test: add test for modififed walletprocesspsbt calls

This test checks that we can successfully process PSBTs and opt out of
finalization.

Previously trying to call `walletprocesspsbt` would attempt to
auto-finalize (as a convenience), and would not permit opt-out of
finalization, instead aborting via `CHECK_NONFATAL`.

Author: willcl-ark

test/functional/rpc_psbt.py

@@ -68,6 +68,28 @@ def set_test_params(self):
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()
 
+    def test_psbt_incomplete_after_invalid_modification(self):
+        self.log.info("Check that PSBT is correctly marked as incomplete after invalid modification")
+        node = self.nodes[2]
+        wallet = node.get_wallet_rpc(self.default_wallet_name)
+        address = wallet.getnewaddress()
+        wallet.sendtoaddress(address=address, amount=1.0)
+        self.generate(node, nblocks=1, sync_fun=lambda: self.sync_all(self.nodes[:2]))
+
+        utxos = wallet.listunspent(addresses=[address])
+        psbt = wallet.createpsbt([{"txid": utxos[0]["txid"], "vout": utxos[0]["vout"]}], [{wallet.getnewaddress(): 0.9999}])
+        signed_psbt = wallet.walletprocesspsbt(psbt)["psbt"]
+
+        # Modify the raw transaction by changing the output address, so the signature is no longer valid
+        signed_psbt_obj = PSBT.from_base64(signed_psbt)
+        substitute_addr = wallet.getnewaddress()
+        raw = wallet.createrawtransaction([{"txid": utxos[0]["txid"], "vout": utxos[0]["vout"]}], [{substitute_addr: 0.9999}])
+        signed_psbt_obj.g.map[PSBT_GLOBAL_UNSIGNED_TX] = bytes.fromhex(raw)
+
+        # Check that the walletprocesspsbt call succeeds but also recognizes that the transaction is not complete
+        signed_psbt_incomplete = wallet.walletprocesspsbt(signed_psbt_obj.to_base64(), finalize=False)
+        assert signed_psbt_incomplete["complete"] is False
+
     def test_utxo_conversion(self):
         self.log.info("Check that non-witness UTXOs are removed for segwit v1+ inputs")
         mining_node = self.nodes[2]
@@ -589,6 +611,7 @@ def run_test(self):
 
         if self.options.descriptors:
             self.test_utxo_conversion()
+        self.test_psbt_incomplete_after_invalid_modification()
 
         self.test_input_confs_control()