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From: ericblake AT comcast DOT net (Eric Blake)
To: Alex Riesen <raa DOT lkml AT gmail DOT com>
Cc: cygwin AT cygwin DOT com
Subject: Re: cygwin-1.5.19-2: mkdir returns inconsistent errno
Date: Thu, 26 Jan 2006 15:25:57 +0000
Message-Id: <012620061525.1038.43D8EA05000B44680000040E22007358340A050E040D0C079D0A@comcast.net>
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> > > This is highly unexpected, does not match linux behaviour (it returns 
> EEXIST),
> > > and actually breaks git (git clone, creation of pathnames, to be precise).
> >
> > Then git has a bug.  Report it there.  To be portable
> > when making pathnames, you must first check
> > for directory existance rather than relying on an
> > errno of EEXIST to tell you the directory exists.
> 
> How do you do it race-free?

chdir().  If chdir() succeeded, then the directory existed,
regardless of the errno that mkdir() reported.
Take a look at gnulib's mkdir-p.c module:
http://cvs.savannah.gnu.org/viewcvs/gnulib/lib/mkdir-p.c?rev=1.5&root=gnulib&view=auto

> 
> No, I don't think it is a bug in git, or in any code which
> expects EEXIST from mkdir in case an entry actually
> just plain does exist.

POSIX does not guarantee EEXIST when calling mkdir()
on an existant directory.  Get used to that - any code
that assumes that mkdir MUST fail with EEXIST is
inherently unportable, according to POSIX.

> 
> Are you sure? Linux 2.6.15. It correctly reports EEXIST
> on /usr and /usr/local and creates /usr/local/dir.
> I'd consider that a bug, if it were otherwise. The reason
> of racing alone if enough.

Maybe the setup was slightly different than I remembered,
Paul Eggert described it here:
http://lists.gnu.org/archive/html/bug-coreutils/2005-10/msg00155.html

In fact, that entire thread is where this mkdir not failing
with EEXIST issue came up.

--
Eric Blake

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