Merge JoinMarket-Org/joinmarket-clientserver#1700: Support payjoin PSBT with multiple sender inputs

865247cf8d Support payjoin PSBT with multiple sender inputs (spacebear)

Pull request description:

  [Compatibility testing with PDK](https://github.com/payjoin/rust-payjoin/issues/51#issuecomment-2100765588) revealed that the JoinMarket payjoin receiver doesn't support signing for PSBTs that contain multiple sender inputs. This patch fixes that.

ACKs for top commit:
  AdamISZ:
    tACK 865247cf8d
  kristapsk:
    re-ACK 865247cf8d

Tree-SHA512: cf25e161be229bc440e1f0a5ad16d529e30c1470e7f6ba450ab912458f6c4db05b3ed9e463549685c5a52f37c234809f4d042522f358a0293a36932c005bbe0d

src/jmclient/payjoin.py

@@ -852,6 +852,7 @@ class PayjoinConverter(object):
         # to create the PSBT we need the spent_outs for each input,
         # in the right order:
         spent_outs = []
+        sender_indices = []
         for i, inp in enumerate(unsigned_payjoin_tx.vin):
             input_found = False
             for j, inp2 in enumerate(payment_psbt.unsigned_tx.vin):
@@ -862,7 +863,7 @@ class PayjoinConverter(object):
                     inp.nSequence = inp2.nSequence
                     spent_outs.append(payment_psbt.inputs[j].utxo)
                     input_found = True
-                    sender_index = i
+                    sender_indices.append(i)
                     break
             if input_found:
                 continue
@@ -900,10 +901,11 @@ class PayjoinConverter(object):
         assert not signresult.is_final
 
         # with signing successful, remove the utxo field from the
-        # counterparty's input (this is required by BIP78). Note we don't
+        # counterparty's inputs (this is required by BIP78). Note we don't
         # do this on PSBT creation as the psbt signing code throws ValueError
         # unless utxos are present.
-        receiver_signed_psbt.inputs[sender_index] = btc.PSBT_Input(index=sender_index)
+        for sender_index in sender_indices:
+            receiver_signed_psbt.inputs[sender_index] = btc.PSBT_Input(index=sender_index)
         log.debug("Receiver signing successful. Payjoin PSBT is now:\n{}".format(
             self.wallet_service.human_readable_psbt(receiver_signed_psbt)))
         # construct txoutset for the wallet service callback; we cannot use