Rootfs and layers

Apply layers in order and honour whiteouts while building the rootfs. Preserve earlier capabilities and use the fixture only to make observation deterministic.

Contract

The harness receives a JSON process description and returns a syscall, mount, and limit plan. Real privileged execution stays outside the runner, but the plan must match Linux.

{
  "op": "merge-layers",
  "layers": [
    { "a": "1", "x": "old" },
    { ".wh.x": "", "b": "2" }
  ]
}

produces exactly:

{ "files": { "a": "1", "b": "2" } }

Emit compact JSON with a final newline. Invalid input leaves stdout empty, writes error to stderr, and exits non-zero.

Design and limits

Separate policy, mutable state, and system effects. Make resource ownership, cleanup order, and pre/post-operation invariants explicit. Do not replace the central mechanism with a simulation: the fixture controls inputs and time while your code implements isolation, memory, or synchronisation as appropriate.

Test empty boundaries, mid-operation failures, and idempotent cleanup. Sort output only at the presentation boundary.

Acceptance

• The example produces the exact output.
• Resources are not leaked and work is not duplicated.
• Failure leaves the system coherent.