TxBuilder::add_psbt

[LLFourn]

The idea would be that instead of calling add_recipients and add_utxo and eventually add_foreign_utxo, you could just add a PSBT and the wallet would then figure out which of those it owns and which it doesn't and then fill in all the details it can.

Does this make sense?

[afilini]

I guess you are referring to the Updater role as described by BIP174, basically an entity that can add the previous utxos, witness script, derivation paths, etc to a PSBT.

We currently have a method that does that (Wallet::complete_transaction), but it's not public yet. I guess we could update the interface a little bit (for instance, it takes the full TxBuilder just to read a couple of parameters that we could just pass as arguments individually) and then we could make it public. Or we could keep that one private and add a new public method that wraps it and does some pre-processing before calling complete_transaction (mainly the creation of prev_script_pubkeys), which is probably what I would do.

I don't know if the best way is to go through TxBuilder, I would just make a method takes a PSBT and returns a PSBT since you wouldn't need pretty much any of the other options exposed by TxBuilder.

[LLFourn]

I don't know if the best way is to go through TxBuilder, I would just make a method takes a PSBT and returns a PSBT since you wouldn't need pretty much any of the other options exposed by TxBuilder.

I think concretely what I am aiming at is being able to do the naive coinjoin mentioned in BIP174. i.e. I generate a PSBT with my inputs and some outputs (where the coinjoin output is fixed to 0.1 BTC). I pass it onto you can then you add your inputs and your coinjoin output and so on. Once everyone has finished adding stuff we pass it back (with randomly re-ordered coinjoin outputs) and everyone adds their signatures.

I don't know if the best way is to go through TxBuilder, I would just make a method takes a PSBT and returns a PSBT since you wouldn't need pretty much any of the other options exposed by TxBuilder.

For this it looks like I do need TxBuilder options like add_recipient, add_utxo, coin selection filters and so on. In short you want create_tx where you pass in a PSBT as the starting point. I don't really see a straightforward way of turning Wallet::complete_transaction into what I want.

[afilini]

Right, I got it now. Appending inputs/outputs to existing PSBTs, not just metadata.

I'm actually thinking this could be part of a larger refactoring that also improves the current bump_fee: for instance, currently we don't support adding extra recipients when bumping fees, or even merging multiple unconfirmed transactions, which should be allowed by BIP125.

I guess in a way the logic to do that would be very similar, with the only difference that "bump_fee" takes the transaction from the wallet's database, while you are proposing to take a PSBT from the outside. I think we could have a common method that contains all the logic, and bump_fee could call that method after having done its few checks on the new fee rate and stuff like that.

[murchandamus]

Maybe something called amend_transaction or extend_transaction that would take parameters for recipient_outputs, feerate and optionally required_utxos and then essentially treats the existing transaction stub as an additional source of output and input definitions.