Comment 4 for bug 1087526

I've just manually tested on raring, and the following seems to work the whole way through, when having installed psusi's branch at https://code.launchpad.net/~psusi/ubuntu/raring/util-linux/cherry-pick-resize and the new growpart with gpt partition support.

$ img=/tmp/my.img
$ mp=/tmp/mp
$ size=800M
$ osize="400M"
$ sudo rm -f $img; sudo rm -Rf $mp; sudo mkdir $mp

## create a new image $osize big
$ sudo truncate --size $osize "$img"

## add a single GPT partition that takes up the whole image
$ sudo sgdisk --new 1:2048: "$img"

## grow the image (as a hypervisor would do)
$ sudo truncate --size $size $img

## attach the image to a device
$ lodev=$(sudo losetup --show --find "$img")
$ echo $lodev
/dev/loop0

## put a filesystem on it, mount it, then grow the partition
$ sudo mkfs.ext4 ${lodev}p1
$ sudo mount ${lodev}p1 "$mp"
$ sudo growpart ${lodev} 1

## /proc/partitions still has old information

## tell kernel about grown partition
$ sudo partx --update 1 ${lodev}

## /proc/partitions has new information

## filesystem can now be re-sized
$ sudo resize2fs ${lodev}p1