bug#17330: files.el cd-absolute overcome false negative from file-executable-p

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 08 May 2014 07:55:34 +0200
> From: Philip Hodges <philip.hodges@bluewin.ch>
> CC: rgm@gnu.org, 17330@debbugs.gnu.org
> 
> > I prefer to solve the problem rather than ask users work around it.
> 
> So when can we reasonably expect a guarantee of no more false negatives 
> for users of 24.3 without having to inspect the fileio.c and files.el 
> and reinvent an undocumented workaround?

Emacs 24.3 was released more than a year ago, so fixing this in that
version might be possible only by some suitable change to the
directory's security descriptor outside of Emacs (if such a change is
possible).

But we can hope to fix this in future versions of Emacs.

> It will be great if you really can *solve* the problem, even just for 
> this one particular scenario. I already suggested a pathological 
> counterexample. Other sources mentioned do indicate that it is 
> impossible to solve it reliably in general. But perhaps it will be 
> enough in practice.

If we understand the problem in enough detail, we might find a
solution of some sort.

> Only the positive outcome of file-executable-p is documented as "this 
> means you can access files in that directory". The negative outcome is 
> not explicitly documented as meaning you cannot, yet that is how callers 
> are interpreting it. So there is clearly scope for rewriting the 
> documentation and changing the callers' logic to match.

That is a different, although related discussion.  Arguably, if a
directory is not accessible by me, Emacs had better not attempt that,
even if it might succeed, and instead leave it for the user to fix the
access rights by other means.

But even if we accept your views on this, it is better to try to solve
the problem than work around it.