Extract disk devices from image

[melato]

Add a command to copy files from the root filesystem of an image to device directories.

For example:

lxops extract-devices -name alpine-nginx-202324-1518 nginx.yaml

This would:

• define disk device directories for an image with the specified name, using the nginx.yaml lxops file
• copy files from the image specified in the lxops file into these device directories

Even better, instead of adding a new command, incorporate this functionality in the launch command:

lxops launch -name alpine-nginx-202324-1518 nginx.yaml

When it needs to create a disk device directory, launch could extract it from the image, instead of creating an empty one.

Rationale

Let's say I want to build an nginx container that has /var/log/ in an external disk device. When I build the corresponding image, until now, I've been building the image with /var/log/ in an external disk device too. The resulting image has an empty /var/log/ directory. The container depends on it being provided externally.

I would like to build the image with the nginx packages and files without any external disk devices. I can then use this image to extract the devices that I want to use as templates for my containers. That way the image has all the files that it was built with, and I can use it with different disk devices.

Implementation for launch

- create the container from the image without starting it
- find the origin of the container zfs filesystem.
  The origin will be a readonly snapshot of the image root filesystem.
- mount this snapshot to a temporary directory:
  sudo mount -t zfs .../images/...@readonly /tmp/...
- copy the device files from that mount:
  rsync -av /tmp/.../rootfs/var/log/ /.../log/
  umount /the temporary directory

[melato]

I ran into a problem of shifted uids: The image has unshifted uids/gids (root is 0). But I need to copy the shifted ids (root is 1000000) in the device directory.

[melato]

One way around the shifted ids problem is to shift them up. Add an "lxops shiftids" command that would shift the uid,gid of all files in a directory, recursively. Call it with sudo, so it can change file ownerehips. I am calling rsync anyway, calling another program to shift the ids is not a lot more overhead. It would be nice if rsync had an option to shift ids.

[melato]

Implemented "extract" command that extracts disk devices from an image. It would be nice to include this functionality in the "launch" command.