GNU bug report logs - #50387
Possible bug in Tramp or in completions

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Gregory Heytings <gregory <at> heytings.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#50387: closed (Possible bug in Tramp or in completions)
Date: Tue, 07 Sep 2021 17:52:02 +0000

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Your message dated Tue, 07 Sep 2021 17:51:17 +0000
with message-id <11c4e3821a7883ef488a <at> heytings.org>
and subject line Re: bug#50387: Possible bug in Tramp or in completions
has caused the debbugs.gnu.org bug report #50387,
regarding Possible bug in Tramp or in completions
to be marked as done.

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From: Gregory Heytings <gregory <at> heytings.org>
To: bug-gnu-emacs <at> gnu.org
Subject: Possible bug in Tramp or in completions
Date: Sun, 05 Sep 2021 00:43:59 +0000

With a ~/.ssh/config file containing two entries:

Host foo
  ...
Host bar
  ...

emacs -Q
M-: (setq tramp-default-method "ssh")
M-: (add-to-list 'completion-styles 'substring)
C-x C-x /ssh: TAB

displays "Sole completion".  If, instead of having completion-styles set 
to:

(substring basic partial-completion emacs22)

it is set to:

(basic substring partial-completion emacs22)

then two completion candidates are shown (as expected): "ssh:foo:" and 
"ssh:bar:".

This happens because the "substring" completion mechanism is tried first 
by completion--some in completion--nth-completion, and returns (t . 
substring), because

(completion-substring-try-completion "/ssh:" #'completion-file-name-table 
#'file-exists-p 5)

returns t, because

(completion-substring--all-completions "/ssh:" 
#'completion-file-name-table #'file-exists-p 5) returns (("ssh:")

returns (prefix "ssh:") "/" "" 1), because

(completion-boundaries "/ssh:" #'completion-file-name-table 
#'file-exists-p "")

returns (1 . 0).

When the "basic" completion mechanism is tried first,

(completion-basic-try-completion "/ssh:" #'completion-file-name-table 
#'file-exists-p 5)

returns ("/ssh:" . 5).

It is not clear to me whether this is a bug (I always thought that the 
order of completion mechanisms in completion-styles did not matter, and in 
this case it does indeed make no difference with (setq 
tramp-default-method "scp")), and if so if it is a bug in Tramp, or in the 
completion functions.

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From: Gregory Heytings <gregory <at> heytings.org>
To: Michael Albinus <michael.albinus <at> gmx.de>
Cc: Stefan Monnier <monnier <at> iro.umontreal.ca>, 50387-done <at> debbugs.gnu.org
Subject: Re: bug#50387: Possible bug in Tramp or in completions
Date: Tue, 07 Sep 2021 17:51:17 +0000

>>> That means, "scp:" hasn't been passed to 
>>> tramp-completion-handle-file-name-all-completions, and the result of 
>>> that function looks proper to me.
>>
>> Yet it isn't.
>
> Why not? In this example, `completion-styles' contains `substring'. And 
> then you ask for completion of "/scp:". Looking at the candidates 
> returned by `tramp-completion-handle-file-name-all-completions', only 
> "pscp:" and "scp:" have the substring "scp:".
>

Because after typing "/s" or "/sc" or "/scp" it makes sense to see that 
"/scp:" is among the completion candidates, but after typing "/scp:" it 
doesn't make sense anymore, the method has already been entered by the 
user, what they now want to see is a list of hosts, and there is no way to 
see it.

>> - before your attempt to fix the problem, C-x C-f / TAB only displayed 
>> "scp:" and "scpx:" among the completion candidates (or e.g. "ssh:", 
>> "sshfs:" and "sshx:" with tramp-default-method set to "ssh");
>
> Yes, this is changed.
>
>> - now C-x C-f / TAB displays all Tramp methods;
>
> When Tramp is loaded. I regard this to be correct.
>
>> - with my patch, which actually fixes the bug, C-x C-f / TAB does not 
>> display any Tramp method, as it should.
>
> Again, C-x C-f / TAB displays a Tramp method only when Tramp is loaded.
>

I filed a bug report, explained in every detail what the bug is, provided 
a patch, and the end result is a situation that is worse than earlier.  I 
expect other bug reports from confused users, and hope that you'll change 
your mind.

Closing.

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