[clhodapp]

terranix and system-manager were the last two integrations with the old shape, where mkConfiguration returned the evaluation directly, required configModule and took a pkgSets attribute set. They now work like the other integrations. This PR is based on #95, which is based on #94. Merge those first, then change the base of this PR to main. The repository does not delete merged branches, so GitHub does not change the base by itself.

The port

• mkConfiguration returns a configuration. A parent declares it under caisson.<integration>.configurations.<name>, or mkTopConfiguration finalizes it at a top. It is evaluated per system, once for each system in force.
• configModule is optional. When it is absent, the configuration registered under the declared name in configs/terranix or configs/systemManager is used.
• Neither entry point takes pkgSets. The modules get the composition's package sets at the system as pkgSets and the composed library as lib. The inputs argument, which nothing read, is gone.

terranix:

• It runs on the selected package set, so it takes defaultPkgs.
• Its value and its json output are the config.tf.json file, with the evaluated configuration under passthru.config.
• It is published as terranixConfigurations.<name>, the name terranix's flake-parts module uses.
• terranix returns only the keys Terraform reads plus _meta, its documented out-of-band channel. A module of the integration copies config.caisson into _meta.caisson, and the integration reads it back from there. A terranix configuration therefore holds no configurations beneath it.

system-manager:

• The package set and nixpkgs. A system-manager configuration runs on the package set caisson selects (defaultPkgs, else the set named default), as every other integration does. It also runs on the nixpkgs the composition declares, as a NixOS configuration does.
  • caisson calls system-manager's nix/lib.nix, the file its flake builds lib.makeSystemConfig from, with the composition's nixpkgs and the composed library. The NixOS integration calls nixos/lib/eval-config.nix the same way.
  • The selected set is passed as the pkgs special argument. The module system takes a special argument before an entry of _module.args, so the selected set is the pkgs of every module and the default of the read-only nixpkgs.pkgs.
  • userborn, the fork system-manager pins, is built from that source on the selected set.
• What is refused. A configuration is refused if it sets nixpkgs.overlays or nixpkgs.config, or passes overlays in ecosystemArgs, because the selected set makes them ineffective. So is one whose nixpkgs.hostPlatform differs from the platform of that set.
• Its value and its toplevel output are the system. It is published as systemConfigs.<name>, which system-manager switch --flake reads, under default when no name is given.
• The compatibility bridge for nixpkgs' nix.nix is unchanged.

Both classes get caisson's core module (modules/terranix/core and modules/systemManager/core, symlinks to the generic core, as for the other classes).

The cost of two more classes

Each registered class declares caisson.modules.<class>.export.enabled and caisson.modules.<class>.exported in every evaluation, and caisson.exports.modules merged both for every class. The two new classes took caisson's overhead in the eval-weight check from 4.18 MB to 4.27 MB, against a ceiling of 4.28 MB. The first commit merges the options only for a class where a module defines them, and builds their types once. The overhead with this PR is 4.01 MB.

As part of that change, exported is now a function returning lazyAttrsOf raw. Before, each exported module was wrapped in a deferred module ({ imports = [ … ]; } with a location string). Now the modules are exported exactly as the function returns them, so a default selection and a defined one publish the same values.

Tests

• The pinned-world tests for both integrations evaluate a top and a configuration published under a structural top. The terranix test checks the file name and the passthru configuration. The system-manager test checks the platform. Both check that the modules see the composed library.
• ecosystemSrcValidationThrows and the twin test use the new shape.
• The unit tests for ecosystem resolution read terranix through a top, over a library that declares one system. The test of the entry-point signatures covers both integrations.
• The argument-errors check now finds no entry point that requires an argument, and checks that both refuse pkgSets.

Consumers

The only consumer is ch-system-manager, whose test flakes call the old shape. clhodapp/ch-system-manager#15 moves them to mkTopConfiguration and declares systems on mkLib. Merge it after this PR.

All 18 workspace consumers pass nix flake check --no-build with this branch in place of caisson, ch-system-manager on that branch. This time the run also points each consumer's tests/dependencies lock at this branch, so the checks partitions, which read that lock, evaluate against it too. My earlier consumer runs for #91 through #95 overrode only the root caisson input, so those partitions ran against the caisson they pin. This branch contains #94 and #95, so the run covers them as well.

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