Describe the bug

EvalState::lookupPathResolved is a non-thread-safe boost::unordered_flat_map (eval.hh:503-504) accessed without synchronization from resolveLookupPathPath (eval.cc:3187-3196). With parallel eval (eval-cores > 1), concurrent threads race on find/emplace, corrupting the map and crashing during rehash.

Adjacent caches (srcToStore, fileEvalCache, importResolutionCache) were converted to boost::concurrent_flat_map but lookupPathResolved was missed.

Steps To Reproduce

Save as repro.sh and run with bash repro.sh:

#!/usr/bin/env bash
set -euo pipefail
EXPR=$(mktemp --suffix=.nix)
cat > "$EXPR" <<'NIX'
let
  nixpkgs = builtins.getFlake "nixpkgs";
  mkSystem = name: nixpkgs.lib.nixosSystem {
    system = "x86_64-linux";
    modules = [{
      networking.hostName = name;
      fileSystems."/" = { device = "/dev/sda1"; fsType = "ext4"; };
      boot.loader.grub.device = "/dev/sda";
      system.stateVersion = "25.11";
    }];
  };
in {
  a = (mkSystem "a").config.system.build.toplevel.outPath;
  b = (mkSystem "b").config.system.build.toplevel.outPath;
  c = (mkSystem "c").config.system.build.toplevel.outPath;
}
NIX
fails=0
for i in $(seq 1 "${1:-20}"); do
  if ! nix eval --json --impure --file "$EXPR" > /dev/null 2>/dev/null; then
    fails=$((fails + 1))
  fi
done
rm -f "$EXPR"
echo "$fails/${1:-20} crashed"

On 8-core x86_64-linux: ~10-30% crash rate. Adding --option eval-cores 1: 0% crash rate.

Crashes manifest as either:

• Assertion 'num_destroyed==size()||num_destroyed==0' failed in ... unchecked_rehash (boost detects inconsistent state during map growth)
• corrupted size vs. prev_size (glibc detects heap metadata corruption)

Expected behavior

nix eval should not crash.

Metadata

Determinate Nixd daemon version: 3.17.2
Determinate Nixd client version: 3.17.2
nix (Determinate Nix 3.17.2) 2.33.3

Additional context

Fix: convert lookupPathResolved to boost::concurrent_flat_map (same as the other caches) or wrap accesses with a mutex.

Workaround: eval-cores = 1 in nix.conf.

Checklist

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