Code review comment for ~jibel/curtin/+git/add-layer-images-support:master

Hey Scott,

you should have a look at the tests, the sublayering works actually a bit different than you think :)

You can only point to the final layers (the topmost), then, the structure on disk built with livecd-rootfs will find its way to the root layer (all layers stack on top of each other and can have one single children).
This is because there is only a single path to the root layers, and they can not be composed that way. If we force the user to explicitely set them manually, it means we'll have another validation round to do in curtin. For instance, you can't compose "02-main2/sublayer2/sublayera.fs", "02-main2/sublayer2/sublayerb.fs".

If the image is invalid, the current proposed code will raise an exception.

I don't think we have anything against a syntax "foo:///pathtosomesquashfs". However, we spotted that the current curtin syntax, if we are correct, is "fsimage: /path/to/some/squashfs", which contradicts the current published docs. Is that an issue on the code?
Having then one single codepath with our additional tests enable skipping some code duplication, as both only points to one single squashfs file.

Does it make more sense?
(Note: jibel is away for a week and half)