Add `sign_psbt_input`

[benthecarman]

Describe the enhancement
Currently there is a sign_psbt function that works great, however, it does not give us fine grained control over which inputs are signed, it just signs everything it can. This can be unsafe for multi-party protocols where they could give us one of our own inputs and we sign it for them. To get around this currently we need to sign the whole psbt and then just take the info from the input we wanted, which is inefficient.

[notmandatory]

Added to alpha.4 for now so we make sure to discuss it before we wrap up our beta API. We may need to target this to a follow-up 1.1 release since it sounds like a non-breaking enhancement.

[benthecarman]

Thanks! The alternative isn't too bad just need to do this for now:

let mut to_sign = psbt.clone();
wallet.sign(&mut to_sign, sig_options)?;
psbt.inputs[input_index] = to_sign.inputs[input_index].clone();

[LLFourn]

FWIW I would love to remove "signers" from the wallet and just have a bunch of ways to sign PSBT inputs with fine grained control about what is signed. It needs some design work but I'd put it in a new crate like bdk_psbt. A good time to do this would be while integrating the miniscript planning module.

[notmandatory]

Since this can be done as a non-breaking change I think we should push to post 1.0 milestone.

[va-an]

Hi! I'd like to work on this issue.

My plan is to add a sign_input method to Wallet with the following signature:

pub fn sign_input(
    &self,
    psbt: &mut Psbt,
    input_index: usize,
    sign_options: SignOptions,
) -> Result<bool, SignerError>

The internal InputSigner::sign_input infrastructure is already in place, so the implementation would reuse
the existing validation and signing logic from Wallet::sign() but scoped to a single input.

@notmandatory If this approach looks good, could you assign this issue to me?

[notmandatory]

Since this issue was created the direction has changed to removing signing from bdk_wallet see PRs linked to #70. Bottom line is signing is being delegated to Psbt::sign. But before this can be done there is work that @oleonardolima is working on bdk_wallet#235.

[oleonardolima]

Oh, I didn't remember this issue. But, this probably will be achieved with the new Psbt::sign