[users at bb.net] Anecdote about attempting to make scheduelrs reconfigurable.

[Neil Gilmore]

Good morning everyone,

As you remember, we're having a lot of trouble related to schedulers in 
0.9.X not being reconfigurable in any meaningful sense of the word. I 
got the go-ahead to try to make a patch for this. Unfortunately, I've 
run into a fairly serious problem.

The advice I was given was (in a nutshell) to take the code in the 
various schedulers' __init__ functions, and spread it into 2 functions. 
checkConfig() would check its arguments for validity, and 
reconfigService() would put them into the instance. Sure, there were 
some various snags at first. Partly, I'm not used to Python's 
argument-passing model. Partly, figuring out which arguments needed to 
get passed where. Partly, figuring out that name should only exist in 
the __init__'s, not the checkConfig's, etc.

What happens when we create a scheduler is this:

The scheduler's __init__ function is called. This calls it's base 
class's __init__, and so on, up to BuildbotService.__init__. Note that 
all arguments must be preserved during these calls. You'll see why.

BuildbotService.__init__ calls checkConfig. Now, this call goes to the 
scheduler's checkConfig, which checks its own specific arguments, then 
passes everything else up to it's base class's checkConfig, and so on.

Everything,s great, right? Wrong...

At this point, we have validated everything. But only the name is in the 
instance. Because everything else is supposed to get assigned in 
reconfigService.

But in between __init__ and reconfigService, other things happen. Such 
as check_single_master() (at least for single master systems). That 
attempts to check the builder names in the schedulers vs. the builders' 
names in the config. Naturally, this fails because the builder names 
haven't been put into the schedulers yet. And it would go on to check 
other things that probably fail, too.

I'm not sure how to resolve this. The hacktastic way would be to just 
figure out what might ever get checked and put that back into __init__. 
But that's not very clean. The proper way would be to figure out how to 
make a scheduler reconcile it's own builderNames, et al. with the 
builders in the config. But I'd need to be able to get the config being 
checked, and I haven't figured out how to do that. There's other checks 
that go between items like this that I'd need to do, too.

Really, I'm just uncomfortable having this task spread out into other 
areas of buildbot.

Or I just go back to our 80% solution and hack checkConfig...

Neil Gilmore
grammatech.com