fix(sandbox): mount the guest bridge outside the read-only /opt

CHANGELOG.md

@@ -9,6 +9,11 @@ file is the human-readable summary of what changed and why.
 
 ### Desktop / Web UI
 
+- **Sandboxes now start on Linux** — every sandbox on Linux failed immediately
+  with `Can't mkdir /opt/codev-bridge: Read-only file system`. The guest's
+  `/opt` comes from the read-only image, so the sandbox could not create its
+  own mount point inside it; the internal bridge directory moved to `/codev-bridge`.
+  No image download or re-create is needed — existing sandboxes start as they are.
 - **New session inside a sandbox now works** — a sandbox had no project of its
   own in the app, so **New session** had nothing to open there and quietly
   started the session on the local server instead: the sandbox looked selected

packages/codev-sandbox/src/bwrap/args.test.ts

@@ -44,13 +44,27 @@ test("mounts the workspace rw and the bundle/bridge ro after the rootfs binds",
   const joined = buildBwrapArgs(spec).join(" ")
   expect(joined).toContain("--bind /host/project /workspace")
   expect(joined).toContain("--ro-bind /host/bundle /opt/codev")
-  expect(joined).toContain("--ro-bind /data/default/bridge /opt/codev-bridge")
+  expect(joined).toContain("--ro-bind /data/default/bridge /codev-bridge")
   expect(joined).toContain("--bind /data/default/runtime /run/codev")
   expect(joined).toContain("--bind /data/default/home /home/codev")
   expect(joined.indexOf("--bind /host/project")).toBeGreaterThan(joined.indexOf("--ro-bind /data/default/rootfs/usr"))
   expect(joined).toContain(`--chdir ${GUEST_WORKSPACE}`)
 })
 
+// bwrap creates missing mount points, but only where it can write: its own
+// tmpfs root, never inside a read-only rootfs bind. A destination nested under
+// a bound rootfs entry must therefore already exist in the image — which the
+// image only guarantees for /opt/codev. GUEST_BRIDGE lived at /opt/codev-bridge
+// and failed every real start with EROFS; the integration test missed it
+// because its host-rootfs allowlist excludes /opt.
+test("never nests a mount destination inside a read-only rootfs bind", () => {
+  const args = buildBwrapArgs({ ...spec, rootfsEntries: [{ name: "opt", symlinkTarget: null }] })
+  const destinations = args.flatMap((arg, index) =>
+    (arg === "--bind" || arg === "--ro-bind") && !args[index + 1].startsWith(spec.rootfsDir) ? [args[index + 2]] : [],
+  )
+  expect(destinations.filter((destination) => destination.startsWith("/opt/"))).toEqual(["/opt/codev"])
+})
+
 test("clears the environment before setting the boot env", () => {
   const args = buildBwrapArgs(spec)
   const joined = args.join(" ")

packages/codev-sandbox/src/bwrap/args.ts

@@ -2,7 +2,11 @@ import { join } from "node:path"
 
 export const GUEST_WORKSPACE = "/workspace"
 export const GUEST_BUNDLE = "/opt/codev"
-export const GUEST_BRIDGE = "/opt/codev-bridge"
+// Top level, not under /opt: the image's own /opt is bind-mounted read-only
+// from the rootfs, so bwrap cannot create a mount point inside it. Only paths
+// the image already provides (/opt/codev) or that bwrap creates on its tmpfs
+// root (this one, /run/codev, /workspace) can be mounted over.
+export const GUEST_BRIDGE = "/codev-bridge"
 export const GUEST_RUNTIME = "/run/codev"
 export const GUEST_HOME = "/home/codev"
 // Fixed ports inside the guest's private network namespace — no conflicts are