bug#60100: 29.0.60; project-current is not resilient

[Dmitry Gutov <dgutov@yandex.ru>]

On 15/12/2022 21:00, Juri Linkov wrote:
> Is this intentional that
> 
> ```
> (project-current nil "/tmp/nonexistent/")
> ```
> 
> raises the error
> 
>    Debugger entered--Lisp error: (file-missing "Opening directory" "No such file or directory" "/tmp/nonexistent")
> 
> instead of silently returning nil.

I would say this is expected.

How could we avoid this? By adding a file-exists-p check somewhere 
inside, right? And failing silently if it returns nil.

Right now the caller code needs to use that check. Or, more likely, 
figure out why they are passing in a directory that does not exist.

> Because when called with
> an existing non-project dir, it just returns nil:
> 
> ```
> (project-current nil "/tmp/") => nil
> ```

nil is a normal return value in this case: it means "no project here", 
that's a regular occasion. The callers who don't want to deal with nil 
should pass t as the first argument.