Test Case: HEAD2, Interlaced, and Extent Mapping

Overall Status Rule: PARTIAL when HEAD2 and interlaced pass but the explicit mapped-payload subscenario remains SHELVED; see issues/extent-metadata-fixture-unavailable.md

Test ID: TC146-head2-interlaced-extent-mapping

G5 fixture contract: Use G5-MANUAL-SETUP.md. Its generated paths, source comparisons, structured corruption offsets, and cleanup rules supersede placeholder examples in this file. Category: Compression Mapping Priority: Critical Regression: New compressed mapping formats

Objective

Verify FreeBSD kernel reads for HEAD2 and interlaced pclusters, and independently review extent-record mapping when erofs-utils 1.8.6 cannot generate that format.

Preconditions

FreeBSD 15 guest and repo22 module.
HEAD2 fixture with byte-level assertions:
- incompat bit EROFS_FEATURE_INCOMPAT_COMPR_HEAD2 is set;
- Z_EROFS_ADVISE_BIG_PCLUSTER_2 is set;
- a proven full-index HEAD record type is changed from HEAD1 to HEAD2;
- the high algorithm nibble names the decoder used by HEAD2;
- CRC32C is recomputed.
Interlaced fixture generated by erofs-utils 1.8.6:
```
mkfs.erofs -zlz4 -C4096 -Efragments -T0 \
  interlaced.erofs source-dir
```
dump.erofs -e must show real 4096-byte plain extents interspersed with compressed extents.

HEAD2 and Interlaced Steps

Prove the HEAD2 feature, map-header advise bits, algorithm nibbles, and HEAD2 record before guest transfer.
Mount the HEAD2 image, hash the full file, and compare a read crossing the first logical pcluster boundary recorded by the manifest. Do not assume the logical boundary is 65536 merely because the physical pcluster limit is 65536.
Read the targeted corrupted HEAD2 copy and record errno.
Mount the erofs-utils 1.8.6 interlaced image, hash the full file, and compare a range crossing the first compressed/plain transition.
Compare pre/post dmesg and clean all resources.

Extent Metadata Static Review

Run these checks against exactly erofs-utils 1.8.6 and both reference trees:

mkfs.erofs -V
grep -R -E 'Z_EROFS_ADVISE_EXTENTS|z_erofs_extent_recsize|struct z_erofs_extent[[:space:]]*\{' \
  /work/build/erofs-utils-v1.8.6-source/include \
  /work/build/erofs-utils-v1.8.6-source/lib
grep -n "struct z_erofs_extent\|z_erofs_extent_recsize" \
  src/erofs_fs.h /work/dev-src-linux/fs/erofs/erofs_fs.h
grep -n "z_erofs_map_blocks_ext" \
  src/zmap.c /work/dev-src-linux/fs/erofs/zmap.c

Review all four record sizes (4, 8, 16, 32 bytes):

4-byte records use the initial 64-bit physical base and accumulated plen;
8-byte records carry per-record 32-bit physical starts;
16/32-byte records use explicit extent counts and binary search by logical start, with 32-byte records adding lstart_hi;
plen format, partial-reference, interlaced/shifted, and final fragment encodings are decoded in Linux order;
metadata reads route through the metabox when the inode NID has bit 63;
malformed zero-count explicit tables and fragment bounds fail closed.

Expected Results

HEAD2 and interlaced full SHA256 and boundary reads match source bytes.
Targeted HEAD2 corruption returns errno 5 (EIO) without a panic/trap.
Explicit mapped-payload result is SHELVED, not PASS, because erofs-utils 1.8.6 has no extent-record generator or on-disk extent structure and the structured helper cannot validate a relocated mapped payload.
The static FreeBSD/Linux format and control-flow review is recorded separately from guest results.
TC146 overall is PARTIAL unless all three subscenarios have dynamic positive coverage.

Cleanup

Unmount all images, detach md units, unload by exact module ID, and verify no new dmesg lines.

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