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

[Glenn Morris <rgm@gnu.org>]

Philip Hodges wrote:

> [using emacs-w32.exe 24.3 in cygwin but also applies to other platforms.]
>
> Symptom:
> When I try to cd to my samba-mounted directory,
> or try to run ediff-files, it refuses with the error
> "Cannot cd to my_directory_name: Permission denied"
> The same directory opens fine in dired-mode, and I can open files within it.

> This is just one way in which file-executable-p can produce a false negative,
> where executing the file or searching the directory may succeed after all.
>
> [False positives, where the operation refuses, have no impact so long as
> the operation fails quickly with an equivalent but authoritative error message.]
>
> This is typical of many situations where it is not good enough just
> to check quickly whether it just looks like it can or cannot be done;
> you have to also actually try it, and report the outcome of
> that particular attempt at that particular moment.
> A preliminary check may still be useful too if it produces a better error
> message quickly up front with no dashed expectations or cleaning up to do.
>
> The comments in fileio.c check_executable (and check_writeable) point out
> that on some filesystems there might be an access control list (ACL) in force,
> and that the effective user might have more permission than the real user.


Perhaps there is something Cygwin-specific at work, because I don't
think I can reproduce such a problem on eg RHEL 6.5 GNU/Linux.

As user A:
mkdir foo1 foo2
chmod 700 foo1 foo2
setfacl -m u:b:r-x foo1

As user B:
In the shell: [ -x foo1 ]      # -> true

In Emacs 24.3: file-executable-p foo1   ; -> true


(There's also file-accessible-directory-p; does that work any better for
you?).