Allow fee bumping for non-BIP125 transactions after mempool expiry #19030

Is your feature request related to a problem? Please describe.

Countless users have encountered the "stuck transaction" problem when the transactions piles up in the mempool, so that transactions with lower fees have to wait.

There's even a wiki page about this problem: https://en.bitcoin.it/wiki/Fee_bumping#I_sent_the_stuck_transaction However, only knowledged/experienced users are able to follow such guidance. What's more, it's obviously an annoying hassle, which also makes the user prone to human errors, like overpaying with different (non-conflicting) UTXOs.

The rule that "only 0-conf transactions with BIP125-enabled are allowed to be replaced (double-spent)" is not really enforcable, so that it's actually a situation that everyone (especially the wallet GUI) just pretends to respect such a rule, which only blocks legitimate fee-bumping attempts by non-tech-savvy users.

Such "rule" also has a expiration time actually - the mempool expiration time of full nodes in the Bitcoin P2P network, like Bitcoin Core nodes.

Edit: The mempool expiration time doesn't mean the transaction itself has a expiration time - that's probably the most tricky part of this problem.

Describe the solution you'd like

Allow Replace-By-Fee for transactions being stuck longer than mempool expiration time.

There are also corner cases (like lack of change output), which requires adding new inputs to the transaction - otherwise the user would probably have to do riskier things like replacing (removing, essentially) existing inputs or potentially immoral things like underpaying or even retracting the payment entirely, by double-spending.

(To my understanding, there could be potential problems or risks like overpaying, if the wallet is allowed to remove existing input)

Describe alternatives you've considered

Child-Pay-For-Parents (CPFP), which also requires change output.

Additional context

https://en.bitcoin.it/wiki/Transaction_expiration

Mining pools also have been providing transaction prioritising services (the so-called "transaction accelerator") years ago, like https://www.pushtx.com/ and https://pushtx.btc.com/

@andronoob alternative that you mentioned here : CPFP is the only option to resolve such issues.

Also wallets can help with better options and UX. Few days back I was thinking even exchanges can start doing CPFP for unconfirmed transactions when users deposit BTC and deduct amount from account balance if available.

Example: I withdraw 0.1 BTC from Bitmex to my Bitfinex deposit address. Bitmex does the withdrawal once in a day and it's not confirmed for hours. Maybe Bitifinex can do CPFP for this transaction.

See #10823 which proposed something similar.

Since #25353, you should be able to replace this tx using -mempoolfullrbf (note that local acceptance does not necessarily translate to propagation on the network, but if the original tx has expired from other mempools, I expect the replacement should work).

Hopefully this helps, closing this now. Feel free to reopen if these options don't work.