Generalized fee bumping

[jb55]

Please describe the feature you'd like to see added.

It would be nice if we had a GUI implementation of this new RPC feature:

• New outputs argument for bumpfee/psbtbumpfee bitcoin/bitcoin#25344

Maybe the lowest common denominator here is:

• PSBT Operations View: Edit PSBT? #558

This would allows us to edit transactions pre-flight and in-flight?

The main usecase here is that let's say you have a transaction that isn't
going to confirm for a couple hours, days, weeks. Then all of a sudden
you remember that you wanted to add another output or remove an existing
one.

In my case, I had a single input and single output, and wanted to add another
output so that the output amount is split. This is pretty common for
transferring between different accounts with different security levels, such as
coldstore to multisig.

More specifically I created a transaction from my cold storage to
multisig, but wanted to update it to [coldstore input -> multisig +
lightning wallet output]. I controlled every wallet involved in the
transaction, and just wanted to add another output that split the funds
and bumped the fee on a single input.

Fee-bumping is just inflight-transaction editing without user-modification of
the number of input/outputs. So this would be in some sense "generalized fee
bumping"?

The main thing that would be needed is restoring the send transaction view from
an arbitrary transaction. Then bumping the fee could ask if you would like to
modify the transaction in any way, dropping you back into that send transaction
view to re-create the transaction. It would also have to keep track of the
original fee so that it can warn that editing the transaction may not confirm.

Design-wise, one might be tempted to just call this "edit transaction"
instead of bump fee, but this may be too confusing. If the user doesn't
bump the fee during transaction editing it may not have any higher
priority over the previous one, leading to confusion when the original tx
confirms, so it would make sense to make this a fee bumping feature.

Thoughts? or is this a too complex/niche use case? I find myself wanted to do
this a lot, at least in the pre-flight case, so I would be happy with PSBT
editing in the meantime 😅

[hebasto]

cc @achow101 @Sjors @GBKS