Achhina's Digital Garden

2026-08-27 Every Error Named the Wrong Component

4 min read

2026-08-27 Every Error Named the Wrong Component

What I set out to do

Run update -c. That was the whole plan.

What I actually did

Three failures, chained, none of which announced itself honestly.

1. update -c died twice, silently. The run stopped mid-build with no error under set -e. My first read was that it had been interrupted; that was wrong. /var/log/nix-daemon.log had the real story, twice:

Assertion failed: (success.builtOutputs.count(wantedOutput) > 0),
function haveDerivation, file ../build/derivation-goal.cc, line 185.

The daemon was aborting. Known upstream (NixOS/nix#14130, root-caused in PR #14137): a multi-output derivation where some outputs are already valid and some are not. The goal can’t return early, builds, then asserts because the wanted output was never re-registered. Each crash leaves more partial outputs behind, so it feeds itself.

Diffing derivation-goal.cc across tags showed 2.32.1 has the bare assert, 2.32.8 replaced it with a workaround, and 2.33.6+ has the real fix. Upgraded the root profile to 2.34.8 and restarted the daemon. nix store info --store daemon confirmed it, which matters — nix --version only proves the client moved.

2. SillyTavern 503s. upstream connect error or disconnect/reset before headers is Envoy’s phrasing and names neither side. Walked it outward: envoy → nothing on host :4000 → ai.litellm.proxy crash-looping, 108 times → ImportError: cannot import name 'get_flat_dependant' from 'fastapi.dependencies.utils'.

Tonight’s nixpkgs bump moved fastapi 0.139.0 → 0.141.1, which dropped get_flat_dependant after inlining its walk into get_flat_params. litellm stayed at 1.97.0 and still imports it. Neither package is broken; nixpkgs shipped an incompatible pair. Confirmed by evaluating both revs directly rather than guessing which side moved.

Fixed with a local patch reimplementing the walk for query params only — get_flat_params is not a drop-in, it concatenates path/query/header/cookie into one list, which would have quietly widened the /management/v1 allowlist to accept header names. Unit-tested the patched function against a real FastAPI route with a nested sub-dependency: recursion works, path and header params stay out.

3. A critical alert pointing at the wrong subsystem. Docker container egress broken fired alongside the litellm warning. Its own description tells you to restart Docker Desktop. But probing all three canaries from inside a container: SigNoz loopback 200, internet 204, litellm 000. Guest networking was fine.

The rule groups by http.url, so every probe URL is its own series and all_the_times fires on any one going dark — and litellm had been added to httpcheckTargets while the rule’s text still described a two-probe design. A dead host service was raising a critical network alert. Filtered the litellm probe out of that query.

What was striking

Every error message named the wrong component. The daemon crash looked like an interrupt. The SillyTavern 503 looked like SillyTavern. The egress alert looked like Docker. In all three cases the honest signal was one layer down and in a different log: /var/log/nix-daemon.log, ~/.local/share/litellm/stderr.log, and a hand-run curl.

Also a lesson in reading lockfiles: I claimed the nixpkgs pin hadn’t moved, because I read .nodes.nixpkgs. That’s a transitive node. The real input resolves through .nodes.root.inputs.nixpkgs → nixpkgs_2, and it had moved exactly as the update output said. A frozen-looking pin that isn’t frozen is a good way to misdiagnose an entire evening.

The alerting failure is the one worth sitting with. The two rules were designed to discriminate — the file says so in a comment, and the litellm alert’s own description explains “fires alone” vs “fires with”. The design was sound and the implementation quietly stopped matching it the moment a third probe target was appended to the list. Nothing tested the discrimination.

Top 3 tomorrow

  1. Watch whether nixpkgs picks up a litellm that drops the get_flat_dependant import; the build-time marker will say so loudly.
  2. Consider a test that asserts the two alert rules actually discriminate, rather than trusting the comment.
  3. Compact MEMORY.md — it’s at 19.7KB against a 17.1KB target and further trimming means dropping curated entries.

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