bug#57418: 28.1; Projects not saved when vc-handled-backends is nil

[Dmitry Gutov <dgutov@yandex.ru>]

On 26.08.2022 10:46, Philip Kaludercic wrote:
> Louis-Guillaume Gagnon<gagnonlg@protonmail.com>  writes:
> 
>> Good day,
>>
>> I've recently been migrating from projectile to the built-in
>> project.el. Since then, one major annoyance is that the projects are
>> never saved into project-list-file, so I have to re-enter the projects'
>> paths everytime I make a query.
>>
>> I have narrowed it down to vc-handled-backends being set to nil in my
>> init-file. If it's left at its default, it works as expected. I could
>> just leave the variable at its default but (iirc) it's part of a bunch
>> of optimizations I made to speedup my TRAMP setup, so I'm reluctant to
>> do that.  And just to be perfectly clear, the rest of project mode seemingly
>> works as expected if that variable is nil, it's just the projects are
>> not saved.
>>
>> Steps to reproduce from emacs -Q:
>>
>> + (setq 'vc-handled-backends nil)
>>
>> + Use project.el to find a file in any project not in your
>> project-list-file, e.g. with C-x p p
>>
>> + Open your project-list-file, "/.emacs.d/projects" in my case
>>
>> Expected result: The recently opened project is listed
>> Actual result: The recently opened project is*not*  listed
> This is to be expected, as by default `project-find-functions' only
> includes one function that uses and relies on VC.  If it were just this,
> I would therefore not consider this to be a bug.  Does anything else
> work?  `project-find-file', `project-shell', `project-compile', ...?

I'm honestly surprised that things work with vc-handled-backends disabled.

Louis-Guillaume, does (project-current) ever evaluate to non-nil on your 
system? I'm guessing you end up picking the project directory manually 
every time.

> What might be worth doing is issuing a warning if `project-try-vc' (the
> default project-finding-function) is invoked while VC is effectively
> disabled.

Probably not: after all, the user can have some next backend configured. 
And we allow setting vc-handled-backends to nil.

> You could try solving this by either adding your own alternative to
> `project-try-vc' (e.g. if you just use Git, then this
> 
> --8<---------------cut here---------------start------------->8---
> (defun project-try-git (dir)
>    "Find a super-directory of DIR containing a root file."
>    (let ((repo (locate-dominating-file dir ".git")))
>      (and repo (list 'git dir))))
> 
> (cl-defmethod project-root ((project (head git)))
>    "Return the root of a explicit PROJECT."
>    (cadr project ))
> 
> (with-eval-after-load 'project
>    (add-hook 'project-find-functions #'project-try-git))
> --8<---------------cut here---------------end--------------->8---
> 
> might be enough, but still be optimised a bit more).

This should work, yes.

We could also add a variable which would substitute vc-handled-backends 
for the purposes of project.el (we'd temporarily use its value). Not 
sure it's worth the added complexity, though.

> Another idea might be to just disable VC for Tramp, or rather just some
> Tramp connections?

(setq-local vc-handled-backends) inside find-file-hook behind a 
file-remote-p check could do the trick.