Race in `.drv` import helper initialization with parallel eval

[omarjatoi]

Describe the bug

Importing many .drv files concurrently can race under parallel-eval, especially through nix eval --json, which deep-forces attributes in parallel.

The issue appears to be in derivationToValue: EvalState::vImportedDrvToDerivation is assigned before imported-drv-to-derivation.nix.gen.hh has finished evaluating. Another worker can observe the non-null root and call forceFunction while the helper is still a thunk.

Observed failure:

error: expected a function but found a thunk: «thunk»

Older builds (specifically 3.17.0) have also crashed in Value::type() from the same path.

Steps To Reproduce

1. Create repro.nix:

let
  mkDrv = n:
    derivation {
      name = "parallel-drv-import-repro-${toString n}";
      system = builtins.currentSystem;
      builder = "/bin/sh";
      args = [ "-c" "echo ok > $out" ];
    };

  mkEntry = n:
    let
      drvPath = builtins.storePath (mkDrv n).drvPath;
    in {
      name = "x${toString n}";
      value = (import drvPath).outPath;
    };
in
builtins.listToAttrs (map mkEntry (builtins.genList (n: n) 1000))

2. Confirm the single-core control case succeeds:

NIX_CONFIG=$'extra-experimental-features = parallel-eval\neval-cores = 1' \
  nix eval --json --impure --show-trace --file repro.nix >/dev/null

3. Run with parallel eval enabled:

NIX_CONFIG=$'extra-experimental-features = parallel-eval\neval-cores = 0' \
  nix eval --json --impure --show-trace --file repro.nix >/dev/null

4. Observe intermittent failure, often immediately.

Expected behavior

Parallel evaluation should behave like eval-cores = 1; concurrent .drv imports should not observe a partially initialized helper value.

Metadata

Observed on (reproducible on both Darwin and Linux):

nix (Determinate Nix 3.19.1) 2.34.6

Additional context

Likely fix: serialize lazy initialization of vImportedDrvToDerivation, evaluate and validate the helper as a function first, then publish the RootValue. The per-.drv conversion can remain outside the lock.

I have a patch that I've tested which resolves the issue omarjatoi@0171ee2. Happy to open a PR, but wanted to make sure there was a bug report first.

Disclaimer: I worked through this bug with Codex, and the patch above was generated by Codex as well.

Checklist

• checked latest Determinate Nix manual (source)
• checked open bug issues and pull requests for possible duplicates