btc(#744): unpack_gentx_coinbase -- gentx bytes -> MutableTransaction codec - #822
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… codec First landed sub-slice of the faithful share->block reconstructor the #744 dispatch handler (won_block_dispatch.hpp) still carries only as an injected std::function stub. btc::coin::unpack_gentx_coinbase turns a share's SSOT non-witness gentx (coinbase) bytes back into the MutableTransaction + txid that reconstruct_won_block will inject at block tx index 0. Parsed TX_NO_WITNESS (coinbase vin count 0x01, never the segwit 0x00 dummy) so the recovered tx is empty-witness and re-serializes byte-identically -- the txid (SHA256d of the non-witness bytes == p2pool gentx_hash) is what the merkle_root walk consumes, so any drift would corrupt the assembled block. Trailing bytes past a complete tx throw std::out_of_range (fail-closed; the reconstruct closure catches and never broadcasts a partial block). gentx_unpack_test.cpp: 4 KATs on btc_share_test (rides the allowlisted target, no build.yml change) -- oracle round-trip bytes-exact + txid-stable for the no-segwit and witness-commitment-first layouts, coinbase-shape sanity, and the trailing-byte throw. Ground-truth (bytes,txid) vectors are coin-agnostic and shared verbatim with the DGB sibling KAT; BTC 72/72 green. Mirrors src/impl/dgb/coin/gentx_unpack.hpp. Per-coin isolation: src/impl/btc/ only.
This was referenced Jul 24, 2026
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First landed sub-slice of the faithful share->block reconstructor. The #744 dispatch handler (won_block_dispatch.hpp, merged in #820) still carries the reconstructor only as an injected std::function stub; this adds btc::coin::unpack_gentx_coinbase, the codec that turns a share's SSOT non-witness gentx (coinbase) bytes back into the {MutableTransaction, txid} that reconstruct_won_block will inject at block tx index 0.
Parses TX_NO_WITNESS (coinbase vin count 0x01, never the segwit 0x00 dummy) so the recovered tx is empty-witness and re-serializes byte-identically; the txid (SHA256d of the non-witness bytes == p2pool gentx_hash) is what the merkle_root walk consumes. Trailing bytes past a complete tx throw std::out_of_range (fail-closed).
gentx_unpack_test.cpp adds 4 KATs riding the allowlisted btc_share_test target (no build.yml change): oracle round-trip bytes-exact + txid-stable for the no-segwit and witness-commitment-first layouts, coinbase-shape sanity, trailing-byte throw. Ground-truth (bytes,txid) vectors are coin-agnostic, shared verbatim with the DGB sibling KAT. Local build: btc_share_test 72/72 green. Mirrors src/impl/dgb/coin/gentx_unpack.hpp. Per-coin isolation: src/impl/btc/ only; p2pool-merged-v36 surface: NONE.