BIP370: add Input Finalizer section; BIP371: remove finalizer clause from an output field - #2277
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BIP 174 tells the Input Finalizer to clear all other data except the UTXO and unknown fields from a finalized input. It was written when the unsigned transaction was a global field. PSBTv2 moves the data that defines the input into the input map, and a finalizer that follows the sentence literally removes it: without PSBT_IN_PREVIOUS_TXID or PSBT_IN_OUTPUT_INDEX the result is invalid, and without PSBT_IN_SEQUENCE and the locktime fields the Transaction Extractor builds a different transaction. BIP 370 amends Creator, Updater, Signer and Transaction Extractor for PSBTv2, adds the Constructor, and says nothing about the Input Finalizer. Add the section: the five fields are kept. Bitcoin Core does this today: at 19b0ff2fa0, PSBTInput::Serialize in src/psbt.h writes them for every PSBTv2 input, finalized or not.
The six taproot input fields say "Finalizers should remove this field after PSBT_IN_FINAL_SCRIPTWITNESS is constructed", and so does the output field PSBT_OUT_TAP_BIP32_DERIVATION. An output has no PSBT_IN_FINAL_SCRIPTWITNESS, the clause names no input, and its two neighbours in the output table carry no such clause. Drop it; Bitcoin Core does not clear output fields on finalization.
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Two independent commits.
BIP370: add an Input Finalizer section
In a version 2 PSBT, the fields that define an input live in the input map:
PSBT_IN_PREVIOUS_TXID,PSBT_IN_OUTPUT_INDEX,PSBT_IN_SEQUENCE, and the two required-locktime fields.BIP174 tells the Input Finalizer to clear every field of a finalized input except the UTXO and unknown fields. Followed literally on a version 2 input, this deletes the fields above. Without
PSBT_IN_PREVIOUS_TXIDorPSBT_IN_OUTPUT_INDEXthe PSBT is invalid; without the sequence and locktime fields the Transaction Extractor builds a different transaction.BIP370 defines every other role for version 2 but has no Input Finalizer section. This adds one, stating that the finalizer keeps those five fields. Bitcoin Core already does this: its input serializer writes them whether or not the input is finalized.
BIP371: remove a finalizer clause from an output field
PSBT_OUT_TAP_BIP32_DERIVATIONcarries the note "Finalizers should remove this field afterPSBT_IN_FINAL_SCRIPTWITNESSis constructed".This is an output field. An output has no
PSBT_IN_FINAL_SCRIPTWITNESS, and the clause names no input to act on. The two neighbouring output fields carry no such note. This removes it.Both are minor amendments to Deployed BIPs.