PSA: Automated tree closing migrating from Gatekeeper to LUCI-Notify

[Owen Rodley]

Hi,

Today we're moving automated tree closing from Gatekeeper (the legacy system) to LUCI-Notify. The new system should be faster, easier to configure and easier to maintain. This will make no difference to manual tree closing. If you have any concerns about tree closures post-migration, please reach out to me. We'll also migrate email notifications to LUCI-Notify shortly.

Going forward, tree closing will be configured via starlark along with the rest of the builder configs. The main "ci.builder" function has a parameter "tree_closing" which causes that builder to close the tree if set to True. See for example https://crrev.com/c/2289271 which enables tree closing for most memory builders, with a few exceptions.

Aside:

You may have noticed that the chromium tree has been very green lately. This is because Gatekeeper has been down. This change will restore automated tree closing and hence most likely make the tree more red.