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Re: [Nmh-workers] Locking (specifically, sequences)

2013-03-10 12:04:38
- The "complex" case, where the sequence file could
  potentially be changed from the time that it's first
  read until the sequence file needs to be manipulated.
  "inc" is an example; "pick" is another.

Is sortm in the same category as folder -pack?

Yeah, I think so.

I'm worried about something else:  concurrent read/write of
the sequences file.  If two or more processes read it and
write it concurrently, the last one to write will destroy
everything that the other processes wrote.  Locked-read-
write will detect that, but I'm not sure we can always
correct it.  I don't have a concrete example of where we
can't, but it feels analogous to a merge conflict.

AFAICT, if you reread the sequences file, that will solve that
problem.  I cannot think of a scenario when it does not; can
you think of one that doing that will fail?

How so?  It depends what "adding" means, but I would limit
that to just nmh commands that can add a new message.  I
think that's inc, refile, rcvstore, burst, and thing that
fcc's (comp, dist, forw, repl, mhmail).  And I don't think
they'll overwrite a message that any other, even non-nmh,
program had created.

Programs that use folder_addmsg() are safe to run concurrently.
inc does not.

--Ken

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