Re: how to "kill" a job

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Bruce Robertson writes:
> Is there such an API point?
> If not, do you have something under development?
> Else, any pointers on how to do it?

There isn't yet such a thing, no, and I haven't planned it. (yet)

Kill an as-yet unprocessed job would be easy -- simply an unlink.
But if the job is active, it'd be quite a bit harder; it's easy in the
LPR-style of queueing, because that has (a) a central server and (b) each
job is a separate process, so a kill(3) of the process can be used to kill
the job, but in DQ, there's no separate process per job -- one process
can deal with many jobs sequentially.

I would say the cleanest way to do it would be to arrange that dq servers
can provide a new constructor switch indicating a signal that should
be delivered if a job kill is to be requested; if that switch isn't
present, killing an active job isn't supported.

The DQ server code would then write that to a flag file in the queue
directory, so that DQ clients (ie. a new "dq-kill" script) can be aware
that the server(s) support this.

The server code should then implement a signal handler for that signal;
that handler will check to see if it is currently processing a killed job
(ie. if the data file has been unlinked before it's complete), and give up
the remainder of the job's processing if it is.  

(alternatively, the server could just ignore the signal, if it's not an
issue for that server's design, e.g. if the jobs are very lightweight, and
completing processing of a killed job will not cause problems.)

If that constructor switch was not specified, I think it would be unsafe
to assume that DQ kill is supported by that queue; we can't go unlinking
active jobs without some kind of "this is supported" negotiation, as a
disappearing job file may cause a die() in server code that wasn't aware
this was likely to happen.

- --j.
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