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+# 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](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:
+  ```sh
+  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
+1. Prove the HEAD2 feature, map-header advise bits, algorithm nibbles, and HEAD2
+   record before guest transfer.
+2. 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.
+3. Read the targeted corrupted HEAD2 copy and record errno.
+4. Mount the erofs-utils 1.8.6 interlaced image, hash the full file, and compare
+   a range crossing the first compressed/plain transition.
+5. 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:
+
+```sh
+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.