bug: home-manager not respecting build-dir when set in /etc/nix/nix.conf?

[johanneskastl]

Are you following the right branch?

• My Nixpkgs and Home Manager versions are in sync

Is there an existing issue for this?

• I have searched the existing issues

Issue description

Dear maintainers,

as discussed in https://discourse.nixos.org/t/permanently-use-a-sandbox-build-directory-outside-of-tmp/60353/, I am using home-manager on a Fedora-based device with only 8GB of RAM. Hence the tmpfs in /tmp is too small, while /var/tmp/ has enough space.

Setting build-dir in /etc/nix/nix.conf (or /etc/nix/nix.custom.conf) shows this setting when running nix config show:

$ nix config show|grep build-dir
build-dir = /var/tmp
sandbox-build-dir = /build

(As far as I found this obseletes setting the TMPDIR variable in the nix-daemon systemd unit file)

However, when using home-manager, I always need to prefix the command with TMPDIR=/var/tmp/, otherwise the build fails due to missing space in /tmp.

# working
$ nix-channel --update && TMPDIR=/var/tmp/ home-manager switch
# failing
$ nix-channel --update && home-manager switch
$

I searched a lot on how to set this in the home-manager configuration itself somewhere, but of course those are lousy search terms so I might just have missed it.

Is there an option that I need to set so this is being respected? Or is this a bug that needs to be fixed?

Thanks in advance!

Kind Regards,
Johannes

Maintainer CC

No response

System information

- system: `"x86_64-linux"`
 - host os: `Linux 6.12.10-200.fc41.x86_64, Aurora, 41.20250302 (Kinoite), ddd565a`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.26.1`
 - nixpkgs: `/nix/store/w3ghsxcwcynh88fgr7j841drw08fyzn0-nixpkgs-24.11/nixpkgs`

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/permanently-use-a-sandbox-build-directory-outside-of-tmp/60353/6