Hide resources from showing on the store

[Colin Watson]

On Sun, Dec 11, 2016 at 12:57:35PM -0800, Mark Shuttleworth wrote:
> On 05/12/16 07:12, Colin Watson wrote:
> > As suggested in this thread and on IRC, I fell back to accepting
> > zero-sized resources instead and uploading those to the store, and that
> > works OK:
> >
> >   https://jujucharms.com/u/launchpad/launchpad-buildd
> >
> > It seems like a fairly nasty hack though.  Resources are pretty nice,
> > and this feels like a wart.
> 
> I think the point is that "juju deploy foo" should always give you a
> reasonable result. In your case, that's the PPA option. *if* a resource
> is specified, then it should be used. What we want to avoid is charms
> which don't work at all unless a resource is specified.

Right, I can definitely see that, and ensuring that there's always a
reasonable result is exactly what I'm trying to do.

I'm not sure that the current mechanism particularly helps with that,
though.  Charm authors can still easily upload a zero-sized resource to
the store and then have their charm refuse to do anything unless you
attach a "real" resource.  If we assume that authors will legitimately
want some way of doing optional resources (it seems like a handy way to
have a payload which normally comes from some centralised source but
which you might instead attach directly for quick iteration during
development), then the zero-sized resource thing seems likely to become
a fairly widespread pattern that people will copy.  Given that, making
authors resort to non-declarative hacks to achieve the goal of an
optional resource might just increase the chance of mistakes.

I can see how resources should default to being required at the charm
store level to avoid making it easy to publish broken-by-default charms
by accident.  But how about something like this:

 * Add an "optional: true" option to resources, which the charm store
   would interpret as allowing a charm to be released without that
   resource.

 * Add some kind of affordance to charm-helpers to make this easy to use
   correctly.  For instance, resource_get might be changed to take a
   "default" parameter, which would be accepted and required iff the
   resource in question is optional, or something along those lines.

 * Perhaps also add something to layer-apt and layer-snap to encapsulate
   the "install from this source by default, but use a resource if it's
   attached" pattern.

I don't have a broad enough view of charms to be able to tell whether
this is a good idea in general or overkill, but it would certainly have
saved me some effort the other day if I could have just written
something like this instead of a bunch of reactive code that took me
several tries to get right:

  options:
      apt:
          packages:
              - launchpad-buildd
              - bzr-builder
              - git-build-recipe
              - quilt
          optional-package-resources:
              - launchpad-buildd
              - python-lpbuildd

-- 
Colin Watson                                       [cjwatson at ubuntu.com]