Add a pure sfdisk resizer for cc_growpart

[Adam Williamson]

The 'growpart' tool itself is actually replicating functionality sfdisk already has. When editing a partition, the size can be specified as '+' to indicate 'make this partition as large as possible'.

sfdisk < 2.26 would fail when attempting to do this on a GPT-labelled disk (which is a reasonable enough justification for growpart's existence, I guess), but from 2.26 onwards it works (in my testing, anyway) for both MBR- and GPT-labelled disks.

I've written a patch that introduces a new resizer (using cc_growpart's existing support for multiple resizer backends) which uses sfdisk directly instead of growpart. The new resizer is only considered to be 'available' if the util-linux version appears to be 2.26 or higher.

The effect should be that native sfdisk resizing will be used if util-linux is new enough, but growpart will be used if it's older.

I would envisage that at some point we could decide that everyone's had enough of a chance to update util-linux and growpart could shuffle off this mortal coil, and the code could probably then be substantially simplified.

I'm going to try and tag a launchpad branch which has the patch applied, hope I get it right, my first time trying this...

[Scott Moser (smoser)]

a couple bits of information on this:
 * currently 2.26 has that dump/restore alters partition table on some disks
   https://bugzilla.redhat.com/show_bug.cgi?id=1211405
 * sfdisk in 2.26 supports gpt disks also

[Scott Moser (smoser)]

added support for using sfdisk > 2.26 to growpart.
growpart can now resize gpt or dos/mbr with sfdisk.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/2538