feat(sandbox): mount extra virtiofs tags from mounts.json in the guest

codev-init now reads /codev-config/mounts.json (written by the vz
provider every start) and mounts each listed tag, non-fatally per entry
so one bad share cannot brick the boot. This is the one-time image
change: all future mount-list changes are host-side manifest writes.

Compatibility is safe in both directions — an old image ignores the
extra shares (they sit unmounted and inert), and a new image without a
manifest skips the block. Existing macOS sandboxes pick the new init up
via Reset, which re-provisions from the current image.

e2e-vz.ts gains a mounts probe: the guest reads a marker from
/mnt/extra and writes back into it, exercising both directions.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Quickbeard

packages/codev-sandbox/image/guest/codev-init

@@ -16,6 +16,17 @@ mount -t tmpfs tmpfs /tmp
 mount -t virtiofs workspace /workspace
 mount -t virtiofs bundle /opt/codev
 mount -t virtiofs config /codev-config
+# Extra host folders arrive as additional virtiofs tags listed in mounts.json,
+# which the host rewrites on every start. Tags and guest paths are sanitized
+# host-side (no spaces or quotes); a failed mount is logged and skipped so one
+# bad share cannot brick the boot.
+if [ -f /codev-config/mounts.json ]; then
+  node -e '
+    for (const m of require("/codev-config/mounts.json").mounts ?? []) console.log(m.tag, m.path)
+  ' | while read -r tag path; do
+    mkdir -p "$path" && mount -t virtiofs "$tag" "$path" || echo "codev-init: mount $tag at $path failed" >&2
+  done
+fi
 ip link set lo up
 
 [ -f /etc/codev-guest.env ] && . /etc/codev-guest.env

packages/codev-sandbox/script/e2e-vz.ts

@@ -24,6 +24,11 @@ const work = await mkdtemp(join(tmpdir(), "codev-vz-e2e-"))
 const projectDir = argValue("--project") ?? join(work, "project")
 await mkdir(projectDir, { recursive: true })
 await writeFile(join(projectDir, "hello.txt"), "hello from the host project\n")
+// A second host folder mounted on top of the project dir, to prove the
+// mounts.json path: the guest must see it at /mnt/extra.
+const extraDir = join(work, "extra")
+await mkdir(extraDir, { recursive: true })
+await writeFile(join(extraDir, "marker.txt"), "extra-mount-ok\n")
 
 const payload = Bun.file(image)
 const hash = createHash("sha256")
@@ -55,7 +60,13 @@ const provider = createVzProvider({
 })
 
 console.log("availability:", await provider.availability())
-const config: SandboxConfig = { id: "sandbox:e2e", name: "e2e", backend: "vz", projectDir }
+const config: SandboxConfig = {
+  id: "sandbox:e2e",
+  name: "e2e",
+  backend: "vz",
+  projectDir,
+  mounts: [{ path: extraDir }],
+}
 
 console.time("create")
 await provider.create(config)
@@ -106,6 +117,37 @@ const brokerOk = await (async () => {
 })()
 console.log(`broker: ${brokerOk ? "OK" : "FAILED"}`)
 
+// Extra mount: the guest reads the marker from /mnt/extra (mounted by
+// codev-init from mounts.json) and writes proof both into the workspace and
+// back into the extra mount itself, exercising read and write.
+console.log(`guest mounts: ${JSON.stringify(sandbox.mounts)}`)
+await fetch(`${sandbox.url}/api/pty`, {
+  method: "POST",
+  headers: { authorization: auth, "content-type": "application/json" },
+  body: JSON.stringify({
+    command: "/bin/sh",
+    args: [
+      "-c",
+      "cat /mnt/extra/marker.txt > /workspace/mount-probe.txt 2>&1; echo written-from-guest > /mnt/extra/guest-write.txt 2>/dev/null",
+    ],
+    cwd: "/workspace",
+  }),
+}).catch(() => null)
+const mountOk = await (async () => {
+  for (let attempt = 0; attempt < 30; attempt++) {
+    const read = await Bun.file(join(projectDir, "mount-probe.txt"))
+      .text()
+      .catch(() => "")
+    const written = await Bun.file(join(extraDir, "guest-write.txt"))
+      .text()
+      .catch(() => "")
+    if (read.trim() === "extra-mount-ok" && written.trim() === "written-from-guest") return true
+    await new Promise((resolve) => setTimeout(resolve, 500))
+  }
+  return false
+})()
+console.log(`extra mount: ${mountOk ? "OK" : "FAILED"}`)
+
 if (hold) {
   console.log("holding the VM up; Ctrl-C to stop")
   await new Promise(() => {})
@@ -117,7 +159,7 @@ await Promise.race([exited, new Promise((resolve) => setTimeout(resolve, 10_000)
 await provider.destroy(config)
 server.stop(true)
 await rm(work, { recursive: true, force: true })
-if (!healthy || !pathsOk || !brokerOk) process.exit(1)
+if (!healthy || !pathsOk || !brokerOk || !mountOk) process.exit(1)
 console.log("e2e OK")
 process.exit(0)