Nixpkgs: Kernel tries to recompile when building system from nix-shell

[Synthetica9]

Issue description

When building a recent nixpkgs version kernel with nixos-rebuild, nix build, and probably others, nix wants to recompile the kernel when we are currently in a nix-shell.

This seems to be related to the fact that these two packages get added to the derivation of the kernel:

• ncurses-6.1-20181027
• pkg-config-0.29.2

Full diff

This doesn't happen for older kernel/nixpkgs combinations, specifically linux-4.14.76, nixpkgs rev 81f5c2698a8 (2018 Oct 17).

cc @tilpner, who was also able to reproduce this on irc.

Steps to reproduce

Minimal failing example:

# Doesn't try to build kernel:
nixos-rebuild build

# Tries to rebuild kernel:
nix-shell -p [] --run "nixos-rebuild build"

Technical details

• system: "x86_64-linux"
• host os: Linux 4.14.85, NixOS, 19.03pre161818.776f084cf1b (Koi)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.1.3
• channels(root): "nixos-19.03pre161797.c26dbef830a"
• channels(synthetica): "nixos-19.03pre158246.6141939d6e0"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

[pbogdan]

It's probably because of https://github.com/NixOS/nixpkgs/pull/47909 which adds some build inputs when in nix-shell.

[Synthetica9]

Yes! Thank you! Reverting that exact (merge) commit in my local repo gives the expected behaviour:

$ nix-shell -p [] --run "nix build -f nixos system"
trace: Optimising i3-gaps
trace: Optimising ag
trace: Optimising htop
trace: Optimising netcat-gnu
[1/13/23 built, 0.0 MiB DL]
error: interrupted by the user
$ # Building kernel
$ git revert -m 1 c67ccd34c08fdb4e97c647c3fb09c1eb6e5431c6
[detached HEAD 9a59ea57b52] Revert "Merge pull request #47909 from teto/menuconfig"
 1 file changed, 2 deletions(-)
$ nix-shell -p [] --run "nix build -f nixos system"
trace: Optimising i3-gaps
trace: Optimising ag
trace: Optimising htop
trace: Optimising netcat-gnu
[18 built, 0.0 MiB DL
$ # Not building kernel! 

That still leaves open the question what to do with this problem.

[tilpner]

nixos-rebuild could unset IN_NIX_SHELL

[Synthetica9]

I'ts not just nixos-rebuild, it's also nix-build, nix build, nix-instantiate, and probably more

[FRidh]

cc @teto @joachifm @yegortimoshenko

nix-shell is for development and debugging of a package, and should therefore resemble the actual build as close as possible. Automatically bringing in additional dependencies when developing is perhaps not such a good idea. Can't simply -p be used to bring in pkgconfig and ncurses?

[yegortimoshenko]

Short-term answer is probably to revert. That said, it is very neat to have complete development shells like this. Maybe:

{
  linuxShellFor = drv: drv.overrideAttrs (super: {
    nativeBuildInputs = super.nativeBuildInputs ++ [ ncurses pkgconfig ];
  });
}

[yegortimoshenko]

Other than that, would really like to see shellInputs derivation attribute that is lazily evaluated in build context, and merged into nativeBuildInputs in shell context.

[teto]

This is a consequence I realized was possible only after submitting my PR and it was merged by then already. As a short term fix, reverting may be the way to go. As someone who works quite often on the kernel source, or on packages, I believe default shellHooks could definitely be improved for a wide range of applications (https://github.com/NixOS/nixpkgs/pull/42864). So if we had a way to modify shellHooks that play nice with the rest of nix, that would be great.

[Synthetica9]

Closed by https://github.com/NixOS/nixpkgs/commit/6e61418a64233a3939a9017b5d893bae6797c8b0