This builder:

https://uberchromegw.corp.google.com/i/chromeos.infra/builders/CrOS%20Repo%20Sync

Polls for changes to repo branches, and triggers waterfall builds.

It would be really useful for firmware, factory, and possibly other builds, to have support for this in swarming. We have requested that support for this be added to LUCI Scheduler, but because the logic is repo specific, that is unlikely to happen.

And new polling builder (or an updated version of the current one) seems like the most expedient solution.