# Test Case: Targeted Compressed-Stream Corruption

**Test ID**: TC116-truncated-compressed
**Category**: Error Handling
**Priority**: Critical

## Objective

Verify targeted damage inside a proven LZ4 physical extent reaches
`z_erofs_decompress` and returns `EIO`. Provider length must remain identical;
this is not a short-media test.

## Fixture Qualification

```sh
work=$(mktemp -d /tmp/repo22-tc116.XXXXXX)
tests/prepare_error_fixtures.sh "$work/fixtures"
cc -O2 -Wall -Wextra -std=c17 tests/read_probe.c -o "$work/read_probe"
cat "$work/fixtures/compressed.extents"
grep '^compressed-stream-corrupt ' "$work/fixtures/fixture-evidence.txt"
wc -c "$work/fixtures/valid-lz4.erofs" \
  "$work/fixtures/compressed-stream-corrupt.erofs"
cat "$work/fixtures/compressed-corrupt-fsck.txt"
sha256 -q "$work/fixtures/compressed-stream-corrupt.erofs"
```

The generator parses extent 0 for `/compressed.bin`, proves the 64-byte patch
at extent offset +32 lies wholly inside that extent, changes nonzero bytes to
zero, remains outside the superblock checksum window, preserves provider
length, and requires `fsck.erofs --extract` failure. The superblock checksum
remains valid because payload bytes are outside its range.

## FreeBSD Procedure

```sh
mkdir "$work/mnt"
unit=$(mdconfig -a -t vnode \
  -f "$work/fixtures/compressed-stream-corrupt.erofs")
mount -t erofs -o ro "/dev/$unit" "$work/mnt"
cmp "$work/fixtures/source/control.txt" "$work/mnt/control.txt"
probe_module=$(dtrace -l -f z_erofs_decompress | awk \
  '$4 == "z_erofs_decompress" && $5 == "entry" { print $3; exit }')
test -n "$probe_module"
dtrace -l -n "fbt:$probe_module:z_erofs_decompress:entry"
set +e
dtrace -q -n "fbt:$probe_module:z_erofs_decompress:entry {
  @calls = count(); } END { printa(\"decompress_calls=%@d\\n\", @calls); }" \
  -c "$work/read_probe expect-error $work/mnt/compressed.bin 5" \
  >"$work/dtrace.out" 2>"$work/dtrace.err"
dtrace_status=$?
set -e
cat "$work/dtrace.out"
test "$dtrace_status" -eq 0
grep -Eq 'decompress_calls=[1-9][0-9]*' "$work/dtrace.out"
truss -o "$work/read.truss" "$work/read_probe" expect-error \
  "$work/mnt/compressed.bin" 5 | tee "$work/error.out"
grep -q 'expected_errno=5' "$work/error.out"
grep -E 'read.*ERR#5' "$work/read.truss"
cmp "$work/fixtures/source/control.txt" "$work/mnt/control.txt"
umount "$work/mnt"
mdconfig -d -u "${unit#md}"
kldunload dtraceall
```

## Expected Results

Mount succeeds, proving media-size validation passed. FBT records at least one
`z_erofs_decompress` entry for the target command, and the target read returns
errno 5 (`EIO`). Control data remains readable and cleanup is complete.