ADR 0023 — M14 K4: building a Nova kernel where novac does not run

• Status: Accepted (project owner, 2026-10-29, taking every recommendation in §5). The wording is the agent's.
• Scope: M14 K4, OD-31. CosmoOS's roadmap 2.0 makes two choices that conflict once its kernel is Nova. Its rule 7 stops committing generated C, so every build runs novac. Its SW0 makes CosmoOS build itself. novac does not run on CosmoOS.

1. Context

• novac is a .NET program, distributed as NativeAOT binaries for Linux, macOS and Windows on x64 and arm64 (release.md). NativeAOT needs a .NET runtime port for each operating system, and CosmoOS has none. It runs Linux programs through its personality, but not the .NET runtime's full use of Linux: threads, mmap and signal-based GC suspension (inferred; untested).
• Today CosmoOS commits the generated C beside each .nova file, so any machine with a C compiler builds the kernel. That is what makes a self-hosted build possible now.
• Generated C is a pure function of the sources and novac's version (c-backend §1; CosmoOS checks it in CI).

2. Options

	A. Source releases carry the generated C (proposed)	B. novac on CosmoOS	C. A Nova compiler in Nova	D. Keep committing generated C
Self-hosted build	from a release tarball, no novac	yes	yes	yes
Changing a .nova file on CosmoOS	needs a host with novac	yes	yes	needs a host with novac
Cost	a make dist target and a CI check	a .NET runtime port, or the Linux personality running .NET	years	the tree doubles as the kernel becomes Nova

3. Proposed decision: A

• The repository holds .nova sources only, as roadmap 2.0's rule 7 says. make dist (CosmoOS) writes a source tarball with the generated C beside each .nova file, made by the pinned novac. A build from the tarball runs no novac.
• CI checks that the tarball builds without novac on both architectures, and that its C matches a fresh generation (the reproducibility check rule 7 already plans).
• A self-hosted CosmoOS builds its kernel from a release tarball. Developing the kernel's Nova code on CosmoOS itself needs a machine with novac until B or C exists. Both stay open as later decisions; C is the long-term direction for a self-hosting Nova.
• Nova needs nothing new for A: novac build --emit c already writes the files, with ADR 0022's --split layout when the kernel uses it.

4. Consequences

• SW0's self-hosting acceptance builds the kernel from the release tarball.
• CosmoOS's roadmap 2.0 and docs/build/nova.md record the choice (a CosmoOS PR once this is accepted).

5. Questions for the owner

1. Option A? (Recommended.)
2. Record B (novac on CosmoOS) and C (a Nova compiler written in Nova) as open decisions for later? (Recommended.)