ibd: cut pre-milestone CPU (fence Arc, densify watermark, v2 decode, MTP ring) - #245
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August 25, 2026 13:21
Leftover TipOnly cloned O(blocks) runs on every get_fk_by_txid_batch. runs is Arc; clone is pointer-sized. extend/pop COW via make_mut so a live leftover snapshot does not block fence updates.
collect_height_band restarted at path_lo every assign tick (up to 65k heights). Advance densify_scan_lo over claim-ready / inflight / archived; pending is still walked so zombies demote. Tip shrink resets.
BIP324 plaintext is command+payload. Hashing a synthetic checksum and rebuilding a v1 frame ran sha256d twice plus a 4 MB copy per block. Dispatch Decodable on the payload; checksum is unused zeros.
Sequential confirm re-read 11 headers for every MTP. Push timestamp on height_fence_extend; pop rebuilds from confirmed. Historical and cold-ring heights keep the 11-read path.
Fence snapshot is Arc, densify skips the BQ-ready prefix, v2 decode skips checksum reframe, and tip MTP is an 11-slot ring. ibd-memory and CHANGELOG note the same.
v2 decode no longer hashes or verifies a checksum. The leftover field failed -D warnings on production builds.
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Why
Default mainnet IBD (
--milestone 840000) skips scripts and mempool relay. algo-review still had four CPU cliffs on that path:What
HeightFenceruns areArc; leftover snapshot is O(1); extend/pop COW.densify_scan_loskips BQ-ready / inflight / archived prefix. Pending is still walked so zombies demote. Tip-hole cover unchanged.parse_v2_contentsdoes not sha256d.FramedMessage::decodeis command+payload (checksum unused zeros).Closes algo-review P7, P8, P10, P16 in the same PR. Soft densify remains request-limited only. No store lock across leftover IO. No uring flatten.
Test plan
cargo test -p rbitcoin-store --lib height_fencecargo test -p rbitcoin-net --lib densify_/codec/v2cargo test -p rbitcoin-consensus --lib median_time_past