psbt: set global_xpubs (at least for multisig descriptors) #27583

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

The walletcreatefundedpsbt, walletprocesspsbt and send* RPC, as well as the send dialog* in the GUI should populate the PSBT_GLOBAL_XPUB field (defined in BIP 174).

At least when used in a multisig context, e.g. when spending from a multi() descriptor.

The Ledger Bitcoin app enforces this as of version 2.1.1., see https://github.com/bitcoin-core/HWI/issues/671

* = when used with an external signer

Is your feature related to a problem, if so please describe it.

Multisig funds (P2WSH and P2SH-P2WSH) are trapped on a Ledger if you update it's Bitcoin app to 2.1.1, unless you use some additional tooling to set this field.

Describe the solution you'd like

No response

Describe any alternatives you've considered

No response

Please leave any additional context

No response

See also this discussion: https://github.com/cryptoadvance/specter-desktop/issues/1886#issuecomment-1246963927 and their implementation: https://github.com/cryptoadvance/specter-desktop/blob/master/src/cryptoadvance/specter/wallet/wallet.py#L1862-L1875

What's the progress with this?

@achow101 what do you think of this?

This is more reasonable to do now that legacy wallets are no longer supported.

I got here reading the PSBT code while doing a review, and went to check whether this still applies. From what I checked today, on a wsh(multi(2,...)) wallet whose descriptors already carry origin and extended key, walletcreatefundedpsbt, walletprocesspsbt and send all return an empty global_xpubs. The field already serializes, deserializes and merges when combining PSBTs, it's just that nothing populates it. @achow101, this is assigned to you, are you still planning to pick it up? If so, I'll leave it with you. If it's stalled, I can prepare a patch.

@jeanpablojp I would not worry about assignment. Feel free to work on this.

That said, I'm not sure if my initial motivation still matters, since Ledger uses BIP388 policies now. See also https://github.com/Sjors/bitcoin/pull/91.

@Sjors I went to check. For multisig, the policies are built by HWI from the PSBT's global xpubs, so the field still feeds that path. I saw in your bitcoin-core/HWI#794 that you add a registration path, and that you leave the Trezor out of it, with parse_multisig still reading psbt.xpub. Without registration the information has to come inside the PSBT, and Specter fills it in itself after getting the PSBT from Core. And even with registration everywhere, whoever signs with just the file, without a wallet loaded, has no policy and no registration to pass.

If it makes any sense to go ahead, do we fill it always, or only when the descriptor is multisig?

@jeanpablojp so it sounds like we still need global xpubs for at least Trezor?

@Sjors Yes, and from what I saw it's not only the Trezor, in your PR digitalbitbox.py, keepkey.py and ledger.py with the legacy app raise the same exception.