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

[Michael Albinus]

Gregory Heytings <ghe@sdf.org> writes:

Hi,

> Apparently my previous last note was not the last one ;-)
>
> I still don't know how this bug should be fixed (except by using (setq
> completion-at-point-functions '(pcomplete t))), but here is a more
> detailed explanation of what is happening, at least how I understand
> it:
>
> 1. TAB calls completion-at-point
> 2. completion-at-point calls pcomplete-completions-at-point, which
> calls pcomplete/cd; this completes the directory name
> 3. completion-at-point let-binds completion-in-region-mode-predicate
> to a lambda, which contains pcomplete-completion-at-point
> 4. completion-at-point calls completion-in-region, which adds
> completion-in-region--postch to post-command-hook
> 5. post-command-hook calls completion-in-region--postch
> 6. completion-in-region--postch funcalls completion-in-region-mode-predicate
> 7. this calls pcomplete-completions-at-point a second time, which
> again calls pcomplete/cd (and adds a '/' after the directory name (?))
> 8. RET is pressed
> 9. post-command-hook still contains completion-in-region--postch: it
> is called again
> 10. completion-in-region--postch funcalls
> completion-in-region-mode-predicate again
> 11. this calls pcomplete-completions-at-point a third (!) time
> 12. at this point pcomplete-completions-at-point considers (for some
> reason, possibly because the last character of the input is '/' (?))
> that it is a command that it must now complete
> 13. therefore instead of calling pcomplete/cd a third time,
> pcomplete-completions-at-point now calls eshell-complete-commands-list
> 14. this loops through all possible command names (on the local or remote 
> host)
> 15. if all command names had had a common prefix, that prefix would
> have been inserted now, but this is not the case, so the effect of
> this loop (apart from a waste of time) is nil
>
> The fact that default-directory is remote is not important here, the
> exact same steps take place when it is local, except that step 15 is
> executed much faster.
>
> At step 15 it is possible to interrupt the loop with C-g.  At step 8
> it is possible to remove completion-in-region--postch from
> post-command-hook for example by switching buffers with C-x C-b.
>
> Why such a overly complicated mechanism is used, or what should be
> done to avoid this behavior, is beyond my understanding.

I don't know the completion machinery, so I'm adding Stefan who might
know better.

However, the remote case could be improved. Tramp uses caches. They
expire after a while (10 seconds per default), but this might be
improved. The appended patch disables Tramp cache expiry while being in
eshell-complete-commands-list, so completion might be faster once the
cache has been filled. Could you pls check?

Best regards, Michael.

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