workshops snap cannot be seeded about workshops

[jpnurmi]

Thanks, the plug was removed by 235ea64 and de4efe8.

[tillkamppeter]

@mvo @jhenstridge This problem occurs in general if we have a Snap A which needs another Snap B be installed in order to work correctly.

In my case it is any Snap which has print functionality. Those Snaps plug the recently released cups interface and the cups interface's slot is provided by the CUPS Snap, so these Snaps require the CUPS Snap to be installed.

As snapd has no general support for dependencies between Snaps we use the workaround of the fake content interface, putting the following into the snapcraft.yaml of Snap A:

plugs: 
   foo-install-B: 
     default-provider: B 
     interface: content 
     content: foo 
     target: $SNAP_DATA/foo 

This works for Snaps in general which we install from the Snap Store or from a *.snap file. If Snap A is seeded it breaks the system installation as described in the initial posting, even if Snap B is also seeded.

What we need in snapd is general support for default-provider: in any plug, not only content interfaces, so that one could do:

plugs: 
   use-feature-of-B: 
     default-provider: B 

or in the snapcraft.yaml of an application which prints:

plugs: 
   cups: 
     default-provider: cups 

to install the CUPS Snap (cups), so that the application can plug the cups interface (or even better, as the cups interface needs the CUPS Snap anyway, define the dependency internally in snapd).