Notice of new option for transaction replacement policies

This version of Bitcoin Core adds a new mempoolfullrbf configuration option which allows users to change the policy their individual node will use for relaying and mining unconfirmed transactions.  The option defaults to the same policy that was used in previous releases and no changes to node policy will occur if everyone uses the default.

Some Bitcoin services today expect that the first version of an unconfirmed transaction that they see will be the version of the transaction that ultimately gets confirmed---a transaction acceptance policy sometimes called "first-seen".

The Bitcoin Protocol does not, and cannot, provide any assurance that the first version of an unconfirmed transaction seen by a particular node will be the version that gets confirmed.  If there are multiple versions of the same unconfirmed transaction available, only the miner who includes one of those transactions in a block gets to decide which version of the transaction gets confirmed.

Despite this lack of assurance, multiple merchants and services today still make this assumption.

There are several benefits to users from removing this first-seensimplification.  One key benefit, the ability for the sender of a transaction to replace it with an alternative version paying higher fees, was realized in Bitcoin Core 0.12.0 (February 2016) with the introduction of BIP125 opt-in Replace By Fee (RBF).

Since then, there has been discussion about completely removing the first-seen simplification and allowing users to replace any of their older unconfirmed transactions with newer transactions, a feature called full-RBF.  This release includes a mempoolfullrbf configuration option that allows enabling full-RBF, although it defaults to off (allowing only opt-in RBF).

Several alternative node implementations have already enabled full-RBF by default for years, and several contributors to Bitcoin Core are advocating for enabling full-RBF by default in a future version of Bitcoin Core.

As more nodes that participate in relay and mining begin enabling full-RBF, replacement of unconfirmed transactions by ones offering higher fees may rapidly become more reliable.

Contributors to this project strongly recommend that merchants and services not accept unconfirmed transactions as final, and if they insist on doing so, to take the appropriate steps to ensure they have some recourse or plan for when their assumptions do not hold.

Notable changes

P2P and network changes

To address a potential denial-of-service, the logic to download headers from peers has been reworked.  This is particularly relevant for nodes starting up for the first time (or for nodes which are starting up after being offline for a long time).

Whenever headers are received from a peer that have a total chainwork that is either less than the node's -minimumchainwork value or is sufficiently below the work at the node's tip, a "presync" phase will begin, in which the node will download the peer's headers and verify the cumulative work on the peer's chain, prior to storing those headers permanently. Once that cumulative work is verified to be sufficiently high, the headers will be redownloaded from that peer and fully validated and stored.

This may result in initial headers sync taking longer for new nodes starting up for the first time, both because the headers will be downloaded twice, and because the effect of a peer disconnecting during the presync phase (or while the node's best headers chain has less than -minimumchainwork), will result in the node needing to use the headers presync mechanism with the next peer as well (downloading the headers twice, again). (#25717)

With I2P connections, a new, transient address is used for each outbound connection if -i2pacceptincoming=0. (#25355)

Updated RPCs

The -deprecatedrpc=softforks configuration option has been removed.  The RPC getblockchaininfo no longer returns the softforks field, which was previously deprecated in 23.0. (#23508) Information on soft fork status is now only available via the getdeploymentinfo RPC.

The deprecatedrpc=exclude_coinbase configuration option has been removed. The receivedby RPCs (listreceivedbyaddress, listreceivedbylabel, getreceivedbyaddress and getreceivedbylabel) now always return results accounting for received coins from coinbase outputs, without an option to change that behaviour. Excluding coinbases was previously deprecated in 23.0. (#25171)

The deprecatedrpc=fees configuration option has been removed. The top-level fee fields fee, modifiedfee, ancestorfees and descendantfees are no longer returned by RPCs getmempoolentry, getrawmempool(verbose=true), getmempoolancestors(verbose=true) and getmempooldescendants(verbose=true). The same fee fields can be accessed through the fees object in the result. The top-level fee fields were previously deprecated in 23.0. (#25204)

The getpeerinfo RPC has been updated with a new presynced_headers field, indicating the progress on the presync phase mentioned in the "P2P and network changes" section above.

Changes to wallet related RPCs can be found in the Wallet section below.

New RPCs

The sendall RPC spends specific UTXOs to one or more recipients without creating change. By default, the sendall RPC will spend every UTXO in the wallet. sendall is useful to empty wallets or to create a changeless payment from select UTXOs. When creating a payment from a specific amount for which the recipient incurs the transaction fee, continue to use the subtractfeefromamount option via the send, sendtoaddress, or sendmany RPCs. (#24118)

A new gettxspendingprevout RPC has been added, which scans the mempool to find transactions spending any of the given outpoints. (#24408)

The simulaterawtransaction RPC iterates over the inputs and outputs of the given transactions, and tallies up the balance change for the given wallet. This can be useful e.g. when verifying that a coin join like transaction doesn't contain unexpected inputs that the wallet will then sign for unintentionally. (#22751)

Updated REST APIs

The /headers/ and /blockfilterheaders/ endpoints have been updated to use a query parameter instead of path parameter to specify the result count. The count parameter is now optional, and defaults to 5 for both endpoints. The old endpoints are still functional, and have no documented behaviour change.

For /headers, use GET /rest/headers/<BLOCK-HASH>.<bin|hex|json>?count=<COUNT=5>instead of GET /rest/headers/<COUNT>/<BLOCK-HASH>.<bin|hex|json> (deprecated)

For /blockfilterheaders/, use GET /rest/blockfilterheaders/<FILTERTYPE>/<BLOCK-HASH>.<bin|hex|json>?count=<COUNT=5>instead of GET /rest/blockfilterheaders/<FILTERTYPE>/<COUNT>/<BLOCK-HASH>.<bin|hex|json> (deprecated)

(#24098)

Build System

• Guix builds are now reproducible across architectures (x86_64 & aarch64). (#21194)

New settings

• A new mempoolfullrbf option has been added, which enables the mempool to accept transaction replacement without enforcing BIP125 replaceability signaling. (#25353)

Wallet

The -walletrbf startup option will now default to true. The wallet will now default to opt-in RBF on transactions that it creates. (#25610)

The replaceable option for the createrawtransaction and createpsbt RPCs will now default to true. Transactions created with these RPCs will default to having opt-in RBF enabled. (#25610)

The wsh() output descriptor was extended with Miniscript support. You can import Miniscript descriptors for P2WSH in a watchonly wallet to track coins, but you can't spend from them using the Bitcoin Core wallet yet. You can find more about Miniscript on the reference website. (#24148)

The tr() output descriptor now supports multisig scripts through the multi_a() and sortedmulti_a() functions. (#24043)

To help prevent fingerprinting transactions created by the Bitcoin Core wallet, change output amounts are now randomized. (#24494)

The listtransactions, gettransaction, and listsinceblockRPC methods now include a wtxid field (hash of serialized transaction, including witness data) for each transaction. (#24198)

The listsinceblock, listtransactions and gettransaction output now contain a new parent_descs field for every "receive" entry. (#25504)

A new optional include_change parameter was added to the listsinceblock command.

RPC getreceivedbylabel now returns an error, "Label not found in wallet" (-4), if the label is not in the address book. (#25122)

Migrating Legacy Wallets to Descriptor Wallets

An experimental RPC migratewallet has been added to migrate Legacy (non-descriptor) wallets to Descriptor wallets. More information about the migration process is available in the documentation.

GUI changes

A new menu item to restore a wallet from a backup file has been added (gui#471).

Configuration changes made in the bitcoin GUI (such as the pruning setting, proxy settings, UPNP preferences) are now saved to <datadir>/settings.jsonfile rather than to the Qt settings backend (windows registry or unix desktop config files), so these settings will now apply to bitcoind, instead of being ignored. (#15936, gui#602)

Also, the interaction between GUI settings and bitcoin.conf settings is simplified. Settings from bitcoin.conf are now displayed normally in the GUI settings dialog, instead of in a separate warning message ("Options set in this dialog are overridden by the configuration file: -setting=value"). And these settings can now be edited because settings.json values take precedence over bitcoin.conf values. (#15936)

Low-level changes

RPC

The deriveaddresses, getdescriptorinfo, importdescriptors and scantxoutset commands now accept Miniscript expression within a wsh() descriptor. (#24148)

The getaddressinfo, decodescript, listdescriptors and listunspent commands may now output a Miniscript descriptor inside a wsh() where a wsh(raw()) descriptor was previously returned. (#24148)

Credits

Thanks to everyone who directly contributed to this release:

• /dev/fd0
• 0xb10c
• Adam Jonas
• akankshakashyap
• Ali Sherief
• amadeuszpawlik
• Andreas Kouloumos
• Andrew Chow
• Anthony Towns
• Antoine Poinsot
• Antoine Riard
• Aurèle Oulès
• avirgovi
• Ayush Sharma
• Baas
• Ben Woosley
• BrokenProgrammer
• brunoerg
• brydinh
• Bushstar
• Calvin Kim
• CAnon
• Carl Dong
• chinggg
• Cory Fields
• Daniel Kraft
• Daniela Brozzoni
• darosior
• Dave Scotese
• David Bakin
• dergoegge
• dhruv
• Dimitri
• dontbyte
• Duncan Dean
• eugene
• Eunoia
• Fabian Jahr
• furszy
• Gleb Naumenko
• glozow
• Greg Weber
• Gregory Sanders
• gruve-p
• Hennadii Stepanov
• hiago
• Igor Bubelov
• ishaanam
• Jacob P.
• Jadi
• James O'Beirne
• Janna
• Jarol Rodriguez
• Jeremy Rand
• Jeremy Rubin
• jessebarton
• João Barbosa
• John Newbery
• Jon Atack
• Josiah Baker
• Karl-Johan Alm
• KevinMusgrave
• Kiminuo
• klementtan
• Kolby Moroz
• kouloumos
• Kristaps Kaupe
• Larry Ruane
• Luke Dashjr
• MarcoFalke
• Marnix
• Martin Leitner-Ankerl
• Martin Zumsande
• Michael Dietz
• Michael Folkson
• Michael Ford
• Murch
• mutatrum
• muxator
• Oskar Mendel
• Pablo Greco
• pasta
• Patrick Strateman
• Pavol Rusnak
• Peter Bushnell
• phyBrackets
• Pieter Wuille
• practicalswift
• randymcmillan
• Robert Spigler
• Russell Yanofsky
• S3RK
• Samer Afach
• Sebastian Falbesoner
• Seibart Nedor
• Shashwat
• Sjors Provoost
• Smlep
• sogoagain
• Stacie
• Stéphan Vuylsteke
• Suhail Saqan
• Suhas Daftuar
• t-bast
• TakeshiMusgrave
• Vasil Dimov
• W. J. van der Laan
• w0xlt
• whiteh0rse
• willcl-ark
• William Casarin
• Yancy Ribbens

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