[what to do on a Kernel-unlock failure]
What would be "safe"? Just writing the message about isn't safe, and
neither is truncating and returning an error. What's that leave?
I confess I don't know enough about procmail's internals for error
handling, so I'm not sure I know what the right thing for procmail
would be. I was suprised that procmail didn't try any of its usual
error recovery strategies - writing to the default mailbox, or if that
fails writing to /usr/spool/mail, or if that fails bouncing the mail.
I think the real question is does a Kernel-unlock failure indicate
that the message failed to deliver? Technically in my case, no. So
maybe procmail doesn't need to do anything about this. Still, it might
be best to err on the side of caution - the worst that you get is a
duplicated email.
Thanks for pointing me at the lockfile program.