test: replace ValidWitnessMalleatedTx class with function

Simplify the witness malleation test helper by converting the
ValidWitnessMalleatedTx class to a standalone function
build_malleated_tx_package() and updating call sites.

Co-authored-by: rkrux <rkrux.connect@gmail.com>

test/functional/test_framework/script_util.py

@@ -36,7 +36,10 @@ from test_framework.script import (
     hash160,
 )
 
-from test_framework.util import assert_equal
+from test_framework.util import (
+    assert_greater_than_or_equal,
+    assert_equal,
+)
 
 # Maximum number of potentially executed legacy signature operations in validating a transaction.
 MAX_STD_LEGACY_SIGOPS = 2_500
@@ -164,43 +167,44 @@ def check_script(script):
     assert False
 
 
-class ValidWitnessMalleatedTx:
+def build_malleated_tx_package(*, parent: CTransaction, rebalance_parent_output_amount, child_amount):
     """
-    Creates a valid witness malleation transaction test case:
-    - Parent transaction with a script supporting 2 branches
-    - 2 child transactions with the same txid but different wtxids
+    Returns a transaction package with valid witness:
+    - Parent transaction whose last output contains a script that has two spending conditions
+    - Two malleated child transactions with same txid but different wtxids because of different witnesses
+
+    Args:
+        parent: Transaction with modifiable outputs. Either unsigned (sign after
+        calling this function) or anyone-can-spend (e.g., MiniWallet's OP_TRUE).
     """
-    def __init__(self):
-        hashlock = hash160(b'Preimage')
-        self.witness_script = CScript([OP_IF, OP_HASH160, hashlock, OP_EQUAL, OP_ELSE, OP_TRUE, OP_ENDIF])
+    hashlock = hash160(b'Preimage')
+    witness_script = CScript([OP_IF, OP_HASH160, hashlock, OP_EQUAL, OP_ELSE, OP_TRUE, OP_ENDIF])
+    witness_program = sha256(witness_script)
+    script_pubkey = CScript([OP_0, witness_program])
 
-    def build_parent_tx(self, funding_txid, amount):
-        # Create an unsigned parent transaction paying to the witness script.
-        witness_program = sha256(self.witness_script)
-        script_pubkey = CScript([OP_0, witness_program])
+    # Append to the transaction the vout containing the script supporting 2 spending conditions
+    assert_greater_than_or_equal(len(parent.vout), 1)
+    last_output = parent.vout[len(parent.vout) - 1]
+    assert_greater_than_or_equal(last_output.nValue, rebalance_parent_output_amount)
+    last_output.nValue -= rebalance_parent_output_amount
+    parent.vout.append(CTxOut(rebalance_parent_output_amount, script_pubkey))
 
-        parent = CTransaction()
-        parent.vin.append(CTxIn(COutPoint(int(funding_txid, 16), 0), b""))
-        parent.vout.append(CTxOut(int(amount), script_pubkey))
-        return parent
 
-    def build_malleated_children(self, signed_parent_txid, amount):
-        # Create 2 valid children that differ only in witness data.
-        # 1. Create a new transaction with witness solving first branch
-        child_witness_script = CScript([OP_TRUE])
-        child_witness_program = sha256(child_witness_script)
-        child_script_pubkey = CScript([OP_0, child_witness_program])
+    # Create 2 valid children that differ only in witness data.
+    # 1. Create a new transaction with witness solving first branch
+    child_witness_script = CScript([OP_TRUE])
+    child_witness_program = sha256(child_witness_script)
+    child_script_pubkey = CScript([OP_0, child_witness_program])
+    child_one = CTransaction()
 
-        child_one = CTransaction()
-        child_one.vin.append(CTxIn(COutPoint(int(signed_parent_txid, 16), 0), b""))
-        child_one.vout.append(CTxOut(int(amount), child_script_pubkey))
-        child_one.wit.vtxinwit.append(CTxInWitness())
-        child_one.wit.vtxinwit[0].scriptWitness.stack = [b'Preimage', b'\x01', self.witness_script]
-
-        # 2. Create another identical transaction with witness solving second branch
-        child_two = deepcopy(child_one)
-        child_two.wit.vtxinwit[0].scriptWitness.stack = [b'', self.witness_script]
-        return child_one, child_two
+    child_one.vin.append(CTxIn(COutPoint(int(parent.txid_hex, 16), len(parent.vout) - 1), b""))
+    child_one.vout.append(CTxOut(child_amount, child_script_pubkey))
+    child_one.wit.vtxinwit.append(CTxInWitness())
+    child_one.wit.vtxinwit[0].scriptWitness.stack = [b'Preimage', b'\x01', witness_script]
+    # 2. Create another identical transaction with witness solving second branch
+    child_two = deepcopy(child_one)
+    child_two.wit.vtxinwit[0].scriptWitness.stack = [b'', witness_script]
+    return parent, child_one, child_two
 
 
 class TestFrameworkScriptUtil(unittest.TestCase):