Date: 24 Aug 1999 11:34:24 +0300 From: Ville-Pertti Keinonen <will@iki.fi> To: grog@lemis.com (Greg Lehey) Cc: hackers@freebsd.org Subject: Re: Mandatory locking? Message-ID: <86r9ktk25r.fsf@not.demophon.com> In-Reply-To: grog@lemis.com's message of "24 Aug 1999 06:43:30 %2B0300" References: <19990823223645.A14001@netmonger.net> <19990824131036.B83273@freebie.lemis.com>
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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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