[clhodapp]

Drift detection used to cover one pair: a home inside a NixOS machine compared /etc/caisson-home-manager/source.json with the machine it was built for. This PR moves it into the framework. Every configuration with an activation now checks the nearest configuration above it that also has an activation, whatever the two types are.

Stacked on #102.

How it works

• The integrations record it. An integration whose configurations are activated passes activation to mkModuleConfiguration, and the manifest records it. identity is the store path of the childless evaluation, meaning the configuration evaluated without the configurations declared inside it. marker is the file the activation writes into. The marker holds the SHA-256 digest of that store path, not the path itself. Computing the path evaluates the childless evaluation and builds nothing. The digest names no store path, so no string context is dropped, and nothing can read the marker as a path.

  Integration	Identity	Marker
  nixos, darwin, system-manager	toplevel	/etc/caisson/marker.json
  home-manager	activation package	$XDG_STATE_HOME/caisson/markers/home-manager-<name>.json
  hjem	manifest file	.local/state/caisson/markers/hjem-<name>.json in the user's directory

  A machine's record also sets standsAlone. A machine is applied on its own host, so a machine declared inside another, such as an image, checks nothing above it.

• The library computes it. lib.caisson.integrations gains activatingAncestorOf, markerOf, checksMarkerOf and driftCheckOf. A child finds the nearest activating configuration among its manifest's ancestors, so it never names that configuration's type.

• Each class writes its marker. The default module of nixos, darwin, systemManager, homeManager and hjem imports activation-marker. It writes the marker only when a configuration declared inside checks it, because computing the identity costs a second evaluation.

• A home checks it. The home-manager default module imports activation-drift, which runs driftCheckOf as an activation step. A home inside a darwin or system-manager machine is checked the same way as one inside NixOS.

Breaking changes (pre-1.0)

• The NixOS module home-manager-source-marker and the home-manager module nixos-source-marker are removed. A home that selects its modules itself names caisson/activation-drift instead.
• caisson.home-manager.machineDrift.warn becomes caisson.drift.warn.
• The marker file moves to /etc/caisson/marker.json, and its field baseSystemOutPath, a store path, becomes identity, a digest.

Paired consumer fixes: clhodapp/ch-personal-machines and clhodapp/ch-home-manager, branch fix/activation-drift in each.

Testing

• nix flake check passes, including the unit tests, the pinned-world suite and the eval-weight gate. The system-manager pinned-world test now checks that the machine writes the marker and that the home's check reads it.
• ch-home-manager's drift-warning-vm passes against this branch on fix/activation-drift. In a NixOS VM it shows five things:
  • the machine writes /etc/caisson/marker.json;
  • the machine's own activation of the home prints no warning;
  • a home built for a different machine warns;
  • a change to the home alone doesn't warn;
  • a missing marker warns.
• nix flake check --no-build passes in all 18 consumers, with both caisson pins pointed at this branch and ch-home-manager and ch-personal-machines on their fix branches.

Not covered

A hjem configuration declared inside a machine records a marker for the configurations inside it, but it doesn't check the machine itself. hjem's activation runs no step that a configuration can add. Its one hook is the systemd unit actions it reports after a switch, which would put the warning in the journal rather than on the terminal.

Docs: nix-caisson/nix-caisson.github.io, branch docs/drift-everywhere.

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