# Test Case: Fragment-backed compressed metabox

**Test ID**: TC142-fragment-backed-compressed-metabox
**Fixture**: `metabox-fragment.erofs` and its four negative variants

## Objective

Independently regress packed-carrier initialization before a fragment-backed
metabox read and recursion/range rejection. Do not reuse TC140 guest results.

## Layout qualification

`g4_fixtures.py verify` must prove a compressed metabox carrier, whole-file
fragment header with bit 63 set, `fragment_offset + carrier_size <= packed_size`,
ordinary metabox/packed NIDs, bit-63 synthetic inode NIDs, CRC-valid images,
and one-field mutations for self-loop, range, metabox recursion, and packed
recursion. Record source and all image SHA256 values.

## Positive steps

```sh
unit=$(mdconfig -a -t vnode -f /tmp/repo22-g4/images/metabox-fragment.erofs)
mount -t erofs -o ro /dev/${unit} /mnt/repo22-g4
stat -f 'mode=%Sp size=%z inode=%i' /mnt/repo22-g4/metabox-a
sha256 -q /mnt/repo22-g4/metabox-a
lsextattr user /mnt/repo22-g4/metabox-a
getextattr -qq -x user shared-prefix-key /mnt/repo22-g4/metabox-a
getextattr -qq -x user item /mnt/repo22-g4/metabox-a
umount /mnt/repo22-g4
mdconfig -d -u "${unit#md}"
```

## Negative steps

Trace-mount each of the four negative images in a fresh md cycle. Capture the
exact `nmount` errno/text, pre/post dmesg, `vmstat -m`, guest responsiveness,
and zero mount/md cleanup.

## Expected results

Positive file SHA256 and values `shared-value`/`value-000` match source. Every
negative returns `EINTEGRITY` (97), with no leak, hang, panic, trap, or residue.
