provide a way to force a full image download via D-Bus

[James Hunt]

It is possible to force a full system-image version to be downloaded (opposed to a delta image) via the command line using '-b':

$ sudo system-image-cli -b 0 -i

However, there doesn't seem to be a way to request this behaviour via the D-Bus api. This is required for snappy.

[Barry Warsaw (barry)]

Could you provide a little more rationale? Why does snappy need it? (Not that it would affect the implementation, but I'd like to know for scheduling purposes.)

[James Hunt (jamesodhunt)]

There are 2 main reasons:

1) To test the system behaviour is consistent when given delta updates vs. full-system updates.
2) To simplify the logic when applying the very first system image update to the "other" partition (which is initially empty).
     If we force a full system update, we can:
    a) guarantee we get a fresh kernel + initrd for the "other" partition.
    b) avoid copying a -> b to then apply the downloaded delta to the "other" partition.

[Barry Warsaw (barry)]

I wonder if it also makes sense to allow for a channel switch in the API that will be required to support a forced full download. If not, then let's open a new bug for that.

[Barry Warsaw (barry)]

James, unless this is still a priority, I'd like to defer it for later.