test: Assert PSBT change type

Make sure the wallet's default change type is respected by default when
funding a PSBT but can be overwritten by the "change_type" option.

Author: guggero

test/functional/rpc_psbt.py

@@ -30,7 +30,7 @@ def set_test_params(self):
         self.num_nodes = 3
         self.extra_args = [
             ["-walletrbf=1"],
-            ["-walletrbf=0"],
+            ["-walletrbf=0", "-changetype=legacy"],
             []
         ]
         self.supports_cli = False
@@ -83,6 +83,14 @@ def test_utxo_conversion(self):
         connect_nodes(self.nodes[0], 1)
         connect_nodes(self.nodes[0], 2)
 
+    def assert_change_type(self, psbtx, expected_type):
+        """Assert that the given PSBT has a change output with the given type."""
+
+        # The decodepsbt RPC is stateless and independent of any settings, we can always just call it on the first node
+        decoded_psbt = self.nodes[0].decodepsbt(psbtx["psbt"])
+        changepos = psbtx["changepos"]
+        assert_equal(decoded_psbt["tx"]["vout"][changepos]["scriptPubKey"]["type"], expected_type)
+
     def run_test(self):
         # Create and fund a raw tx for sending 10 BTC
         psbtx1 = self.nodes[0].walletcreatefundedpsbt([], {self.nodes[2].getnewaddress():10})['psbt']
@@ -301,6 +309,17 @@ def run_test(self):
         # when attempting BnB coin selection
         self.nodes[0].walletcreatefundedpsbt([], [{self.nodes[2].getnewaddress():unspent["amount"]+1}], block_height+2, {"changeAddress":self.nodes[1].getnewaddress()}, False)
 
+        # Make sure the wallet's change type is respected by default
+        small_output = {self.nodes[0].getnewaddress():0.1}
+        psbtx_native = self.nodes[0].walletcreatefundedpsbt([], [small_output])
+        self.assert_change_type(psbtx_native, "witness_v0_keyhash")
+        psbtx_legacy = self.nodes[1].walletcreatefundedpsbt([], [small_output])
+        self.assert_change_type(psbtx_legacy, "pubkeyhash")
+
+        # Make sure the change type of the wallet can also be overwritten
+        psbtx_np2wkh = self.nodes[1].walletcreatefundedpsbt([], [small_output], 0, {"change_type":"p2sh-segwit"})
+        self.assert_change_type(psbtx_np2wkh, "scripthash")
+
         # Regression test for 14473 (mishandling of already-signed witness transaction):
         psbtx_info = self.nodes[0].walletcreatefundedpsbt([{"txid":unspent["txid"], "vout":unspent["vout"]}], [{self.nodes[2].getnewaddress():unspent["amount"]+1}], 0, {"add_inputs": True})
         complete_psbt = self.nodes[0].walletprocesspsbt(psbtx_info["psbt"])