[Buildbot-devel] Have 'nextSlave' to query lock status

[Jared Grubb]

I control my builds using master and slave locks, and have noticed this scenario:
* Slave A starts a build, grabs slave lock L for 30 mins
* Slave B is idle
* Change comes in and triggers a build (that requires lock L to run)
* New build gets assigned randomly to A
* New build blocks waiting for a lock, even though B could run right now

I'm considering writing a 'nextSlave' functor that will only return slaves that can get the lock now (tested via Lock.isAvailable I think). Will that work? 

It sounds like it will work, but I am concerned that, if all slaves fail the lock-test and nextSlave returns None, there is some corner case where nothing will trigger a new test later. What triggers these maybe-run-build checks? Is it pretty much any action like another build finishing? I've been trying to find this in the code and havent yet found it.

Jared