SignPsbt fails to add partial signature when P2TR output is being spent in a funded PSBT

[alexbosworth]

Background

Given a funded PSBT using FundPsbt, the resulting funded PSBT should be able to be signed with SignPsbt but there is no signature being added in the result of SignPsbt when an input is a P2TR input like there is when an input is a P2WPKH.

Your environment

• lnd v0.15.0-beta.rc3

Steps to reproduce

• Create a funded PSBT using FundPsbt with 1 output and 1 input, where the input is a P2TR
• Pass this funded PSBT to SignPsbt in walletrpc

Also do the same thing, but where the input is a P2WPKH

Expected behavior

Both the P2TR signing and the P2WPKH signing should result in a partially signed PSBT

Actual behavior

For the P2TR funded PSBT the output of SignPsbt will be equal to the output of FundPbst, meaning it is not partially signed

For the P2WPKH funded PSBT it will include a partial signature as expected

[Roasbeef]

I think I spotted the issue, when we sign we skip if the bip32 derivation info isn't present: https://github.com/lightningnetwork/lnd/blob/master/lnwallet/btcwallet/psbt.go#L132-L136

However, when we fund w/ a P2TR input, we only place the new taproot specific info: https://github.com/btcsuite/btcwallet/blob/af5562928b707e0b56c1e44052ece3205bb66552/wallet/psbt.go#L273-L288

The itests we have also always put both and old and new information: https://github.com/lightningnetwork/lnd/blob/7106ea59db9ade78d65bb46605f94637900c0e3f/lntest/itest/lnd_psbt_test.go#L769-L776

[Roasbeef]

If this is correct, then we need to fix this on the btcwallet side, then update the lnd dep.