Date: Tue, 24 Aug 1999 02:14:31 -0700 From: John-Mark Gurney <gurney_j@efn.org> To: Ville-Pertti Keinonen <will@iki.fi> Cc: Greg Lehey <grog@lemis.com>, hackers@FreeBSD.ORG Subject: Re: Mandatory locking? Message-ID: <19990824021431.55023@hydrogen.fircrest.net> In-Reply-To: <86r9ktk25r.fsf@not.demophon.com>; from Ville-Pertti Keinonen on Tue, Aug 24, 1999 at 11:34:24AM %2B0300 References: <19990823223645.A14001@netmonger.net> <19990824131036.B83273@freebie.lemis.com> <86r9ktk25r.fsf@not.demophon.com>
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Ville-Pertti Keinonen scribbled this message on Aug 24:
>
> 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.
what you do is this:
lockf -k $mailfile cat ${mailtmp} >> $mailfile
then you don't have to worry.. that's what lockf is for...
--
John-Mark Gurney Voice: +1 541 684 8449
Cu Networking P.O. Box 5693, 97405
"The soul contains in itself the event that shall presently befall it.
The event is only the actualizing of its thought." -- Ralph Waldo Emerson
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