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Re: [PATCH] Fix hostname completion on MS Windows

From: Michael Albinus
Subject: Re: [PATCH] Fix hostname completion on MS Windows
Date: Thu, 08 Apr 2021 14:40:36 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

Jim Porter <> writes:

Hi Jim,

> This patch resolves an MS Windows-specific issue with file name
> completion for Tramp: currently, when trying to complete a Tramp
> hostname (or method/user), it fails to match anything on MS Windows
> (however, completing the remote path works ok). This is because
> `file-name-completion' provides handlers with an absolute path to
> match against, but on MS Windows, that means the path starts with the
> volume letter, e.g. "c:/ssh:host:/path/to/file".

I don't mind to apply such a patch, but I haven't heard about this kind
of file name completion to "c:/ssh:host:/path/to/file". Could you pls
show an example, and what you have typed in order to get this?

Best might be to enable Tramp traces after loading Tramp, like

--8<---------------cut here---------------start------------->8---
(require 'trace)
(dolist (elt (all-completions "tramp-" obarray 'functionp))
  (trace-function-background (intern elt)))
(untrace-function 'tramp-read-passwd)
--8<---------------cut here---------------end--------------->8---

Then run the test, and disable traces afterwards

--8<---------------cut here---------------start------------->8---
--8<---------------cut here---------------end--------------->8---

There will be a new buffer *trace-output*, which shall tell us the

> The attached patch fixes things for me; it's somewhat hacky, since it
> tries to recognize the mutilated path string as shown above.

That would be OK, but perhaps we make the volume letter optional? Like

--8<---------------cut here---------------start------------->8---
(when (eq system-type 'windows-nt)
--8<---------------cut here---------------end--------------->8---

> However, changing that would (probably) be an invasive change to how
> core Emacs handles file name completion. This also mirrors the
> behavior of `tramp-drop-volume-letter', so hopefully it's ok. I
> *think* this only applies to windows-nt systems and not cygwin (which
> usually uses *nix-style paths), but I don't have Cygwin installed so I
> didn't try it there.

I know, that Eli Zaretskii (maintainer of Emacs) has committed several
patches in order to drop such a volume letter for remote file names, but
I don't remember where they have been applied. Perhaps, this was an
Emacs 28 change.

> I don't have copyright assignment papers filled out, but hopefully
> this patch is small enough that that's not necessary. If you think it
> is, just let me know and I'll get that handled.

This patch is small enough (the upper limit are ~15 lines), but in
general it is a good idea to sign copyright papers for future
changes. To start the assignment process you should download
and return the completed information to the address at the top.

> As a small addendum: this patch still doesn't fix things when
> `default-directory' is a UNC path (e.g. "\\server\mnt\"), but the same
> issue applies to `tramp-drop-volume-letter', and this case should be
> rare in practice anyway.

Yep, we could handle this case afterwards. If needed.

> - Jim

Best regards, Michael.

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