backend: auto picks the engine, once per sandbox (#29)

README.md

@@ -36,7 +36,7 @@ box down devbox
 
 ```yaml
 base: docker.io/library/alpine:latest
-backend: podman        # podman, krun or firecracker
+backend: auto          # firecracker when it fits, else podman; or name one
 isolation: strict      # the VMM runs as a UID that is not yours
 network: bridge        # or none, for no network at all
 warm: 2                # keep VMs booted so runs start in ms

docs/docs.html

@@ -77,7 +77,7 @@ <h2 id="boxfile">Boxfile</h2>
     <p>One YAML file per sandbox at <code>~/.box/sandboxes/&lt;name&gt;/Boxfile</code>. Unknown keys are rejected, so a typo fails loudly.</p>
     <div class="table-wrap build"><table>
       <thead><tr><th>Field</th><th>Values</th><th>Default</th><th>Meaning</th></tr></thead>
-      <tbody><tr><td class="mono">base</td><td>image reference</td><td class="mono">alpine:latest</td><td>The image every VM starts from.</td></tr><tr><td class="mono">backend</td><td>podman, krun, firecracker</td><td class="mono">podman</td><td>What boots the VM. See Architecture.</td></tr><tr><td class="mono">isolation</td><td>standard, strict</td><td class="mono">standard</td><td>strict runs the VMM as a subordinate UID of the sandbox's own.</td></tr><tr><td class="mono">cpus</td><td>1 to 16</td><td class="mono">2</td><td>vCPUs.</td></tr><tr><td class="mono">ram_mib</td><td>128 and up</td><td class="mono">2048</td><td>Guest RAM in MiB.</td></tr><tr><td class="mono">network</td><td>bridge, none</td><td class="mono">bridge</td><td>Internet access, or none at all.</td></tr><tr><td class="mono">readonly</td><td>bool</td><td class="mono">false</td><td>Read-only root; /tmp and /data stay writable.</td></tr><tr><td class="mono">timeout_seconds</td><td>int</td><td class="mono">0</td><td>Per-command limit; exits 124. 0 is unlimited.</td></tr><tr><td class="mono">warm</td><td>0 to 8</td><td class="mono">0</td><td>VMs kept booted for run (podman, krun).</td></tr><tr><td class="mono">warmup</td><td>shell lines</td><td class="mono"></td><td>Run once in each warm VM before it serves a run.</td></tr><tr><td class="mono">packages</td><td>names</td><td class="mono"></td><td>Installed with the image's package manager.</td></tr><tr><td class="mono">run</td><td>shell lines</td><td class="mono"></td><td>Extra build steps.</td></tr><tr><td class="mono">env</td><td>map</td><td class="mono"></td><td>Environment for every command.</td></tr><tr><td class="mono">mounts</td><td>host, guest, mode</td><td class="mono"></td><td>Host directories shared in (podman, krun); ro by default.</td></tr><tr><td class="mono">blueprint</td><td>users, write_files, runcmd</td><td class="mono"></td><td>Provisioning applied at build time.</td></tr><tr><td class="mono">pkgmgr</td><td>apk, apt, dnf</td><td class="mono">inferred</td><td>Override the package manager.</td></tr><tr><td class="mono">passt</td><td>bool</td><td class="mono">false</td><td>A real NIC with a default route (podman, krun).</td></tr><tr><td class="mono">seccomp</td><td>path</td><td class="mono"></td><td>A seccomp profile for the VMM.</td></tr></tbody>
+      <tbody><tr><td class="mono">base</td><td>image reference</td><td class="mono">alpine:latest</td><td>The image every VM starts from.</td></tr><tr><td class="mono">backend</td><td>auto, podman, krun, firecracker</td><td class="mono">podman (new Boxfiles: auto)</td><td>What boots the VM. auto picks firecracker when the sandbox is offline, has no host mounts, no warm pool and no forks, and the host can run it; podman otherwise. The pick is made once and shown by box inspect. See Architecture.</td></tr><tr><td class="mono">isolation</td><td>standard, strict</td><td class="mono">standard</td><td>strict runs the VMM as a subordinate UID of the sandbox's own.</td></tr><tr><td class="mono">cpus</td><td>1 to 16</td><td class="mono">2</td><td>vCPUs.</td></tr><tr><td class="mono">ram_mib</td><td>128 and up</td><td class="mono">2048</td><td>Guest RAM in MiB.</td></tr><tr><td class="mono">network</td><td>bridge, none</td><td class="mono">bridge</td><td>Internet access, or none at all.</td></tr><tr><td class="mono">readonly</td><td>bool</td><td class="mono">false</td><td>Read-only root; /tmp and /data stay writable.</td></tr><tr><td class="mono">timeout_seconds</td><td>int</td><td class="mono">0</td><td>Per-command limit; exits 124. 0 is unlimited.</td></tr><tr><td class="mono">warm</td><td>0 to 8</td><td class="mono">0</td><td>VMs kept booted for run (podman, krun).</td></tr><tr><td class="mono">warmup</td><td>shell lines</td><td class="mono"></td><td>Run once in each warm VM before it serves a run.</td></tr><tr><td class="mono">packages</td><td>names</td><td class="mono"></td><td>Installed with the image's package manager.</td></tr><tr><td class="mono">run</td><td>shell lines</td><td class="mono"></td><td>Extra build steps.</td></tr><tr><td class="mono">env</td><td>map</td><td class="mono"></td><td>Environment for every command.</td></tr><tr><td class="mono">mounts</td><td>host, guest, mode</td><td class="mono"></td><td>Host directories shared in (podman, krun); ro by default.</td></tr><tr><td class="mono">blueprint</td><td>users, write_files, runcmd</td><td class="mono"></td><td>Provisioning applied at build time.</td></tr><tr><td class="mono">pkgmgr</td><td>apk, apt, dnf</td><td class="mono">inferred</td><td>Override the package manager.</td></tr><tr><td class="mono">passt</td><td>bool</td><td class="mono">false</td><td>A real NIC with a default route (podman, krun).</td></tr><tr><td class="mono">seccomp</td><td>path</td><td class="mono"></td><td>A seccomp profile for the VMM.</td></tr></tbody>
     </table></div>
 
     <h2 id="cli">CLI</h2>