Dispatch time estimation for build farm jobs

Introduction

Please note that job dispatch times are estimated. Alas, we have not mastered the art of writing psychic software yet.

For the purpose of this description a 'platform' is considered to be the combination of a

• processor (e.g. i386 or amd64) and a
• virtualization setting (one of: true, false or null)

Build farm jobs can either target a specific platform (e.g. binary builds) or be platform-independent (e.g. "generate a source package from a recipe" builds).

The former can only make use of build machines (or "builders" in Soyuz parlance) of the given platform while platform-independent jobs may run on any available builder.

Jobs with an unspecified virtualization setting will be dispatched to virtual builders only.

Builders can -- roughly speaking -- either be idle or building. For any job running on a particular builder its estimated duration and its start time is available allowing us to estimate the job's remaining execution time.

By the way, did I already mention that job dispatch times are an estimation only?

Given:

• a queue of pending (i.e. ready to build) build farm jobs sorted in descending order according to their score
• a pool of builders

Wanted: the estimated dispatch time for a specific job in the pending queue i.e. for the job of interest (aka JOI).