nixFlakes: can't figure out nix-instantiate equivalent

[colemickens]

Describe the bug

I can't figure out the equivalent for many nix-instantiate commands. Another bug #???? describes how nix eval and nix eval --{raw|json} behave differently. And the latter that works for me, returns the out path, rather than the derivation (this feels similar to how nix copy aggressively builds instead of just copying drvs #3696).

How do I get the drv path for a flake output? This is what I have now, I don't like it:

function instantiate() {
  machine="${1}"; shift
  # do it
  true
  #drv="$(nix eval --pure-eval ".#nixosConfigurations.${machine}.config.system.build.toplevel")" # TODO: why????
  set -e
  drv="$(set -eu; nix --experimental-features 'nix-command flakes' --pure-eval \
    eval \
    --raw ".#machines.${machine}")"
  drv="$(set -euo pipefail; nix --experimental-features 'nix-command flakes' --pure-eval \
    show-derivation "${drv}" | jq -r 'to_entries[].key')"
  echo -e "${drv}"
}

nix (Nix) 2.4pre20200721_ff314f1

EDIT (workaround)

You can do something like this instead:

thing=".#packages.x86_64-linux.testPackage" # pkg example
thing=".#nixosConfigurations.yourHostName.config.system.build.toplevel" # nixos cfg example
nix build \
  --experimental-features 'nix-command flakes' \
  "${thing}.drvPath"

This gives you a /nix/store/abc123..thing.drv path that you can then nix copy --derivation or nix-copy-closure to a remote machine to build, etc.

[zimbatm]

I think it's on purpose, Eelco wanted to hide the .drv implementation detail. With the idea that it will allow us to change the format down the line.

[colemickens]

This blocks "the good way" of doing large remote nix builds. I sympathize with the desire to iterate on the format, but can that be done in a backward compatible way that still allows all of the places where I've been using drvs?

I know I'm not the only one that nix-instantiates, copies drvs, and then uses cachix to move binaries.

[xbreak]

I know I'm not the only one that nix-instantiates, copies drvs, and then uses cachix to move binaries.

We do the same but on-premise cache.

[colemickens]

I remembered that --derivation works for nix copy and was hoping that it might apply here, but it appears not. While --derivation appears in the help for nix eval, it still outputs the build output path, rather than the path to derivation. This is another thing that is forcing me to keep extra tooling around to work with flakes.

[thufschmitt]

It should always possible to get the drv path with something like nix eval --raw foo#bar.drvPath. But indeed as @zimbatm pointed out, this might not be a viable long-term solution and there should be a cleaner way of doing that

[nixos-discourse]

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

https://discourse.nixos.org/t/to-flake-or-not-to-flake/10047/4

[stale]

I marked this as stale due to inactivity. → More info

[dhess]

Please un-stale.

[virchau13]

It should always possible to get the drv path with something like nix eval --raw foo#bar.drvPath.

It's worth saying that this is the current approach taken by nixos-rebuild when --build-host is specified. So it's got precedent, at least.

[disassembler]

My primary use case is for nix show-derivation. In non-flakes I used to do nix-instantiate foo.nix -A bar which gives me the drv without building and I could then run nix show-derivation on that file to inspect it. Now with flakes, I've been doing nix build .#bar, building the entire thing, then nix-store -q --deriver result/ and then I can show-derivation that. I'm assuming there has to be a better way that doesn't build the derivation. It also makes it is hard to automate if an error occurs in the build, as you have to go through the logs to find the text output of the drv file.

[thufschmitt]

I'm assuming there has to be a better way that doesn't build the derivation

nix show-derivation .#bar? 😛

More generally, most of the new-style commands that act on a derivation or a store path indifferently accept a litteral store path or a Nix expression/flake ref as input (and will transparently build it or not depending on whether it needs to)

[thufschmitt]

Going back to the original motivation for the issue (fast remote builds by instantiating locally, copying the drv and building remotely), it's worth pointing out that this is possible with:

$ nix build --eval-store auto --store ${remoteMachine} .#foo

[AleXoundOS]

I think it's on purpose, Eelco wanted to hide the .drv implementation detail.

.drv files is quite an easy concept (way more easier than flakes itself, imho). I doubt that hiding .drv from an end user is worth...

[plietar]

I've found out nix path-info --derivation .#foo does exactly this.

[hraban]

I've found out nix path-info --derivation .#foo does exactly this.

This is brilliant, thank you @plietar , this effectively answers the question and closes the ticket. Solid!

[nixos-discourse]

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

https://discourse.nixos.org/t/tool-for-evaluating-stateversion-changes/31043/6

[nixos-discourse]

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

https://discourse.nixos.org/t/webkit-2-36-3-keeps-trying-to-build-from-source-nixos-22-05/19874/12

[Yeshey]

I've found out nix path-info --derivation .#foo does exactly this.

How would this be used in a flake with only nixosConfigurations? like this:

git+file:///home/yeshey/.setup
└───nixosConfigurations
    ├───arm-oracle: NixOS configuration
    ├───laptop: NixOS configuration
    ├───surface: NixOS configuration
    └───vm: NixOS configuration

it complains that it does not provide an atribute:

> nix path-info --derivation /home/yeshey/.setup#laptop                    
warning: Git tree '/home/yeshey/.setup' is dirty
warning: input 'nur' has an override for a non-existent input 'nixpkgs'
error: flake 'git+file:///home/yeshey/.setup' does not provide attribute 'packages.x86_64-linux.laptop', 'legacyPackages.x86_64-linux.laptop' or 'laptop'

> nix path-info --derivation /home/yeshey/.setup#nixosConfigurations.laptop
warning: Git tree '/home/yeshey/.setup' is dirty
warning: input 'nur' has an override for a non-existent input 'nixpkgs'
error: 'nixosConfigurations.laptop.type' is not a string but a set

[Patryk27]

I think you're looking for:

nix path-info --derivation .#nixosConfigurations.systemName.config.system.build.toplevel

[nixos-discourse]

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

https://discourse.nixos.org/t/webkit-2-36-3-keeps-trying-to-build-from-source-nixos-22-05/19874/13