Allow running nix without a /etc/passwd entry

[viceice]

Is your feature request related to a problem? Please describe.
We are using the nix flake update to update the nix lockfiles from renovatebot.
The executable is run inside docker containers which a custom user.
But sometimes the image needs to be run with an arbitrary userid, which is not inside the /etc/passwd of the prebuild image.
This happpens when running on openstack, then the userid is random and the primary group is root, so we've prepared our image for that usecase.
We also explicit set HOME to our prebuild user which is writable by root group.
This works fine for most tools but not for the nix cli, which fails with cannot determine user's home directory here

nix/src/libutil/util.cc

Line 553 in e37f436

throw Error("cannot determine user's home directory");

• Add support for nix manager renovatebot/renovate#8000
• nix: support running without explicit home containerbase/base#799
• Nix lockFileMaintenance is broken renovatebot/github-action#713

Describe the solution you'd like
I like to skip that check somehow so the existing writable HOME can be used by nix cli.

Describe alternatives you've considered
No workaround possible.

Additional context
Not relevant

Priorities

Add 👍 to issues you find important.

[SuperSandro2000]

But sometimes the image needs to be run with an arbitrary userid, which is not inside the /etc/passwd of the prebuild image.

Sidenote regardless of the feature request: This is usually a bad idea because it breaks all kinds of programs because this is expected to exist. I would recommend to use something similar to the systemd DynamicUser feature instead.

[viceice]

systemd isn't available inside docker container by default

[viceice]

and as described you can't influence the userid used by Openstack.

[thufschmitt]

This might be a consequence of #6676 which will cause $HOME to be ignored if it doesn't belong to the calling user (mostly because Darwin's defaults for sudo was making this a bit of a pain otherwise).

Any chance you could run nix with a $HOME pointing to a folder owned by the current user?

[viceice]

sure, but the current user still has no entry inside /etc/passwd file, wouldn't it fail then anyways?

[thufschmitt]

I don't think it would. /etc/passwd should only be looked at if Nix believes that $HOME isn't good-enough (unless it's used somewhere else in the code, but I don't think so)

[viceice]

ok, let me test if we can change the owner of the home dir on the fly, because it should be writable by current user anyways
🤞

[viceice]

ok, only root can change ownership. so i would need to create the home on the fly 😕