[clhodapp]

Implements the caisson-core part of section 21 of the caisson design document (deferred evaluations, cross-child references).

Deferred configurations. A deferred configuration is a function of exactly { name, parent, args }, where a configuration is a function of { name, parent }. The args are values a deployment tool supplies after the flake is evaluated, and they reach the configuration only through args. mkDeferredConfiguration { type; class; record; configuration; } makes one. A parent cannot call it, because it has no args to give, so it holds an entry that deferChild makes: the record, the name, type, class and parent, and finalize { args, argsSource }, which returns the manifest. The parent's history, its inputs and the grouping by system leave the entry out.

Args on the manifest. Every manifest carries argsNeeded, the args its evaluation takes as an attrset of mkArg { cacheable } leaves, and discoverArgs, a function of { system } returning a derivation whose program prints the args it finds as JSON, or null. Both are empty or null for a configuration that takes no args. finalize fills the args against the argsNeeded of the manifest it returns: a given arg is passed on unevaluated, and an arg the caller did not give throws caisson-deferred: needs <path> when it is read. The manifest finalize returns carries argsSource, a function from an arg path to where its value came from, and never the values. A tool and a parent that gives its deferred children their args both call finalize; there is no other way to finalize a deferred configuration.

forDirectChildren. A third container beside forChildren and forNearestChildren, with the per-class extraDefaultModuleImports. It reaches the direct children of the class only (a child evaluated per system included), after the nearest additions, marked direct = true, and nothing beneath them.

unsafeCrossChildReferences. Each container may hold it by class. A configuration it reaches reads the full evaluation of the level that set it, as crossChildLevel (and crossChildLevelPath). forDirectChildren beats forNearestChildren, which beats forChildren; within one container the nearest parent wins; a forNearestChildren setting connects at its class whether or not it wins. The childless manifest of a level that sets it carries its full manifest as crossChildFull.

The hermetic tests cover these, including args that depend on the args above them and an arg that was not given failing where it is read; all 137 pass. caisson's use of these follows in nix-caisson/caisson#141 (the colmena nodes fix) and #142 (the NixOps4 integrations), pinned to this branch.

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