Add an exact flake registry override for nixpkgs

[grahamc]

Never see this again:

Users of CI platforms that aren't GitHub Actions frequently see rate-limiting errors while fetching Nixpkgs.

This is because GitHub aggressively rate-limits tarball downloads for requests without a GitHub Token. In GitHub Actions, this is largely solved by providing a token in the nix.conf.

This change causes nixpkgs and therefore nix run nixpkgs#... commands to fetch Nixpkgs from FlakeHub, making this work out of the box across the board.

Note that users are encouraged to avoid using the flake registry for flake inputs: #37

Motivation

Context

Summary by CodeRabbit

• Chores

  • Switched the nixpkgs flake registry to an exact tarball source on FlakeHub (https://flakehub.com/f/DeterminateSystems/nixpkgs-weekly/0.1) instead of the previous GitHub-based reference, providing a fixed retrieval endpoint.

• Tests

  • Updated regression expectations to reflect the new tarball-based fallback resolution.

[coderabbitai]

📝 Walkthrough

Walkthrough

The nixpkgs builtin flake registry entry is changed from a GitHub ref to an exact FlakeHub tarball URL (https://flakehub.com/f/DeterminateSystems/nixpkgs-weekly/0.1), and the functional test expectation is updated to match.

Changes

Builtin Flake Registry Update

Layer / File(s)	Summary
Nixpkgs FlakeHub migration src/libfetchers/builtin-flake-registry.json	nixpkgs registry target replaced: previously a GitHub mapping pinned to ref: "nixpkgs-unstable", now an exact tarball with to.type: "tarball" and to.url: "https://flakehub.com/f/DeterminateSystems/nixpkgs-weekly/0.1".
Fallback resolution test update tests/functional/flakes/flakes.sh	Test expectation updated to assert the builtin fallback resolution for nixpkgs returns the FlakeHub tarball URL instead of the prior github:NixOS/nixpkgs/nixpkgs-unstable flakeref.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

I’m a rabbit with a flake in paw,
Swapping GitHub for a tarball law,
Nixpkgs now hops to a steady URL,
Tests updated, all is well. 🐇

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: adding an exact flake registry override for nixpkgs, which switches from GitHub to a FlakeHub tarball target.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch grahamc-patch-1

---

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[coderabbitai]

🧹 Nitpick comments (1)

src/libfetchers/builtin-flake-registry.json (1)

334-356: ⚡ Quick win

Confirm exact precedence: bare nixpkgs will match the FlakeHub entry; qualified refs will use the GitHub fallback.

lookupInRegistries treats "exact": true as entry.from == input, and otherwise uses entry.from.contains(input) (with a restart on first match). With both entries sharing the same from, a bare nixpkgs input should hit the exact-equality FlakeHub mapping, while qualified refs (e.g., nixpkgs/nixos-23.11) won’t be equal to the bare from and should fall through to the non-exact contains mapping to GitHub.
Consider adding a brief comment/documentation explaining why the registry intentionally contains two nixpkgs entries.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/libfetchers/builtin-flake-registry.json` around lines 334 - 356, Add a
brief explanatory comment in builtin-flake-registry.json adjacent to the two
entries that share "from": {"id":"nixpkgs"} clarifying that lookupInRegistries
treats entries with "exact": true as entry.from == input (so a bare "nixpkgs"
will match the FlakeHub tarball entry) and that the second, non-exact/github
entry exists as a fallback for qualified refs (e.g., "nixpkgs/nixos-23.11")
because those will not equal the bare from and will match via contains/restart;
also ensure the exact-true FlakeHub entry appears before the non-exact GitHub
entry to preserve precedence.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@src/libfetchers/builtin-flake-registry.json`:
- Around line 334-356: Add a brief explanatory comment in
builtin-flake-registry.json adjacent to the two entries that share "from":
{"id":"nixpkgs"} clarifying that lookupInRegistries treats entries with "exact":
true as entry.from == input (so a bare "nixpkgs" will match the FlakeHub tarball
entry) and that the second, non-exact/github entry exists as a fallback for
qualified refs (e.g., "nixpkgs/nixos-23.11") because those will not equal the
bare from and will match via contains/restart; also ensure the exact-true
FlakeHub entry appears before the non-exact GitHub entry to preserve precedence.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 01a83314-49a1-469f-a0c3-90f076a05b29

📥 Commits

Reviewing files that changed from the base of the PR and between 0a5b634 and 4478ce3.

📒 Files selected for processing (1)

• src/libfetchers/builtin-flake-registry.json

[github-actions]

🚀 Deployed on https://6a0e69510f6853a4e5a5e080--determinate-nix.netlify.app

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@tests/functional/flakes/flakes.sh`:
- Line 287: The test currently hardcodes the fallback URL to
https://flakehub.com/f/DeterminateSystems/nixpkgs-weekly/0.1 which pins the
builtin registry to a stale weekly release; update the test in
tests/functional/flakes/flakes.sh so it does not hardcode "0.1"—either resolve
to a floating ref like "nixpkgs-unstable" or read the weekly version from a
configurable variable (e.g., WEEKLY_PIN or FLAKEHUB_WEEKLY_VERSION) and
document/update that variable; ensure the assertion that uses the exact string
`https://flakehub.com/f/DeterminateSystems/nixpkgs-weekly/0.1` is replaced with
a dynamic expectation or the unstable alias, and add a short comment explaining
the upgrade cadence/process for bumping the pinned weekly if you keep pinning.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: c26b089d-9444-40b6-8979-dd6cba054699

📥 Commits

Reviewing files that changed from the base of the PR and between 4478ce3 and 108fc0b.

📒 Files selected for processing (1)

• tests/functional/flakes/flakes.sh