wrapper for `pcomplete'."
   (interactive "p")
   (setq this-command 'pcomplete)
   (condition-case nil
       (if interactively
          (call-interactively 'pcomplete)
        (pcomplete))
     (text-read-only (completion-at-point))))

IOW, pcomplete-completions-at-point was called only if `pcomplete' failed, 
and is now called by default.  (`completion-at-point-functions' was set to 
`(pcomplete-completions-at-point t)', as in Emacs 27.)

A simple fix is to eval

(setq completion-at-point-functions '(pcomplete t))

after starting eshell (or to put this in one of the eshell hooks), which 
will restore the previous default behavior.

I have no idea how `pcomplete-completions-at-point' should be adapted to 
avoid this bug.