Fix bind mount unmount breaking host /dev/pts

[Copilot]

Using MNT_DETACH (lazy unmount) on bind-mounted pseudo-filesystems like /dev/pts leaves the host in a broken state where PTY allocation fails (sudo: unable to allocate pty: No such device).

Changes

• Unmount strategy: Try normal umount2(flags=0) first, fall back to MNT_DETACH only on EBUSY
• Error handling: Cache raw_os_error() results to avoid redundant calls, properly distinguish EINVAL/ENOENT/EBUSY

Before:

let ret = unsafe { libc::umount2(target_cstr.as_ptr(), libc::MNT_DETACH) };

After:

// Try normal unmount first (safer for pseudo-filesystems like /dev/pts)
let ret = unsafe { libc::umount2(target_cstr.as_ptr(), 0) };
if ret == 0 { return Ok(()); }

let err_code = Error::last_os_error().raw_os_error();

// Fall back to lazy unmount only if busy
if err_code == Some(libc::EBUSY) {
    let ret = unsafe { libc::umount2(target_cstr.as_ptr(), libc::MNT_DETACH) };
    // ... handle errors ...
}

This prevents host filesystem corruption while maintaining backward compatibility for busy mounts.

Original prompt

使用docker导出乌班图 然后用这个BINARY="/root/rk8s/project/target/debug/examples/overlayfs_example"

sudo "$BINARY"
--mountpoint /root/merged
--upperdir /root/upper
--lowerdir /root/ubuntu-rootfs
--bind "proc:/proc"
--bind "sys:/sys"
--bind "dev:/dev"
--bind "dev/pts:/dev/pts"
--bind "etc/resolv.conf:/etc/resolv.conf"
--privileged接下来再开一个窗口 chroot ~/merged/ /bin/bash在里面进行apt update等操作都正常 但是ctrl+c退出overlayfs之后 再次运行（或者别的复杂操作）我宿主机就sudo: unable to allocate pty: No such device 只有运行mount -t devpts devpts /dev/pts才能解决 修复一下

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