RFC: our kernel snap "build" easily packages the wrong kernel version

[Kamal Mostafa]

This version: string in snapcraft.yaml *looks* like a kernel version number
and it is what sets the version of the snap itself...

    $ grep version snapcraft.yaml
    version: 4.4.0-71.92

But it actually has no correlation to the actual kernel version which
gets downloaded (not actually "built" at all *cough*) and packaged into
the snap.

Our Makefile blindly fetches and apt-install's the "latest published" kernel
version from the archive.  Thus, it is painfully easy to accidentally
produce a kernel snap with version 'A' but which actually contains kernel
version 'B'.  If I simply git fetch the current snap repo[0] and build
it right now, I get this crazy result:

    $ grep version snapcraft.yaml
    version: 4.4.0-71.92
    $ sudo snapcraft
    [...] Unpacking linux-image-4.4.0-72-generic (4.4.0-72.93) ...
    [...] Snapped pc-kernel_4.4.0-71.92_amd64.snap

The snap version is "4.4.0-71.92" but really contains the 72.93 kernel!  If
I build this snap from the exact same SHA next week, then I'll get the 73.94
kernel!

I propose that we fix this by tweaking the Makefile to apt-install the
*specific* kernel version matching the snap version number.  That's an easy
change to the PACKAGE line(s) in the Makefile, e.g.:

    -PACKAGE := linux-signed-generic
    +PACKAGE := linux-signed-generic=$(SNAPCRAFT_PROJECT_VERSION)

However -- because the $(PACKAGE) being installed there is the actually the
meta package -- that then also means that all of the snapcraft.yaml's
versions will need to be set to the desired kernel *meta* version
instead of the image version, e.g. "version: 4.4.0.72.78" instead of
"version: 4.4.0-72.93".

With that change in place, snapcraft will now (sanely!) fail if you try to
package a kernel (meta) version that it cannot download, instead of blindly
packaging some arbitrary kernel version du-jour.

Otherwise, the only change in the result is that the snap version number will
of course now match the kernel meta version instead of the the image version:

    $ grep version snapcraft.yaml
    version: 4.4.0.72.78
    $ sudo snapcraft
    [...] Unpacking linux-image-4.4.0-72-generic (4.4.0-72.93) ...
    [...] Snapped pc-kernel_4.4.0.72.78_amd64.snap

Comments on this proposal?

 -Kamal

[0] git://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux-snap/+git/xenial pc