virt-resize from an NBD source

New in libguestfs ≥ 1.23.23 is the ability to use virt-resize from a remote source.

One use for this is to use nbdkit’s xz plugin as a source, to easily create new images from highly compressed templates. In the example below nbdkit transparently (and in RAM) uncompresses the disk image, serving it up over /tmp/sock (a socket):

$ nbdkit -r -f -U /tmp/sock \
    /usr/lib64/nbdkit/plugins/nbdkit-xz-plugin.so \
    file=f17x64.img.xz

Virt-resize fetches the image from the socket^† and writes & resizes it to the destination file:

$ truncate -s 20G /tmp/output.img   # adjust target size
$ virt-resize 'nbd://?socket=/tmp/sock' /tmp/output.img
$ killall nbdkit; rm /tmp/sock

You can then use virt-sysprep to set the hostname and run configuration scripts in the output image.

^† Unfortunately you also have to temporarily disable SELinux for this to work, because SELinux prevents qemu from connecting to the NBD socket. Grrrr.