bug#73801: 31.0.50; project-try-vc sometimes set wrong cache project-vc-extra-root-markers

[Dmitry Gutov <dmitry@gutov.dev>]

Hi! Thanks for the report.

On 14/10/2024 09:29, Zhengyi Fu wrote:
> Consider the following project:
> 
>     test-project/
>     ├── .git/
>     └── subproject/
>         ├── marker
>         └── subdir/
> 
> If `project-vc-extra-root-markers' is set to `("marker")' and
> `project-try-vc' is invoked with `test-project/subproject/subdir', it will set the `project-vc'
> VC property of the `test-project/subproject' directory to
> `(".../test-project" project-vc nil)', so if later `project-try-vc' is
> invoked with that directory, it will return a wrong result.
> 
> This is because project-vc tries to detect the VC backend by invoking
> `project-try-vc' on the subproject while let binding
> `project-vc-extra-root-markers' to nil and the result is cached.
> 
> The following patch can fix the problem:
> 
> --- a/lisp/progmodes/project.el
> +++ b/lisp/progmodes/project.el
> @@ -595,7 +595,7 @@ project-try-vc
>               (let* ((project-vc-extra-root-markers nil)
>                      ;; Avoid submodules scan.
>                      (enable-dir-local-variables nil)
> -                   (parent (project-try-vc root)))
> +                   (parent (project-try-vc dir)))
>                 (and parent (setq backend (nth 1 parent)))))
>             (setq project (list 'vc backend root))

I've pushed a different fix in commit 29b30eb49f8 (with slightly better 
performance, I think).

Please try it when you can.

It would be nice to get either of the patches into Emacs 30, too, but it 
might be a little late given where it is in the pretest.