schroot.conf(5) reads:
union-mount-options=options
[…] Note: One can use the variables
“${CHROOT_UNION_OVERLAY_DIRECTORY}” and
“${CHROOT_UNION_UNDERLAY_DIRECTORY}” to refer to the writable
overlay session directory and read-only underlying directory which
are to form the union.
However, it looks like these (or any other, for that matter)
variables are never substituted within the code, and the value
of the ‘union-mount-options’ option is instead passed to
mount(8) “as-is”, leading to an error like:
Nov 28 06:30:49 crabs kernel: [938142.164904] aufs opt_add:714:mount[3093]: lookup failed ${CHROOT_UNION_OVERLAY_DIRECTORY} (-2)
As an (untested) workaround, the following code could be added
to either setup.d/10mount (do_mount_fs_union), or a file
processed prior to one.
CHROOT_UNION_MOUNT_OPTIONS=${CHROOT_UNION_MOUNT_OPTIONS//\${CHROOT_UNION_UNDERLAY_DIRECTORY\}/${CHROOT_UNION_UNDERLAY_DIRECTORY}}
CHROOT_UNION_MOUNT_OPTIONS=${CHROOT_UNION_MOUNT_OPTIONS//\${CHROOT_UNION_OVERLAY_DIRECTORY\}/${CHROOT_UNION_OVERLAY_DIRECTORY}}
(Per my reading of [1], the problem is not resolved as of the
current ‘master’.)
[1] https://github.com/codelibre-net/schroot/blob/master/etc/setup.d/10mount