Exclude /nix from ostree changes and effects

[orowith2os]

Nix, as a packaging format, requires that /nix exist, and be mutable.

However, ostree prevents /nix from being created, and even if it were, it would be immutable, preventing the use of Nix on ostree systems.

I suggest taking the appropriate measures to ensure that Nix is usable on an ostree-based system. Some folks from the Nix community may be able to detail everything that would be required to support this, I am just making this issue, as it's one of my (few!) complaints about using an ostree-based system (Fedora Silverblue).

[cgwalters]

Hi, I think Nix has a ton of great ideas, and I definitely want ostree to play nicely with it.
This issue touches on #2681

Try this today:

mkdir /var/nix
chattr -i /
ln -sr /var/nix /nix

Does that work? IOW can /nix be a symlink? If so, then things get simpler. If it can't be, can it be a mountpoint? i.e. does

chattr -i /
mkdir -p /nix /var/nix
mount --bind /var/nix /nix

work?

[lucab]

There is some related discussion in coreos/rpm-ostree#337 as well.
Over there, it was mentioned that the symlink may have some negative side-effects on the artefacts purity, and that the mountpoint approach is recommended instead: https://nixos.org/manual/nix/stable/command-ref/env-common.html#env-NIX_IGNORE_SYMLINK_STORE.

So overall I think the deployment should carry an empty top-level /nix directory, to be bind-mounted to node-local storage.

[orowith2os]

What's the current status of this? I can see some work being done over in nix-community/nix-installers#8 that installs to /var/nix, but some utilities might still use /nix if they're used to it.

[jcdickinson]

@orowith2os nix-installers (specifically that PR) installs to /var/nix and mounts that to /nix using systemd. No changes are required for rpm-ostree.

[orowith2os]

Closing this issue, since the PR seems to handle everything needed.