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+# repo22 compact LZ4 root-cause verification
+
+- Date: 2026-08-08 UTC
+- Source parent: `c939c472134c69fa4665f52ab1aec9a17cd583b9`
+- FreeBSD guest: 15.0-RELEASE-p8 amd64, QEMU TCG
+- Image tool: erofs-utils 1.8.6
+- Scope: the existing minimal compact LZ4 fixture only; no CI or wrapper was added
+
+## Changes under verification
+
+1. Accept incompat bit `0x8` for compressed HEAD2 while rejecting images with
+   external device slots until true multi-device I/O exists.
+2. Widen zmap `clusterofs` so the 64 KiB NONHEAD sentinel is representable.
+3. Count compact indexes using the logical cluster size.
+4. Apply leading zero padding according to the algorithm and the LZ4
+   `0padding` feature instead of stripping it unconditionally for all streams.
+5. Zero decompression targets and require exact output lengths.
+6. Apply DEFLATE `windowbits` and ZSTD `windowlog` to the decoder calls.
+7. Separate LZ4 full and partial completion rules and validate input/output
+   boundaries.
+8. Permit valid compact NONHEAD deltas to cross compact-pack boundaries.  The
+   previous BSD-only restriction caused the first real image read to fail at
+   the second compressed extent head.
+
+## Build and module lifecycle
+
+`./build.sh` completed successfully.  The resulting module had no unresolved
+`bcmp` symbol and contained no temporary zmap/zdata diagnostic strings.
+
+On the guest, all lifecycle operations succeeded:
+
+```text
+kldload_rc=0
+mount_rc=0
+umount_rc=0
+md_detach_rc=0
+kldunload_rc=0
+```
+
+## Real image result
+
+The fixture was generated by the existing manual-test command:
+
+```sh
+mkfs.erofs -T0 --all-root --ignore-mtime -x-1 -Uclear \
+  -zlz4 -C4096 lz4-root-compact-4k.erofs src-root-lz4
+```
+
+`dump.erofs` identifies `/test-lz4.txt` as layout 3 (compact), 1 MiB logical,
+8 KiB on disk, with two compressed extents.  The same image previously failed
+its first read with `EINTEGRITY` on `f2301295`.
+
+Final FreeBSD results:
+
+```text
+SHA256 (/mnt/repo22-rootfix/test-lz4.txt) = 370eb0a8df86868c4842ca535ed64670f0277ea2ed47a703f089bbb13ee4ac52
+SHA256 (source test-lz4.txt)               = 370eb0a8df86868c4842ca535ed64670f0277ea2ed47a703f089bbb13ee4ac52
+SHA256 (first 4096 bytes from EROFS)       = 3d6d283a95f80b3e4a9c243cf82dc0644283910b6a3be931fede5d771e132b14
+SHA256 (first 4096 bytes from source)      = 3d6d283a95f80b3e4a9c243cf82dc0644283910b6a3be931fede5d771e132b14
+full_read_rc=0
+first_4k_read_rc=0
+```
+
+Result: the minimal real compact LZ4 mount/read gate passes.
+
+## Integration follow-up
+
+The integration review after `6c9543892` found that the leading-padding
+predicate still selected every non-LZ4 algorithm.  The predicate was narrowed
+to LZ4 streams with the `0padding` incompat feature, matching the stated
+algorithm-specific behavior.  `./build.sh` and the same FreeBSD 15 compact LZ4
+mount/read lifecycle were rerun after this correction; the full-file SHA256
+remained `370eb0a8df86868c4842ca535ed64670f0277ea2ed47a703f089bbb13ee4ac52`,
+the first 4096-byte read returned 4096 bytes, and module unload succeeded.
+
+## Remaining gaps
+
+This batch does not claim runtime completion for legacy full indexes, 64 KiB
+or larger pclusters, ztailpacking, interlaced pclusters, fragments, extents,
+partial references, MicroLZMA, DEFLATE, or ZSTD images.  DEFLATE and ZSTD
+configuration handling was corrected and the module loaded on this FreeBSD 15
+kernel, but those algorithms were not image-tested in this converged batch.
+The LZ4 partial-decoding branch was code-reviewed but is not exercised by the
+current synchronous full-extent read path.