bug#12632: file permissions checking mishandled when setuid

[Paul Eggert <eggert@cs.ucla.edu>]

On 10/22/2012 05:40 PM, Stefan Monnier wrote:
>>> We use IS_DIRECTORY_SEP in other places, so why avoid it here?
>> > If it's not needed, it makes the code a bit harder to read.
> I don't see why.  The name says very clearly what it is intended to do.
> On the contrary, it seems more clear than comparing with '/' which only
> makes sense if you happen to know that / is the special char used as
> directory-separator.

Using IS_DIRECTORY_SEP indicates that it's necessary to
process a file name using a system-dependent interpretation
rather than the standard POSIX interpretation.  When
plain X=='/' would do, it can mislead the reader if the code
does IS_DIRECTORY_SEP (X), as the reader may naturally (and
mistakenly) infer that the code was written that way because
it must treat '\' specially on Windows.  It's primarily
this clarity, not the brevity, that I was referring to.