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By: Tom Sydney Kerckhove <syd@cs-syd.eu>

nix-ci-eval: a fast, parallel flake-output discoverer

A small tool that discovers a flake's buildable outputs for one system,
purpose-built for NixCI's show jobs. It links only the LGPL-2.1 Nix
evaluator libraries.

Given a flake reference and a system it streams one newline-delimited
JSON object per discovered output, tagged by type (job / error /
dependency), then the dependency edges between the outputs. Each
attribute path carries its system component so it names the attribute
exactly as `nix build` takes it, and a failing attribute becomes an
error line rather than aborting the run.

A coordinator forks worker processes, each with its own Nix evaluator.
--workers is a ceiling, not a count: the pool grows on demand, sized from
what the run measures an evaluation and a worker to cost, so a small
flake never forks a pool it cannot keep busy and a large one still ramps
to the ceiling.

Tested by pure unit tests for the Nix-free helpers and a hermetic
integration test that drives the built binary against a fixture flake
with no inputs and diffs a golden.

Timeline