wallet: Precompute Txdata after setting PSBT inputs' UTXOs

If we are given a PSBT that is missing one or more input UTXOs, our
PrecomputedTransactionData will be incorrect and missing information
that it should otherwise have, and therefore we may not produce a
signature when we should. To avoid this problem, we can do the
precomputation after we have set the UTXOs the wallet is able to set for
the PSBT.

Also adds a test for this behavior.

src/wallet/wallet.cpp

@@ -1969,7 +1969,6 @@ TransactionError CWallet::FillPSBT(PartiallySignedTransaction& psbtx, bool& comp
     if (n_signed) {
         *n_signed = 0;
     }
-    const PrecomputedTransactionData txdata = PrecomputePSBTData(psbtx);
     LOCK(cs_wallet);
     // Get all of the previous transactions
     for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
@@ -1993,6 +1992,8 @@ TransactionError CWallet::FillPSBT(PartiallySignedTransaction& psbtx, bool& comp
         }
     }
 
+    const PrecomputedTransactionData txdata = PrecomputePSBTData(psbtx);
+
     // Fill in information from ScriptPubKeyMans
     for (ScriptPubKeyMan* spk_man : GetAllScriptPubKeyMans()) {
         int n_signed_this_spkm = 0;

test/functional/rpc_psbt.py

@@ -21,6 +21,7 @@
     assert_greater_than,
     assert_raises_rpc_error,
     find_output,
+    find_vout_for_address,
 )
 from test_framework.wallet_util import bytes_to_wif
 
@@ -764,5 +765,15 @@ def test_psbt_input_keys(psbt_input, keys):
             psbt = self.nodes[0].walletprocesspsbt(psbt)["psbt"]
             self.nodes[0].sendrawtransaction(self.nodes[0].finalizepsbt(psbt)["hex"])
 
+            self.log.info("Test that walletprocesspsbt both updates and signs a non-updated psbt containing Taproot inputs")
+            addr = self.nodes[0].getnewaddress("", "bech32m")
+            txid = self.nodes[0].sendtoaddress(addr, 1)
+            vout = find_vout_for_address(self.nodes[0], txid, addr)
+            psbt = self.nodes[0].createpsbt([{"txid": txid, "vout": vout}], [{self.nodes[0].getnewaddress(): 0.9999}])
+            signed = self.nodes[0].walletprocesspsbt(psbt)
+            rawtx = self.nodes[0].finalizepsbt(signed["psbt"])["hex"]
+            self.nodes[0].sendrawtransaction(rawtx)
+            self.generate(self.nodes[0], 1)
+
 if __name__ == '__main__':
     PSBTTest().main()