failure when /etc/resolv.conf is a symlink

[tych0]

Stacker bind mounts in /etc/resolv.conf so that the container can resolve things as the host would.

However, when /etc/resolv.conf is a symlink in the container, we get,

lxc build-c3-minbase 20210330225039.565 ERROR    utils - utils.c:open_without_symlink:1062 - Too many levels of symbolic links - resolv.conf in /home/tycho/c3/build/stacker/rootfsPivot/etc/resolv.conf was a symbolic link!

because the kernel doesn't like mounting over a symbolic link.

[tych0]

A few options here:

1. we could write the file contents. but if e.g. it's a symbolic link to a file somewhere in /run that doesn't exist, that's not so great since we create that file for posterity. Additionally, we modify /etc even if there are no other changes to it, so we'd have to make some sort of exclusion for that sort of like we do for / and /stacker.

2. we could add an additional "stacker" overlay layer, that includes "/stacker" and "/etc/resolv.conf" on top of whatever the rootfs is. This would make it totally invisible in the final image. However, this would be a fair amount of work to do in the btrfs backend. Perhaps we can delete the btrfs backend? :)

[raharper]

I had also entertained the idea of also bind mounting the target of the
symlink

% mkdir -p /tmp/wark/rootfs/etc
% ls -al /etc/resolv.conf
lrwxrwxrwx 1 root root 39 Mar 14  2020 /etc/resolv.conf -> ../run/systemd/resolve/stub-resolv.conf
% cp -a /etc/resolv.conf /tmp/wark/rootfs/etc/resolv.conf
# once we copy this to a different rootfs path, the link is relative and invalid
% test -f /tmp/wark/rootfs/etc/resolv.conf; echo $?
1
% sudo mount -t tmpfs tmpfs /tmp/wark/rootfs/run
% mountpoint /tmp/wark/rootfs/run
/tmp/wark/rootfs/run is a mountpoint
% mkdir -p /tmp/wark/rootfs/run/systemd/resolve
% sudo mount -o bind /run/systemd/resolve /tmp/wark/rootfs/run/systemd/resolve
ls -al /tmp/wark/rootfs/etc/
total 3
drwxrwxr-x 2 rharper rharper  3 Mar 30 18:37 .
drwxrwxr-x 4 rharper rharper  4 Mar 30 18:36 ..
lrwxrwxrwx 1 rharper rharper 39 Mar 14  2020 resolv.conf -> ../run/systemd/resolve/stub-resolv.conf
# after bind mount it's valid
% test -f /tmp/wark/rootfs/etc/resolv.conf; echo $?
0

Vetting the source path and not masking that with the tmpfs mount might be
a bit of work (check if /run is empty) but I think for resolv.conf handling
it's typically file in /etc or symlink into /run/