Add option to skip downloading

[phillbaker]

Similar to #14, adds an option to the params to skip any downloading on a put - there's usually no need for this as it's immediately discarded and results in additional bandwidth.

Example usage:

resources:
- name: <resource name>
  type: <resource type name>
  source:
    access_key_id: {{aws-access-key}}
    secret_access_key: {{aws-secret-key}}
    bucket: {{aws-bucket}}
jobs:
- name: <job name>
  plan:
  - put: <resource name>
     params:
       skip_download: true

[afeld]

Sorry for the slow response. I like it for efficiency's sake, but don't love the additional complexity, variance in behavior, or additional option that we need to explain to the user. @LinuxBozo Thoughts?

[LinuxBozo]

Ok, if I'm understanding correctly, this option is solely useful for when doing a put on the resource, since the Concourse lifecycle does a get right after a put to verify that it receives the same "version" that it just placed. In most cases where you don't care, you can do a no-op, which is the intent here it appears. Unfortunately, by using the params to a put, the get/in has no clue about this param, so it would have no effect. See: vmware-archive/atc#72

[phillbaker]

Hm, thanks for the link @LinuxBozo, I'm not sure how, but it is working 😀 .

In my mind this parallels the skip_download on the official docker image resource, which was worth an optional parameter.

I'll close this PR for now and if there're other folks interested we can re-open. Thanks!

[da1nerd]

I think this would be a valuable feature. We have a server set up that is generating builds for all of our github branches which we cache for 30 days. Right now this resource is needlessly downloading 65GB of data my pipeline will never use.