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

From: Ken Hornstein
Subject: Re: [Nmh-workers] Locking (specifically, sequences)
Date: Sun, 10 Mar 2013 13:04:26 -0400

>> - 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.


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