Buildbot: trigger virtual builders problem

Question

I wish to trigger builders dynamically (depending of list of supported platforms of incoming project). There are virtual builders for this purposes. It's really works but there is a critical problem: when I trigger more virtual builds then number of free workers, other builds are skipped.

On image below 4 builds triggered with one active worker

On success we can see only two executed builds. Other are lost (not executed at all)

Again, for 1 free (active, not paused) worker only 2 (of 4) triggered builds was executed. For N free workers it will be N+1 executed builds. Reproduction 100%.

The code is pretty clear

class MultiplePlatformsBuilder(Trigger):
    def __init__(self, *args, **kwargs):
        ...

    def getSchedulersAndProperties(self):
        sch = self.schedulerNames[0]
        triggered_schedulers = []
        counter = 0
        # trigger scheduler for each requested platform
        platforms_list = self.set_properties.get('buildable_platforms_list')
        for pl in platforms_list:
            counter += 1
            props = dict()
            props["virtual_builder_name"] = "virtual-" + pl
            props["virtual_builder_description"] = "This is dynamic build " + str(counter)
            triggered_schedulers.append([sch, props])

        return triggered_schedulers

# trigger configuration    
c['schedulers'].append(schedulers.Triggerable(name='dynamic_scheduler', builderNames=['factory1']))
factory.addStep(MultiplePlatformsBuilder(
    schedulerNames=['dynamic_scheduler'],
    set_properties={
        'buildable_platforms_list': ["platform1", "platform2", "platform3", "platform4"]
        # platforms_list_var: Property(platforms_list_var),
    },
))

Answer 1

The problem took me several days. It turned out this is not a bug but feature named Collapsing Build Requests

All build requests with the same codebases will collapse if there are no free workers.

All I need is just disable this feature:

c['collapseRequests'] = False

I'm sure this should be mentioned in context of virtual builders because they all have same codebases but shouldn't be collapsed - this makes no sense