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Fix block splitter partition bounds - #4918
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Valid external sequence streams can exhaust the recursive block splitter's budget even within the 128 KiB block-size limit. The budget was checked only on function entry, so a left-child recursion could fill the partition array before its ancestors wrote their own split positions. The terminating block boundary also had no dedicated slot. These writes could silently overwrite adjacent compression-context state. Recheck the split budget after the left recursion, reserve one extra partition entry for the terminating boundary, and assert the final split count. Update the comment that incorrectly described the limit as unreachable. Add a deterministic regression using 38,400 valid external sequences with sequence validation enabled. It verifies the 196-split limit, the in-bounds terminator, and a successful round trip. The unmodified baseline emitted 200 blocks; the fixed implementation emits 197.
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Valid external sequence streams can exhaust the recursive block
splitter's budget even within the 128 KiB block-size limit. The budget
was checked only on function entry, so a left-child recursion could
fill the partition array before its ancestors wrote their own split
positions. The terminating block boundary also had no dedicated slot.
These writes could silently overwrite adjacent compression-context
state.
Recheck the split budget after the left recursion, reserve one extra
partition entry for the terminating boundary, and assert the final
split count. Update the comment that incorrectly described the limit
as unreachable.
Add a deterministic regression using 38,400 valid external sequences
with sequence validation enabled. It verifies the 196-split limit,
the in-bounds terminator, and a successful round trip. The unmodified
baseline emitted 200 blocks; the fixed implementation emits 197.