[WIP] Allow builds for arches that arent currently built

[andymcc]

buildprep will fail if the current arch does not exist in the
builds.json, since the directory structure is not in place. This checks
whether the arch is included in the builds.json already, and if not it
will continue through.

[andymcc]

Perhaps a better approach would be to change is_empty to check if the arch specific build exists rather than the trying to account for every occasion where it checks "is_empty" - since that seems to be the actual fail.

[jlebon]

Thanks for opening this! See related discussions in #673 and coreos/fedora-coreos-tracker#262 about multi-arch orchestration.

What's the model you're pursuing right now? Essentially buildprep from and buildupload back to the same source? That's going to be racy :) I think we're going to need lower-level "primitives" in cosa to do this properly.

Let's work towards resolving these discussions, though in the meantime, probably easier to just target a different source?

[andymcc]

@jebon - thanks for the feedback! I'm updating to target a different source for now.

My approach was to treat "no build for current arch" the same way as "no build" - but after reading the issue you linked and the way s3 handles locking it's pretty clear that won't work too well. I'll close this one out. Thanks again!