Ability to specify cache on mesos artifact

[ssalinas]

/fixes #1416

[bydga]

Hi again, I've just tried the cache from self-compiled singularity (current master). And when I did a deployment with the original approach (no caching, just specifying the artifacts as strings)

uris: ["http://some.url/resource.tgz", "s3n://some-other-resource-from-s3.tgz"]

None of the resources was unpacked in the sandbox directory - which Mesos does by default when not specifying the cache field in the protobuf.

I think the behavior should be the same - see: http://mesos.apache.org/documentation/latest/fetcher/

message CommandInfo {
  message URI {
    required string value = 1;
    optional bool executable = 2;
    optional bool extract = 3 [default = true];
    optional bool cache = 4;
    optional string filename = 5;
  }
}

When I did the deploy with the object specification

uris: [{uri: "http://resource.tgz", extract: true} ]

everything worked just fine

[ssalinas]

Ah, I had been defaulting it to false for all of those parameters, we can update that

[bydga]

Well, since you are breaking previous behaviour - which might cause all tasks that rely on an archive unpacking to fail, plus its against the mesos-default behavior, I would suggest to change the extract flag to true :)

[ssalinas]

Yep, like I said, we can update. I'll get a PR in before we do the next release