# Test Case: Future Compression Algorithm Rejection

**Test ID**: TC115-unsupported-algorithm
**Category**: Error Handling
**Priority**: High

## Objective

Verify mount-time `EOPNOTSUPP` for a legal compressed image whose
`available_compr_algs` field additionally advertises unknown bit `0x8000`.

## Fixture Qualification

```sh
work=$(mktemp -d /tmp/repo22-tc115.XXXXXX)
tests/prepare_error_fixtures.sh "$work/fixtures"
od -An -tx2 -j 1106 -N 2 "$work/fixtures/future-algorithm.erofs"
python3 tests/erofs_fixture.py inspect \
  "$work/fixtures/future-algorithm.erofs"
grep '^future-algorithm ' "$work/fixtures/fixture-evidence.txt"
wc -c "$work/fixtures/valid-lz4.erofs" \
  "$work/fixtures/future-algorithm.erofs"
sha256 -q "$work/fixtures/future-algorithm.erofs"
```

The mutator requires `EROFS_FEATURE_INCOMPAT_COMPR_CFGS`, asserts the old
16-bit field at absolute byte 1106, sets only future bit `0x8000`, recomputes
CRC32C, and preserves provider length.

## FreeBSD Procedure

```sh
mkdir "$work/mnt"
unit=$(mdconfig -a -t vnode -f "$work/fixtures/future-algorithm.erofs")
set +e
mount -t erofs -o ro "/dev/$unit" "$work/mnt" \
  >"$work/mount.out" 2>"$work/mount.err"
status=$?
set -e
test "$status" -ne 0
set +e
truss -o "$work/mount.truss" mount -t erofs -o ro "/dev/$unit" \
  "$work/mnt" >/dev/null 2>&1
set -e
grep -E 'nmount.*ERR#45' "$work/mount.truss"
! mount -p | awk -v path="$work/mnt" '$2 == path { found=1 }
  END { exit found ? 0 : 1 }'
mdconfig -d -u "${unit#md}"
```

## Expected Results

Mount returns errno 45 (`EOPNOTSUPP`), not checksum or media-size failure. No
mount or md consumer remains.
