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Date: Wed, 26 Nov 2003 19:45:54 -0800
From: Claus Assmann <freebsd+stable@esmtp.org>
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Subject: Re: NFS/Sendmail issue since upgrading to 4.9-STABLE
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On Wed, Nov 26, 2003, Adam Maloney wrote:

> This custom application reads a directory list and for each qf file,
> attempts a LOCK_EX.  For the past 2 years this has appeared to work fine
> with Sendmail - if Sendmail is still working on the qf file, we aren't able
> to get an exclusive lock, so we go on to the next file, etc.  (Maybe Claus
> can confirm some of this behavior?)
> 
> It appears that Sendmail changed from 8.12.9 (FreeBSD 4.8R) to 8.12.10
> (FreeBSD 4.9-STA).  What we believe is happening is that the qf file is
> being created, and our application is getting the exclusive lock before
> Sendmail.  This makes Sendmail spit out the "resource temporarily
> unavailable" error, since it can't lock the file, and it leaves our
> application with an empty qf file that we are trying to process.

Yes, there's a race condition. 8.12.10 changed the behavior to avoid
overwriting an existing qf file:

        Better error handling in case of (very unlikely) queue-id conflicts.

                tfd = open(tf, O_CREAT|O_WRONLY|O_EXCL, FileMode);
                if (tfd < 0 ||
                    !lockfile(tfd, tf, NULL, LOCK_EX|LOCK_NB) ||
                    (tfp = sm_io_open(SmFtStdiofd, SM_TIME_DEFAULT,

So if your application accesses the qf file between open() and
lockfile() sendmail will complain and exit() without trying again.
Older versions would try again and eventually overwrite the file...

Fix 1: don't mess with qf files.
Fix 2: check the size before accessing a qf file: don't touch
(open/lock) empty files.