Summary:
This was the major goal of D3859/D3855, and to a lesser degree D3854/D3852.
As Drydock is allocating a resource, it may need to allocate other resources first. For example, if it's allocating a working copy, it may need to allocate a host first.
Currently, we have the process basically queue up the allocation (insert a task into the queue) and sleep() until it finishes. This is problematic for a bunch of reasons, but the major one is that if allocation takes more resources (host, port, machine, DNS) than you have daemons, they could all end up sleeping and waiting for some other daemon to do their work. This is really stupid. Even if you only take up some of them, you're spending slots sleeping when you could be doing useful work.
To partially get around this and make the CLI experience less dumb, there's this goofy `synchronous` flag that gets passed around everywhere and pushes the workflow through a pile of special cases. Basically the `synchronous` flag causes us to do everything in-process. But this is dumb too because we'd rather do things in parallel if we can, and we have to have a lot of special case code to make it work at all.
Get rid of all of this. Instead of sleep()ing, try to work on the tasks that need to be worked on. If another daemon grabbed them already that's fine, but in the worst case we just gracefully degrade and do everything in process. So we get the best of both worlds: if we have parallelizable tasks and free daemons, things will execute in parallel. If we have nonparallelizable tasks or no free daemons, things will execute in process.
Test Plan: Ran `drydock_control.php --trace` and saw it perform cascading allocations without sleeping or special casing.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2015
Differential Revision: https://secure.phabricator.com/D3861
