[MESOS-2056] Refactor fetcher code in preparation for fetcher cache

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 0.22.0
• Component/s: agent, fetcher
• Target Version/s: 0.22.0
• Epic Link: Fetcher Cache
• Sprint: Mesosphere Q4 Sprint 2 - 11/14, Mesosphere Q4 Sprint 3 - 12/7
• Story Points: 1
• Assignee: Bernd Mathiske
• Reporter: Bernd Mathiske
• Shepherd: Benjamin Hindman
• Created: 10/Nov/14 17:30
• Updated: 18/Nov/14 13:56
• Resolved: 18/Nov/14 13:56

Description

Refactor/rearrange fetcher-related code so that cache functionality can be dropped in. One could do both together in one go. This is splitting up reviews into smaller chunks. It will not immediately be obvious how this change will be used later, but it will look better-factored and still do the exact same thing as before. In particular, a download routine to be reused several times in launcher/fetcher will be factored out and the remainder of fetcher-related code can be moved from the containerizer realm into fetcher.cpp.