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Allow previewing an older NixOS release with --downgrade

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Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F
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nils
2026-11-09 08:25:36 +01:00
co-authored by Claude Sonnet 5.5
parent 1efa068443
commit b37d8a0961
5 changed files with 26 additions and 14 deletions
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SPECS.md
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@@ -228,7 +228,7 @@ Upgrading nixpkgs (the kernel and every package) on the router, without a new re
- **Setting** `router.update.channel`: `unstable` or a release like `26.05` (branch `nixos-26.05`); null: the packages that came with the installed release. Read from the saved `settings.json` when upgrading; applying settings does not change nixpkgs.
- **`router-upgrade check`**: the branch's newest revision from `<channels.nixos.org>/<branch>/git-revision` (a redirect to releases.nixos.org, so curl follows it; the last revision Hydra's tests passed), against the running one (`nixos-version --revision`). Writes `/var/lib/router/upgrade/check.json`.
- **`preview [--force]`**: locks a copy of the configuration flake to that revision (`nix flake lock --override-input nixos-router/nixpkgs`), evaluates it and lists the packages of the running system that have another version in the new system's derivations, how much `nix build --dry-run` would download, and how many derivations would be built here. Nothing is downloaded: evaluation fetches the nixpkgs source only. Approximate: a rebuild without a new version is not listed. Refuses a NixOS release older than the running one. A changed `linux` sets `rebootNeeded`. Result in `preview.json`.
- **`preview [--force] [--downgrade]`**: locks a copy of the configuration flake to that revision (`nix flake lock --override-input nixos-router/nixpkgs`), evaluates it and lists the packages of the running system that have another version in the new system's derivations, how much `nix build --dry-run` would download, and how many derivations would be built here. Nothing is downloaded: evaluation fetches the nixpkgs source only. Approximate: a rebuild without a new version is not listed. Refuses a NixOS release older than the running one (unstable calls itself 26.11, so following 26.05 from unstable is one) unless `--downgrade` (the page's "downgrade anyway") is given; it then warns that state written by the newer release may not work. Not exercised in a VM test: the test's nixpkgs copy has the same release. A changed `linux` sets `rebootNeeded`. Result in `preview.json`.
- **`download`**: fetches and builds it (needs 3 GiB free), so a flaky uplink does not matter at `apply`.
- **`apply [timeout]`**: takes the previewed lock into the configuration directory and runs `router-apply` (commit-confirm). The directory as it was is kept in `config.pre-update` like for updates: a rollback or a failed build puts the lock back. Refused if the configuration flake changed since the preview.
- **Pin**: `nixpkgs-pin.json` in the configuration directory (channel, revision, flake reference with `narHash`). `router-update` locks it again after it installs a new release of nixos-router, so a release does not undo an upgrade; the reference is found in the store by its hash, offline.