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Anchored-region patcher

How glix writes to flake.nix without parsing Nix.

Location: glix/internal/flake/patcher.go.

The anchors​

# >>> glix-managed inputs >>>
# <<< glix-managed inputs <<<

# >>> glix-managed hosts >>>
# <<< glix-managed hosts <<<

These markers are placed by flake.nix.tmpl at glix init time. glix promises:

  • Never write outside any pair of markers.
  • Always replace the contents between a matched pair atomically.
  • Refuse to operate if a marker is missing or out of order.

PatchFile(path, regions map[string]string) error​

Reads the file, locates each requested region, replaces the content between markers with the supplied string, and writes back atomically (temp file + fsync + rename).

If multiple regions are passed, the file is read once and rewritten once.

RenderInputs(m *manifest.Manifest) string​

For each enabled package, produces:

<name> = {
  url = "<flake-with-optional-?rev=pin>";
};

Pin application: applyPin(flake, pin) returns:

  • flake unchanged if pin == "" or flake already has rev= in its query.
  • flake + "?rev=" + pin if flake has no ? (no query yet).
  • flake + "&rev=" + pin if flake already has a ? (preserve the existing query).

The output is sorted alphabetically by package name so the diff is deterministic.

RenderHosts(entries []HostEntry) string​

For each host:

<name> = mkHost "<name>" "<system>";

Sorted alphabetically. mkHost is defined statically in flake.nix.tmpl.

Why this, not a real parser​

A full Nix parser is huge, takes a slow dependency, and gives glix the ability to mangle code it shouldn't touch. The anchored-region approach is:

  • Bounded. glix can only write inside the markers; user code outside is untouchable.
  • Auditable. A git diff after any glix command shows exactly what changed.
  • Inspectable. Users can read the markers and the content between them like any other config file.

The cost is that any new piece of state glix needs to push into flake.nix requires a new region, and regions are append-only (you can't remove one without breaking older user repos). In practice we've kept it to two: inputs and hosts.

Tests​

glix/internal/flake/patcher_test.go covers:

  • Round-trip stability (writing the same render twice is a no-op).
  • Pin matrix (applyPin with empty, with existing query, with existing rev=, with sourcehut variant).
  • Marker-missing → error.
  • Multiple regions in one pass.

Future-proofing​

Adding a new region is a small ceremony; see CLI internals → Adding a new region for the exact steps.