Ref writers, server side: the pre-receive hook, run tokens, admission

[nishu-builder]

The server half of design/ref-writers.md. Part 2 of 4, stacked on #298.

• Pre-receive hook. The server installs hooks/pre-receive, which re-runs the server binary with --pre-receive, so it covers smart HTTP and iroh alike. For each ref a push moves:

  • a content-named ref (refs/caos/req/<h>, refs/heads/caos-test/<h>) must point at <h>, or be deleted;
  • a ref under refs/caos/w/<ns>/ needs one of that namespace's writers, proven by a signature or a run token. writers itself changes only by a current writer's signature, as a fast-forward;
  • refs/caos/dev (caos.unguardedRef) is always accepted.

  git config caos.refWriters report on the server's repository only logs what it would refuse.

• Run tokens. A job whose writes arg names namespaces, or *, gets a token at /secret/caos-write, revoked when its container is done. A top-level request is granted what the writer of its signed X-Caos-Write header may write; a continuation or child only what its creator was granted. A token can found a namespace that lists only its own writer, and can never change a writers list.

• conversation_protocol::v3::writers holds what the client and workers share: the list format, genesis commits, the signed messages, and GitStore push auth.

Anything else is still let through. Conversations and actors live outside namespaces until #305 moves them in and refuses the rest, so nothing pushed today is refused.

Verified here

• cargo fmt --check, clippy -D warnings and cargo test pass. server/tests/pre_receive.rs does real git pushes through the hook binary. The one failing test, launcher::checkout_import_completes_partial_history_and_keeps_local_edits, fails the same way on main in this container.
• llm-step, run-and-update-ref and llm-test-tool, which link conversation-protocol, build and pass their tests.
• Both lint scripts and nix build pass.

Not verified here

• The caos-test suite: caosd up can't fetch flake inputs from inside its containers in this sandbox.

Stack

1. Propose ref writers: who may push a ref, and how jobs inherit it #298: design doc
2. this PR: server
3. Ref writers, client side: keys, namespaces and writers from caos-cli #304: caos-cli
4. Ref writers for conversations and actors: namespaces, delegation, and closing the rest #305: conversations and actors

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