[Buildbot-devel] Latent Slave code changes

John Carr johnmcarr at me.com
Tue May 11 14:20:04 UTC 2010

On Tuesday, May 11, 2010, at 12:37AM, <exarkun at twistedmatrix.com> wrote:
>On 9 May, 09:15 pm, johnmcarr at me.com wrote:
>>Hi guys,
>>I'd like to make some changes to latent slaves to better support 
>>starting virtual machines on demand. A working prototype of the changes 
>>i'd like to make are here, but i'd like to clarify a few things. I dont 
>>expect that this is ready to land.
>I haven't looked at your code, so sorry if the answer to this question 
>would have been obvious otherwise.  But.  Why isn't it sufficient to 
>have the Deferred returned from start_instance (which already supports 
>Deferred return values) just fire later, when some previous VMs have 
>been shut down?

start_instance does indeed return a Deferred, and whenever i have said "make start_instance return true or false", what i actually mean is return a Deferred that calls back with True or False. This allows the scheduler a chance to find another slave (it should be possible to control multiple VM hosts with this code), rather than effectively blocking until a slot is available - and with how the code is currently structured, that is really what you are proposing.

start_instance currently fires its callback when a latent slave has started and attached - this is quite neat and i have no intention to change that. What my change does is add a code path where it can return early and say "Nope, you cant use this slave after all" without it being an error condition. The scheduler kicks in and can assign this to other slaves. I should add that error conditions in start_instance currently disable the latent slave all together, which is not desirable.


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