Ville-Pertti Keinonen scribbled this message on Aug 24:
>
> g...@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...
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