"experimental feature 'ca-derivations' is disabled" even though it is enabled

[NobbZ]

Describe the bug

I wanted to try to rebuild the system in question using the new content addressable features, therefore I updated it to a recent stable release.

Whenever I try to build a CA enabled system config on that machine I get the following error:

$ nix build .\#nixosConfigurations.delly-nixos.config.system.build.toplevel
error: experimental Nix feature 'ca-derivations' is disabled; use '--experimental-features ca-derivations' to override
(use '--show-trace' to show detailed location information)

The error does not change, even if I provide --experimental-features 'ca-derivations flakes nix-command' on the CLI explicitely.

Also it is set in the nix.conf:

$ cat /etc/nix/nix.conf| grep experi
experimental-features = nix-command flakes ca-derivations ca-references

Building any CA enabled system config on one of my other systems works fine.

Steps To Reproduce

1. Try to build any system with nixpkgs.config.contentAddressedByDefault = true; on the affected system.
2. See it fail as described above

Expected behavior

A system configuration is built.

nix-env --version output

$ nix-env --version
nix-env (Nix) 2.4pre20210504_fe3a10a

Additional context

The system where the problem occurs is quite an old machine with an Intel Core2Duo CPU and only 4 GiB of RAM, though I wouldn't expect this to cause the actual error.

In general the configuration of my systems is available at https://github.com/NobbZ/nixos-config

The most relevant file for the affected system is probably https://github.com/NobbZ/nixos-config/blob/master/nixos/hosts/legacy/delly-nixos.nix

If any further details about this system or any other in my setup are required just ask.

Please be aware that I won't have physical access to the affected machine for the next couple of days, only SSH until VPN or WiFi decides to crash.

[nixos-discourse]

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

https://discourse.nixos.org/t/content-addressed-nix-call-for-testers/12881/53

[asymmetric]

@NobbZ is there something relevant in your ~/.config/nix/nix.conf?

[NobbZ]

@asymmetric I do not have such a file.

[teto]

I've just rebased on nixos-unstable and experiments something similar:

$ sudo nixos-rebuild --flake ~/home --no-write-lock-file --override-input nixpkgs-teto /home/teto/nixpkgs --override-input hm /home/teto/hm

building the system configuration...
error: experimental Nix feature 'flakes' is disabled; use '--experimental-features flakes' to override
error: getting status of '/tmp/nixos-rebuild.PjbHJF/result': No such file or directory

with ~/.config/nix/nix.conf
experimental-features = nix-command flakes

It used to work fine, I haven't changed anything relevant, I ran two switch in a row, first one succeeded, second failed with the previous error.

nix --version                                                                                                                                                                       ~/nixpkgs
nix (Nix) 2.4pre20210503_6d2553a

[teto]

even stranger

 nix show-config | grep experimental 
experimental-features = nix-command flakes

[teto]

(for anyone hitting this, I was able to work around it with https://discourse.nixos.org/t/how-to-do-a-flake-build-in-non-nixos-system/10450/7)

[thufschmitt]

(for anyone hitting this, I was able to work around it with https://discourse.nixos.org/t/how-to-do-a-flake-build-in-non-nixos-system/10450/7)

Duuuuh, that explains everything! So what happens is that nixos-rebuild --flake sets --experimental-features (here), overriding whatever’s in the config file.

NixOS/nixpkgs#123246 should fix it

[NobbZ]

I did not use nixos-rebuild but directly build the corresponding attribute. The merged fix does not solve the reported problem. Can someone please re-open?

[thufschmitt]

I did not use nixos-rebuild but directly build the corresponding attribute

Duh, I knew I’d missed something 🤦‍♂️

[thufschmitt]

@NobbZ do you also have the error if you run nix build as root to bypass the Nix daemon?

[NobbZ]

Yes, I have, and I also have to add that the error happens for any __contentAddressed = true derivation that I try to build, not only system configuration.

[teto]

I also still have the issue with
error: experimental Nix feature 'flakes' is disabled; use '--experimental-features flakes' to override with 2.4pre20210601_5985b8b even though flakes are enabled

nix show-config|grep flake                                                                                                                                                       
experimental-features = nix-command flakes
flake-registry = https://github.com/NixOS/flake-registry/raw/master/flake-registry.json

[kanashimia]

nix show-config|grep flake                                                                                                                                                       
experimental-features = nix-command flakes
flake-registry = https://github.com/NixOS/flake-registry/raw/master/flake-registry.json

sudo nixos-rebuild

with ~/.config/nix/nix.conf

Do sudo nix show-config | grep flake (:
You change config of your user, and then use sudo to switch to a different user.

BTW here is an undocumented feature - you can pass --option experimental-features "nix-command flakes" to nixos-rebuild

[teto]

good idea, I had checked /etc/nix/nix.conf which enabled flake and nix-command but I also had a file /root/.config/nix/nix.conf with just nix-command in it (I have no recollection of ever creating that file but well it exists). Removing the latter fixed the issue. Thanks for sharing the undocumented feature, it worked as well.

[asymmetric]

BTW here is an undocumented feature - you can pass --option experimental-features "nix-command flakes" to nixos-rebuild

TBF, the nixos-manpage doe say

   In addition, nixos-rebuild accepts various Nix-related flags, including --max-jobs / -j,
   --show-trace, --keep-failed, --keep-going, --impure, and --verbose / -v. See the Nix manual for
   details.

Wether this si sufficient as (indirect) documentaion though, I'm not so sure :)

[kanashimia]

Wether this si sufficient as (indirect) documentaion though, I'm not so sure :)

Note that nixos-rebuild only supports some very specific options of nix, basically the only way to know is by reading the source code.

[klarkc]

@NobbZ It's probably better to open a new issue and reference this one. Otherwise, it may go unnoticed by the maintainers.