bug#47168: 28.0.50; Infinite recursion in project-root

[Dmitry Gutov]

Hi Juri,

On 15.03.2021 19:04, Juri Linkov wrote:
> When for some reason the argument of 'project-root' becomes nil,
> for example, when the current buffer is not under version control,
> then
>
>    (project-root (project-current))
>
> goes into infinite recursion:
>
> Debugger entered--Lisp error: (error "Lisp nesting exceeds 
> ‘max-lisp-eval-depth’")
>    project-root(nil)
>    project-roots(nil)
>    ...
>    project-root(nil)
>    project-roots(nil)
>    ...

Note that the right way to write this code is either

  (project-root (project-current t))

or

  (when-let ((project (project-current)))
    (project-root project))

so we only see this when the programmer failed to account for the 
absence of current project.

Still, it would be nice to never show a backtrace like this, even in 
those cases.

The idea here was to keep compatibility with backends which implement 
project-roots and not project-root (perhaps the built-in vc backend in 
Emacs 27), as well as let the clients call either project-root or 
project-roots, also for backward compatibility purposes. Worst case, we 
can give up on that and require all 3rd party code standardize on 
project-root and have all users install the latest project.el from ELPA.

But perhaps we still can have it both ways?

Stefan, any chance there's a relatively non-dirty way we can check 
whether there is a non-default implementation for the generic function 
with given args, before calling it? That would allow us to break recursion.