Adjust the Drydock allocator to limit each pending lease to one allocating…


Adjust the Drydock allocator to limit each pending lease to one allocating resource

Ref T13677. Currently, one lease may cause multiple resources to allocate simultaneously if it starts allocating one, then wakes up from a yield later on and still sees no available resources.

This is never desired -- or, at least, produces desirable behavior only entirely by accident. Normally, it causes an excess of resources to allocate.

This is not a catastrophic problem: the extra resources usually get used sooner or later or cleaned up; and the total amount of badness is limited by overall resource allocation limits.

However, this behavior is also suppressed by an artificial "25% of current pool size" growth limit throttle which I intend to remove. Removing this throttle without fixing the allocator behavior could make this "too many resources" problem worse.

Change the allocator so that a lease that has started allocating a resource won't allocate another resource until the first resource leaves the "pending" state.

This also fixes some general oddness with the allocator and attempts to simplify the structure.

Test Plan:

  • Ran 8 taskmasters.
  • Destroyed all resources and leases.
  • Leased 4 working copies.
    • Saw exactly 4 resources build and lease, all simultaneously.
  • Destroyed all resources and leases.
  • Leased 32 working copies.
    • Saw exactly 32 resources build and lease, approximately 8 at a time (limited by taskmasters).
  • Destroyed all leases (but not resources).
  • Leased 32 working copies, saw them satisfied by existing resources.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13677

Differential Revision: https://secure.phabricator.com/D21807