bug#59185: Trouble mounting recursive file systems in containers

[Morgan Smith]

Hello!

So I was trying to mount /run/user/1000 in a container so it would have
access to all my wayland sockets and such when I got a very cryptic
error message.

I was trying something like this:

guix shell --share=/run/user/1000 -C coreutils

After far too long tracking down the issue, it turns out that the
directory had submounts within it meaning that the MS_REC flag is
required to bind mount it.

My /run/user/1000 only had a submount because xdg-document-portal was
making one.  To test this yourself you can run `mount` to find something
with some submounts.  I think /sys/fs might fail for me for the same
reason.

Now I have no clue what we should do to enable this use case.  Maybe we
should allow users to specify mount options using something like this?

guix shell -C --mount=rbind,ro=/run/user/1000

Maybe we could always bind with the recursive flag?


Thanks,

Morgan