more machine fixes

My latest guess about what is happening is something like this:

1. request starts, tries to render output file.  in the process it populates some things on the output card including the selected_revision_id.  this probably doesn't get cached in the rails cache but probably does in the local cache(?).  Any way, at this point it's the current_revision_id from the db, here the "rev1".
2. file isn't there.  controller rescue sends us to not_found
3  update_machine_output is called.  this creates a new revision (with a new current_revision_id / rev2) and a new file (with rev1, because it's cached in some form on the output card)
4. machine_output_url is called and uses with rev2

So the url matches the db, but the file is one revision behind, right?

I'm still not clear on where exactly things are getting cleared that they should be.  Probably time to play with byebug...