dotfiles/docs/gitea-binary-cache.md

Gitea Release-backed Nix binary cache

The workflows in .gitea/workflows/ publish the closures of every nixosConfigurations host to Gitea Releases.

• nix-cache-bootstrap.yml is a one-shot manual workflow that creates the initial cache.
• nix-cache-update.yml runs on every branch push. It creates one immutable generation release per commit and uploads only NAR content hashes that have not appeared in an older generation.
• Hosts are built independently. If one host fails, successful host closures and store paths completed during the failed build are published before the job reports the build failure.
• The cache-latest release is the stable cache index. It contains nix-cache-info, cache-public-key, cache-manifest.json, and every <store-hash>.narinfo file.
• Each narinfo has an absolute URL: that points at the generation release containing its immutable NAR. Rewriting URL: does not alter the signed store-path fingerprint.

The operational manifest enumerates all narinfo and NAR URLs. Nix itself does not read that manifest: it requests nix-cache-info and <store-hash>.narinfo directly from the cache URI.

One-time setup

Generate a signing key on a trusted machine:

umask 077
nix key generate-secret --key-name dotfiles-gitea-cache-1 > cache-private-key
nix key convert-secret-to-public < cache-private-key

Add the complete contents of cache-private-key as the repository Actions secret NIX_CACHE_PRIVATE_KEY. Do not commit this file. Ensure the repository Actions token is allowed to write Releases, then run Bootstrap Nix binary cache once from the Actions UI.

The bootstrap log and the following stable asset expose the public key:

https://git.yutakobayashi.com/moons-14/dotfiles/releases/download/cache-latest/cache-public-key

The repository and its Release assets must be publicly readable for ordinary Nix clients to use this as an unauthenticated substituter. The runner needs enough disk for the Nix store plus one compressed copy of all host closures. It also needs bash, curl, jq, and standard GNU userland tools.

The workflows remove /homeless-shelter before building. Nix requires that dummy home path not to exist when the runner performs builds without a sandbox.

Large release assets and Cloudflare

git.yutakobayashi.com is proxied by Cloudflare. Large NAR uploads can receive 413 Request Entity Too Large before they reach Gitea. NAR files cannot be split because the Nix binary-cache protocol downloads each NAR as one object.

Create an HTTPS origin hostname that is DNS-only in Cloudflare, or use a private Gitea URL reachable from the runner. Set that URL as the repository Actions variable NIX_CACHE_API_SERVER_URL, for example:

https://git-origin.yutakobayashi.com

Only Gitea API calls and uploads use this variable. CACHE_SERVER_URL remains the public URL, so the URLs written to narinfo and the client substituter stay under https://git.yutakobayashi.com.

The origin reverse proxy request-body limit and Gitea's [repository.release] FILE_MAX_SIZE must also be larger than the largest NAR. Protect an origin hostname with a firewall or another access control that still allows the Actions runner to reach it.

When server-side limits cannot be changed, the workflows enforce CACHE_MAX_UPLOAD_BYTES=90000000. NARs larger than that limit and their narinfo files are not uploaded. They are recorded under skipped in cache-manifest.json. Nix clients can still substitute every smaller store path and obtain a skipped path from another substituter or build it locally. Set the value to 0 only when the upload path has no smaller request limit.

NixOS client configuration

After bootstrap, copy the exact value from cache-public-key into extra-trusted-public-keys:

{
  nix.settings = {
    extra-substituters = [
      "https://git.yutakobayashi.com/moons-14/dotfiles/releases/download/cache-latest"
    ];
    extra-trusted-public-keys = [
      "dotfiles-gitea-cache-1:REPLACE_WITH_THE_GENERATED_PUBLIC_KEY"
    ];
  };
}

The substituter value is the directory-like cache URI, not the manifest file URL. A quick validation after bootstrap is:

cache=https://git.yutakobayashi.com/moons-14/dotfiles/releases/download/cache-latest
curl --fail "$cache/nix-cache-info"
curl --fail "$cache/cache-manifest.json" | jq '.cache, (.objects | length), (.narinfos | length)'

Because every branch receives the signing secret, only trusted users should be allowed to push branches or modify Actions workflows in this repository.