Bind mounting for cloned containers is too hard; provide equivalent of lxc-start-ephemeral --bind option

[Martin Pitt]

In order to efficiently exchange data with a test bed in a container, I make ample use of lxc-start-ephemeral's --bdir option to bind-mount a temp dir from the host into the container. However, we can't use ephemeral in autopkgtest production as that leads to failure of some tests due to bugs in overlayfs/aufs.

The problem is that with lxc-clone and lxc-start there is no counterpart at all for the --bdir option. So to do the equivalent thing, one needs to detect the LXC path (lxc-config lxc.lxcpath), make assumptions about the file layout in the container dir (/rootfs, /config), manually mkdir -p the bind mount path, and add the bind mount to config. This feels like poking around in internals way too much and thus not very future-proof, and it's also a lot of effort:

  http://anonscm.debian.org/gitweb/?p=autopkgtest/autopkgtest.git;a=commitdiff;h=5fbbe3f05f4

Could lxc-clone or lxc-start grow a --bdir option as well, to make this simpler and keep the knowledge of paths and file layouts within LXC itself?

[Stéphane Graber (stgraber)]

You could use something like:
lxc-start -n my-container -s "lxc.mount.entry=/path/on/host path/in/container none bind,create=dir 0 0"

Which will bind-mount /path/on/host to <rootfs>/path/in/container and create the target path if missing.

[Martin Pitt (pitti)]

Yay, I completely missed that possibility. Thanks!