bug#60188: 30.0.50; zerop cpd-length are not all handled in project--read-file-cpd-relative

[Shuguang Sun via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Juri Linkov <juri@linkov.net> writes:

>> When I call C-x p d (project-find-dir), it reports the error message:
>> `project--read-file-cpd-relative: Wrong type argument: stringp, nil`
>>
>> Finaly I found it is due the the zero length of
>> `common-parent-directory` in `project--read-file-cpd-relative` as I
>> using `fd` to provide the list of dirs which does not include the common
>> project root in the returnd dirs.
>>
>> In `project--read-file-cpd-relative`, the zero length of
>> `common-parent-directory` is handled for `prompt` but not for `abbr-cpd`
>> which will be used as parameter of `string-prefix-p`.
>>
>> Below is the diff and a working patch. But no sure for the impact of HIST.
>>
>> ```
>> diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
>> index 559da6dd649..8c4ea8eeca0 100644
>> --- a/lisp/progmodes/project.el
>> +++ b/lisp/progmodes/project.el
>> @@ -1039,7 +1039,9 @@ project--read-file-cpd-relative
>>           (_ (when included-cpd
>>                (setq substrings (cons "./" substrings))))
>>           (new-collection (project--file-completion-table substrings))
>> -         (abbr-cpd (abbreviate-file-name common-parent-directory))
>> +         (abbr-cpd (if (zerop cpd-length)
>> +                       ""
>> +                     (abbreviate-file-name common-parent-directory)))
>
> Strange, I can't reproduce the problem.  When cpd-length is 0,
> then for the empty string (abbreviate-file-name "") returns "".
>
>
>
>

Actually in my case, it is `(abbreviate-file-name nil)`. As `fd` returns
relative path, common-parent-directory is nil.

-- 
Best Regards
Shuguang Sun