From owner-freebsd-hackers  Tue Aug 24  1:35:14 1999
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To: grog@lemis.com (Greg Lehey)
Cc: hackers@freebsd.org
Subject: Re: Mandatory locking?
References: <19990823223645.A14001@netmonger.net> <19990824131036.B83273@freebie.lemis.com>
From: Ville-Pertti Keinonen <will@iki.fi>
Date: 24 Aug 1999 11:34:24 +0300
In-Reply-To: grog@lemis.com's message of "24 Aug 1999 06:43:30 +0300"
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grog@lemis.com (Greg Lehey) writes:

> an agreement of some kind.  But what if I want to merge the contents
> of another mail folder:

>   cat oldmail >>/var/mail/grog

> That works, but it's playing with fire: if sendmail is delivering a
> message at the same time, it won't see me, and my cat doesn't get a
> lock beforehand, so both an incoming message and part of my mail
> folder could end up getting written to the same location.  With
> mandatory locking, it would work, transparently.

Certainly not with range-locking rather than file-locking.  cat is
certainly not guaranteed to be atomic, and while you shouldn't end up
writing things in the same location, what might happen unless you are
preventing multiple openers is:

    cat writes part of oldmail to /var/mail/grog
    sendmail locks /var/mail/grog
    (cat may try to write more to /var/mail/grog but blocks)
    sendmail delivers new mail
    sendmail unlocks /var/mail/grog
    cat writes the rest of oldmail to /var/mail/grog

You'll still probably end up with a broken mailbox.


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