test: functional test for new coin selection logic

Create a wallet with mixed OutputTypes and send a volley of payments,
ensuring that there are no mixed OutputTypes in the txs. Finally,
verify that OutputTypes are mixed only when necessary.

test/functional/test_runner.py

@@ -156,6 +156,7 @@ BASE_SCRIPTS = [
     'mempool_spend_coinbase.py',
     'wallet_avoidreuse.py --legacy-wallet',
     'wallet_avoidreuse.py --descriptors',
+    'wallet_avoid_mixing_output_types.py --descriptors',
     'mempool_reorg.py',
     'mempool_persist.py',
     'p2p_block_sync.py',

test/functional/wallet_avoid_mixing_output_types.py

@@ -0,0 +1,176 @@
+#!/usr/bin/env python3
+# Copyright (c) 2022 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://www.opensource.org/licenses/mit-license.php.
+"""Test output type mixing during coin selection
+
+A wallet may have different types of UTXOs to choose from during coin selection,
+where output type is one of the following:
+    - BECH32M
+    - BECH32
+    - P2SH-SEGWIT
+    - LEGACY
+
+This test verifies that mixing different output types is avoided unless
+absolutely necessary. Both wallets start with zero funds. Alice mines
+enough blocks to have spendable coinbase outputs. Alice sends three
+random value payments which sum to 10BTC for each output type to Bob,
+for a total of 40BTC in Bob's wallet.
+
+Bob then sends random valued payments back to Alice, some of which need
+unconfirmed change, and we verify that none of these payments contain mixed
+inputs. Finally, Bob sends the remainder of his funds, which requires mixing.
+
+The payment values are random, but chosen such that they sum up to a specified
+total. This ensures we are not relying on specific values for the UTXOs,
+but still know when to expect mixing due to the wallet being close to empty.
+
+"""
+
+import random
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.blocktools import COINBASE_MATURITY
+
+ADDRESS_TYPES = [
+    "bech32m",
+    "bech32",
+    "p2sh-segwit",
+    "legacy",
+]
+
+
+def is_bech32_address(node, addr):
+    """Check if an address contains a bech32 output."""
+    addr_info = node.getaddressinfo(addr)
+    return addr_info['desc'].startswith('wpkh(')
+
+
+def is_bech32m_address(node, addr):
+    """Check if an address contains a bech32m output."""
+    addr_info = node.getaddressinfo(addr)
+    return addr_info['desc'].startswith('tr(')
+
+
+def is_p2sh_segwit_address(node, addr):
+    """Check if an address contains a P2SH-Segwit output.
+       Note: this function does not actually determine the type
+       of P2SH output, but is sufficient for this test in that
+       we are only generating P2SH-Segwit outputs.
+    """
+    addr_info = node.getaddressinfo(addr)
+    return addr_info['desc'].startswith('sh(wpkh(')
+
+
+def is_legacy_address(node, addr):
+    """Check if an address contains a legacy output."""
+    addr_info = node.getaddressinfo(addr)
+    return addr_info['desc'].startswith('pkh(')
+
+
+def is_same_type(node, tx):
+    """Check that all inputs are of the same OutputType"""
+    vins = node.getrawtransaction(tx, True)['vin']
+    inputs = []
+    for vin in vins:
+        prev_tx, n = vin['txid'], vin['vout']
+        inputs.append(
+            node.getrawtransaction(
+                prev_tx,
+                True,
+            )['vout'][n]['scriptPubKey']['address']
+        )
+    has_legacy = False
+    has_p2sh = False
+    has_bech32 = False
+    has_bech32m = False
+
+    for addr in inputs:
+        if is_legacy_address(node, addr):
+            has_legacy = True
+        if is_p2sh_segwit_address(node, addr):
+            has_p2sh = True
+        if is_bech32_address(node, addr):
+            has_bech32 = True
+        if is_bech32m_address(node, addr):
+            has_bech32m = True
+
+    return (sum([has_legacy, has_p2sh, has_bech32, has_bech32m]) == 1)
+
+
+def generate_payment_values(n, m):
+    """Return a randomly chosen list of n positive integers summing to m.
+    Each such list is equally likely to occur."""
+
+    dividers = sorted(random.sample(range(1, m), n - 1))
+    return [a - b for a, b in zip(dividers + [m], [0] + dividers)]
+
+
+class AddressInputTypeGrouping(BitcoinTestFramework):
+    def set_test_params(self):
+        self.setup_clean_chain = True
+        self.num_nodes = 2
+        self.extra_args = [
+            [
+                "-addresstype=bech32",
+                "-whitelist=noban@127.0.0.1",
+                "-txindex",
+            ],
+            [
+                "-addresstype=p2sh-segwit",
+                "-whitelist=noban@127.0.0.1",
+                "-txindex",
+            ],
+        ]
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_wallet()
+
+    def make_payment(self, A, B, v, addr_type):
+        fee_rate = random.randint(1, 20)
+        self.log.debug(f"Making payment of {v} BTC at fee_rate {fee_rate}")
+        tx = B.sendtoaddress(
+            address=A.getnewaddress(address_type=addr_type),
+            amount=v,
+            fee_rate=fee_rate,
+        )
+        return tx
+
+    def run_test(self):
+
+        # alias self.nodes[i] to A, B for readability
+        A, B = self.nodes[0], self.nodes[1]
+        self.generate(A, COINBASE_MATURITY + 5)
+
+        self.log.info("Creating mixed UTXOs in B's wallet")
+        for v in generate_payment_values(3, 10):
+            self.log.debug(f"Making payment of {v} BTC to legacy")
+            A.sendtoaddress(B.getnewaddress(address_type="legacy"), v)
+
+        for v in generate_payment_values(3, 10):
+            self.log.debug(f"Making payment of {v} BTC to p2sh")
+            A.sendtoaddress(B.getnewaddress(address_type="p2sh-segwit"), v)
+
+        for v in generate_payment_values(3, 10):
+            self.log.debug(f"Making payment of {v} BTC to bech32")
+            A.sendtoaddress(B.getnewaddress(address_type="bech32"), v)
+
+        for v in generate_payment_values(3, 10):
+            self.log.debug(f"Making payment of {v} BTC to bech32m")
+            A.sendtoaddress(B.getnewaddress(address_type="bech32m"), v)
+
+        self.generate(A, 1)
+
+        self.log.info("Sending payments from B to A")
+        for v in generate_payment_values(5, 9):
+            tx = self.make_payment(
+                A, B, v, random.choice(ADDRESS_TYPES)
+            )
+            self.generate(A, 1)
+            assert is_same_type(B, tx)
+
+        tx = self.make_payment(A, B, 30.99, random.choice(ADDRESS_TYPES))
+        assert not is_same_type(B, tx)
+
+
+if __name__ == '__main__':
+    AddressInputTypeGrouping().main()