Merge bitcoin/bitcoin#23375: test: MiniWallet: more deterministic coi…

…n selection for coinbase UTXOs (oldest first)

d2c4904 test: MiniWallet: more deterministic coin selection for coinbase UTXOs (oldest first) (Sebastian Falbesoner)

Pull request description:

  The coin selection strategy for MiniWallet is quite straight-forward: simply pick a single UTXO with the largest value:

  https://github.com/bitcoin/bitcoin/blob/ab25ef8c7f767258d5fe44f53b35ad8bd51ed5cd/test/functional/test_framework/wallet.py#L173-L174

  If there are several candidates with the same value, however, it is not clear which one is taken.  This can be particularly problematic for coinbase outputs with fixed block subsidy, since spending could lead to a `bad-txns-premature-spend-of-coinbase` reject if an UTXO from a too-recent block is picked.  Introduce block height as second criteria (saved in `self._utxos` in the methods `generate(...)` and `rescan_utxos(...)`), in order to avoid potential issues with coinbases that are not matured yet. If there is a tie between coinbase UTXOs and non-coinbase UTXOs (the latter are added via `scan_tx(...)`), prefer the non-coinbase UTXOs, since those don't need to mature.

  The issue came up while refactoring the test rpc_blockchain.py, see bitcoin/bitcoin#23371 (comment) (PR #23371).

ACKs for top commit:
  MarcoFalke:
    review ACK d2c4904
  shaavan:
    ACK d2c4904

Tree-SHA512: 15d67b42fb8b77fd53022ea2ab8a6ed2b615567f3ce73bab16c06bfcb687c1a04dcb0360d0c2287c526b604cd3ac5eef7b14ce46fc31e23047ce1a3290027306

test/functional/test_framework/wallet.py

@@ -88,13 +88,13 @@ def rescan_utxos(self):
         res = self._test_node.scantxoutset(action="start", scanobjects=[self.get_descriptor()])
         assert_equal(True, res['success'])
         for utxo in res['unspents']:
-            self._utxos.append({'txid': utxo['txid'], 'vout': utxo['vout'], 'value': utxo['amount']})
+            self._utxos.append({'txid': utxo['txid'], 'vout': utxo['vout'], 'value': utxo['amount'], 'height': utxo['height']})
 
     def scan_tx(self, tx):
         """Scan the tx for self._scriptPubKey outputs and add them to self._utxos"""
         for out in tx['vout']:
             if out['scriptPubKey']['hex'] == self._scriptPubKey.hex():
-                self._utxos.append({'txid': tx['txid'], 'vout': out['n'], 'value': out['value']})
+                self._utxos.append({'txid': tx['txid'], 'vout': out['n'], 'value': out['value'], 'height': 0})
 
     def sign_tx(self, tx, fixed_length=True):
         """Sign tx that has been created by MiniWallet in P2PK mode"""
@@ -115,8 +115,9 @@ def generate(self, num_blocks, **kwargs):
         """Generate blocks with coinbase outputs to the internal address, and append the outputs to the internal list"""
         blocks = self._test_node.generatetodescriptor(num_blocks, self.get_descriptor(), **kwargs)
         for b in blocks:
-            cb_tx = self._test_node.getblock(blockhash=b, verbosity=2)['tx'][0]
-            self._utxos.append({'txid': cb_tx['txid'], 'vout': 0, 'value': cb_tx['vout'][0]['value']})
+            block_info = self._test_node.getblock(blockhash=b, verbosity=2)
+            cb_tx = block_info['tx'][0]
+            self._utxos.append({'txid': cb_tx['txid'], 'vout': 0, 'value': cb_tx['vout'][0]['value'], 'height': block_info['height']})
         return blocks
 
     def get_descriptor(self):
@@ -170,7 +171,7 @@ def send_to(self, *, from_node, scriptPubKey, amount, fee=1000):
 
     def create_self_transfer(self, *, fee_rate=Decimal("0.003"), from_node, utxo_to_spend=None, mempool_valid=True, locktime=0, sequence=0):
         """Create and return a tx with the specified fee_rate. Fee may be exact or at most one satoshi higher than needed."""
-        self._utxos = sorted(self._utxos, key=lambda k: k['value'])
+        self._utxos = sorted(self._utxos, key=lambda k: (k['value'], -k['height']))
         utxo_to_spend = utxo_to_spend or self._utxos.pop()  # Pick the largest utxo (if none provided) and hope it covers the fee
         if self._priv_key is None:
             vsize = Decimal(96)  # anyone-can-spend