on a disconnected client, without running the time-consuming rsyncs.
This is useful when a build is interrupted and needs to be restarted.
* After we have cleaned up the machine, reset the queue counter by using
pollmachine -queue. This has a race condition if other builds are being
dispatched to the machine (e.g. builds on another branch):
getmachine can claim a directory and increment the counter, then the
machine is polled and finds e.g. 0 chroots in use, and resets the
counter to 0, then claim-chroot is run and the build dispatched, with
the counter now off-by-one. This could be fixed by running
claim-chroot with the .lock held, but this turns out to be too
time-consuming. A two-level lock approach might also fix this
efficiently.
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