Re: master 3e4d4f472d: Rework `abbreviate-file-name' in Tramp

[Jim Porter]

On 3/7/2022 4:42 AM, Michael Albinus wrote:
> branch: master
> commit 3e4d4f472d3960a7d18dad76b8d54a66bc5d9f6c
[snip]
> diff --git a/lisp/net/tramp.el b/lisp/net/tramp.el
> index 932dfb3691..5bf6a54020 100644
> --- a/lisp/net/tramp.el
> +++ b/lisp/net/tramp.el
[snip]
> @@ -3360,15 +3362,16 @@ Let-bind it when necessary.")
[snip]
>               ;; Otherwise, just use the cached value.
> -            (tramp-get-connection-property vec "home-directory" nil))))
> +            (tramp-get-connection-property vec "~" nil))))
> +    (when home-dir
> +      (setq home-dir
> +           (tramp-compat-funcall
> +            'directory-abbrev-apply
> +            (tramp-make-tramp-file-name vec home-dir))))
>       ;; If any elt of `directory-abbrev-alist' matches this name,
>       ;; abbreviate accordingly.

I don't know how much this matters in practice, but one benefit of the 
old code is that it cached the *abbreviated* home directory (though I 
gave it a poor name; "home-directory" didn't indicate that it's 
abbreviated). This can significantly improve performance when there are 
many entries in `directory-abbrev-alist', since then Tramp doesn't have 
to abbreviate the home-dir on each call. That's similar to how 
`abbreviate-file-name' works on local files, which also caches the 
abbreviated home dir.

See the attached benchmark/results. I also attached a sketch of a patch 
for a way to avoid that performance cost. I'm not sure it's actually a 
good way to do things (or even if it's correct in all cases), but 
hopefully it serves as a demonstration. It would probably be better to 
cache it somewhere that doesn't persist across sessions though[1]; it 
could get out of sync with `directory-abbrev-alist' and cause problems 
(this was a problem with the old code too).

That said, I wouldn't have a problem if the answer is 
"`directory-abbrev-alist' is short, so caching this value is more 
trouble than it's worth." The `abbreviate-file-name' function even asks 
this question (for local files):

      ;; FIXME Is it even worth caching abbreviated-home-dir?
      ;; Ref: https://debbugs.gnu.org/19657#20

- Jim

[1] At least, I *think* Tramp connection properties are cached across 
Emacs sessions...

benchmark.el
Description: Text document

results.txt
Description: Text document

0001-Cache-abbreviated-home-directory-for-Tramp.patch
Description: Text document