Rewrite the client Reassembler around one whole-frame buffer per frame:
frame_bytes rides in every header and packetize geometry is
deterministic (every non-final block is exactly max_data_per_block data
shards), so a data shard's final AU offset is computable on arrival —
copy it there once, straight from the decrypt ring. New
ErasureCoder::reconstruct_into decodes ONLY the missing shards directly
into the frame buffer's holes (gf16 native; gf8 legacy shim); received
recovery shards ride pooled shard-sized buffers. The completed buffer
IS Frame::data.
Deletes the per-shard to_vec + per-block concat + final AU concat
(~178k allocs and a double copy of every byte per second at 2 Gbps —
the pump wall the 2026-07-14 sweeps measured at 98.9% of an M3 Ultra
core). Reassembly now costs ~0.4 µs/packet in-stream.
The eager buffer changes the hostile-header exposure, so two new
firewalls: derived-geometry validation (a header lying about its
data_shards/block_count vs its own frame_bytes is dropped before it can
scribble across another shard's range) and an in-flight allocation
budget (IN_FLIGHT_BUF_FACTOR × max_frame_bytes) so a window of tiny
first-shards can't commit gigabytes.
Behavior parity pinned by the existing suite (all green unchanged) plus
new end-to-end roundtrips through the real Packetizer (multi-block +
partial tail, loss within budget, reversed delivery, duplicates, empty
frame, unrecoverable block ages out, budget enforcement). loss-harness
recovery curve identical; pipeline bench: gf8/1MB +42%, gf16 neutral
(host-encode dominated). Known pre-existing quirk kept as-is: reversed
delivery reconstructs early (data+recovery ≥ k) and counts late-not-lost
shards into fec_recovered_shards.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
enricobuehler
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