[chxmxii]

Closes #29. Part of the v0.5 milestone.

New Boxfiles say backend: auto: box picks the engine so users don't have to.

How it picks

• firecracker when it fits: the Boxfile is offline (network: none), has no host mounts, no warm pool and no forks, and the host can run it (x86_64 Linux, KVM, vhost-vsock). Its VMM holds no capabilities at all.
• podman otherwise, which supports everything.
• The pick and its reason are recorded with the sandbox. box inspect shows backend chosen by backend: auto (…reason…); box doctor shows what auto picks on this host.

Why the pick sticks

firecracker keeps /data as a disk, podman as a directory. A sandbox that silently changed engine would lose sight of its data, so:

• the recorded engine stays while it can still serve the Boxfile;
• a change it cannot serve switches engines only while /data is empty; otherwise box stops and prints how to move the data (box data export, box reset, box build, box data import);
• an explicit backend: always wins, and a Boxfile without the key keeps podman, so existing sandboxes never move.

Firecracker's limits now live in one function, boxfile.FirecrackerFits, used by validation and by auto, so the two cannot disagree.

Also

• box fork on a firecracker sandbox refuses up front with the reason and the way out, instead of failing at the first run.
• Fixed: box reset did not reset a firecracker sandbox. It emptied the /data directory, but firecracker's /data is a disk, which survived. It now deletes the disk; the next boot makes an empty one.
• Fixed: box inspect created a missing firecracker disk while reporting on it (through fcDataDisk). It only reads now.

Tests

• Unit: picks firecracker when it fits and the host can run it; podman for a networked Boxfile or a host without firecracker, with the reason; stays on podman once data lives there even if firecracker would now fit; switches an empty sandbox, refuses (with the move recipe) one with data; a Boxfile without the key stays podman and records nothing.
• Real VMs: a new offline sandbox lands on firecracker (6.1 kernel) and runs; a networked one on podman; forking the firecracker one is refused with the reason; box reset on firecracker really empties /data; box doctor reports the auto choice.