bug(vm): per-sandbox Unix socket paths exceed macOS sun_path limit

まず、VM compute driver におけるサンドボックスごとのソケットパスの構築から始め、デフォルトの state_dir を使って macOS の bind 失敗を再現します。サンドボックスディレクトリがどのように作成および復元されるかを追跡し、その後、改訂後のパスが macOS の制限内に収まり、検出と永続化を維持し、run/compute-driver.sock が変更されないことを確認します。

User Story

As a developer running the VM compute driver on macOS, I want per-sandbox Unix socket paths to fit within the OS sun_path limit so that sandbox creation does not fail with a socket bind error.

Problem Statement

The VM driver constructs per-sandbox Unix socket paths by joining {state_dir}/sandboxes/{sandbox_id}/{socket_name}. Sandbox IDs are UUID v4 strings (36 chars), and state_dir depends on the user's home directory. On macOS, struct sockaddr_un.sun_path is 104 bytes (103 usable). With the default state_dir (~/.local/state/openshell/vm-driver), the resulting socket path reaches 107 bytes — 3 bytes over the limit — and bind() fails.

/Users/benoitf/.local/state/openshell/vm-driver/sandboxes/3eb2ad45-bead-4c2e-bd10-1a4a7f3a2721/control.sock
└─────────────────────────────────── 107 bytes ──────────────────────────────────────┘

Path length breakdown:

/Users/benoitf/.local/state/openshell/vm-driver   (47)
/sandboxes/                                        (11)
3eb2ad45-bead-4c2e-bd10-1a4a7f3a2721               (36)
/control.sock                                      (13)
                                            total = 107 bytes
                                     macOS limit = 104 bytes
                                       over by 3 bytes

Impact / Why This Matters

When this happens, VM sandbox creation fails at the control socket bind step. The error surfaces as a low-level socket error, not as a clear path-length diagnostic.

The current workaround is to manually configure a short state_dir (e.g. /tmp/os-vm). or rename control.sock for ctl.sock for my case (as I reduced 3 extra chars)
This is non-obvious, fragile, and breaks persistence expectations. Any user whose home directory path is longer than /Users/benoitf (15 chars) would be even further over the limit.

Linux is less affected (108-byte limit) but not immune with deep state directories or longer usernames.

Acceptance Criteria

• Per-sandbox Unix socket paths fit within 103 usable bytes on macOS for any reasonable state_dir
• The fix does not break sandbox state directory layout or persistence/restore
• Existing sandbox directories remain discoverable (sandbox ID must still be recoverable from the filesystem)
• The gateway compute-driver socket path (run/compute-driver.sock) remains unaffected

Reproduction Steps

1. On macOS, start the gateway with --compute-driver vm using the default state_dir (~/.local/state/openshell/vm-driver)
2. Run openshell sandbox create --name test --from ubuntu:24.04
3. Observe the sandbox creation fails at the control socket bind

Environment

• OpenShell: development build (current main branch)
• OS: macOS (Apple Silicon) — sun_path is 104 bytes
• Runtime: VM compute driver (libkrun)

Logs

Error: bind() failed for /Users/benoitf/.local/state/openshell/vm-driver/sandboxes/3eb2ad45-bead-4c2e-bd10-1a4a7f3a2721/control.sock