bug#58784: 28.2; project-buffers incorrect under let-bound default-directory

[Dmitry Gutov]

Hi!

On 26.10.2022 03:13, Sean Devlin wrote:
> The project.el function project-buffers can return incorrect results
> when invoked while a let-binding for default-directory is in
> effect. This is because project-buffers (both the default and vc-based
> implementations) does its work by inspecting the local value of
> default-directory in each buffer, and the let-binding temporarily
> affects this value.
>
> To see this in action, start emacs with -Q and evaluate the following
> forms in order:
>
> (require 'project)
>
> (find-file-noselect "/tmp/tmpfile")
> (setq my-project '(transient . "/tmp/"))
>
> ;; just the tmpfile
> (project-buffers my-project)
>
> ;; both tmpfile and scratch
> (let ((default-directory "/tmp/"))
>    (project-buffers my-project))

Thanks for the report, I can see the problem.

> In the last form, project-buffers includes the current buffer (i.e. the
> scratch buffer in our example) with the results. (This is true even if
> the current buffer is visiting a file in some unrelated directory.)
>
> This matters because the command project-switch-project let-binds
> default-directory before calling project-switch-commands. This means
> that if you set project-switch-commands to some function that calls
> project-buffers, you will get incorrect results.
>
> For example, evaluate the following forms in order:
>
> (defun my-list-project-buffers ()
>    "List the current project's buffers."
>    (interactive)
>    (let ((buffer-list (project-buffers (project-current t)))
>         (buffer-name (project-prefixed-buffer-name "my-project-buffer-list")))
>      (with-current-buffer (get-buffer-create buffer-name)
>        (erase-buffer)
>        (save-excursion
>         (dolist (buffer buffer-list)
>           (insert (buffer-name buffer))
>           (insert ?\n))))
>      (switch-to-buffer buffer-name)))
>
> (setq project-switch-commands #'my-list-project-buffers)

Looks like a reimplementation of projectile-ibuffer, seems useful.

> ;; list tmpfile but also scratch
> (project-switch-project "/tmp/")

Not sure how to fix this, though. In bug#53626 we discussed a somewhat 
similar problem, and a let-binding seems impossible to "escape".

What else can we do? One option is to change the signature of every 
compatible command to take the project object as its first argument. 
Might have been more realistic when the package was first written, too 
much breakage now, probably.

Another would be to add a new var to help override the project choice 
without touch default-directory.

Something like the attached. Please try it out.

project-current-directory-override.diff
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