test: fix "tx-size-small" errors after default address change

Addresses #17043, affects RBF and BIP68 functional tests.

The "tx-size-small" policy rule rejects transactions with a non-witness size of
smaller than 82 bytes (see src/validation.cpp:MemPoolAccept::PreChecks(...)),
which corresponds to a transaction with 1 segwit input and 1 P2WPKH output.

Through the default address change, the created test transactions have segwit
inputs now and sending to short scriptPubKeys might violate this rule. By
bumping the dummy scriptPubKey size to 22 bytes (= the size of a P2WPKH
scriptPubKey), on all occurences the problem is solved.

The dummy scriptPubKey has the format:
    21 <21-byte-long string of 'a' or 1s>

former commit messages, now squashed:
test: rbf, bip68: use constant DUMMY_P2WPKH_SCRIPT for bumped scriptPubKey
test: rbf, bip68: use constant DUMMY_P2WPKH_SCRIPT for dummy scriptPubKeys (b'a' * 35)
test: rbf, bip68: comment DUMMY_P2WPKH_SCRIPT constant, put into common (new) module

3 files changed, 59 insertions, 36 deletions

diff --git a/test/functional/feature_bip68_sequence.py b/test/functional/feature_bip68_sequence.py
index 677362756c..682a00ff4d 100755
--- a/test/functional/feature_bip68_sequence.py
+++ b/test/functional/feature_bip68_sequence.py
@@ -8,7 +8,6 @@ import time
 
 from test_framework.blocktools import create_block, create_coinbase, add_witness_commitment
 from test_framework.messages import COIN, COutPoint, CTransaction, CTxIn, CTxOut, FromHex, ToHex
-from test_framework.script import CScript
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
@@ -17,6 +16,7 @@ from test_framework.util import (
     satoshi_round,
     softfork_active,
 )
+from test_framework.script_util import DUMMY_P2WPKH_SCRIPT
 
 SEQUENCE_LOCKTIME_DISABLE_FLAG = (1<<31)
 SEQUENCE_LOCKTIME_TYPE_FLAG = (1<<22) # this means use time (0 means height)
@@ -29,10 +29,9 @@ NOT_FINAL_ERROR = "non-BIP68-final (code 64)"
 class BIP68Test(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 2
-        # TODO remove output type argument and fix resulting "tx-size-small" errors
         self.extra_args = [
-            ["-acceptnonstdtxn=1", "-addresstype=p2sh-segwit"],
-            ["-acceptnonstdtxn=0", "-addresstype=p2sh-segwit"],
+            ["-acceptnonstdtxn=1"],
+            ["-acceptnonstdtxn=0"],
         ]
 
     def skip_test_if_missing_module(self):
@@ -85,7 +84,7 @@ class BIP68Test(BitcoinTestFramework):
         # input to mature.
         sequence_value = SEQUENCE_LOCKTIME_DISABLE_FLAG | 1
         tx1.vin = [CTxIn(COutPoint(int(utxo["txid"], 16), utxo["vout"]), nSequence=sequence_value)]
-        tx1.vout = [CTxOut(value, CScript([b'a']))]
+        tx1.vout = [CTxOut(value, DUMMY_P2WPKH_SCRIPT)]
 
         tx1_signed = self.nodes[0].signrawtransactionwithwallet(ToHex(tx1))["hex"]
         tx1_id = self.nodes[0].sendrawtransaction(tx1_signed)
@@ -97,7 +96,7 @@ class BIP68Test(BitcoinTestFramework):
         tx2.nVersion = 2
         sequence_value = sequence_value & 0x7fffffff
         tx2.vin = [CTxIn(COutPoint(tx1_id, 0), nSequence=sequence_value)]
-        tx2.vout = [CTxOut(int(value - self.relayfee * COIN), CScript([b'a' * 35]))]
+        tx2.vout = [CTxOut(int(value - self.relayfee * COIN), DUMMY_P2WPKH_SCRIPT)]
         tx2.rehash()
 
         assert_raises_rpc_error(-26, NOT_FINAL_ERROR, self.nodes[0].sendrawtransaction, ToHex(tx2))
@@ -192,7 +191,7 @@ class BIP68Test(BitcoinTestFramework):
                 value += utxos[j]["amount"]*COIN
             # Overestimate the size of the tx - signatures should be less than 120 bytes, and leave 50 for the output
             tx_size = len(ToHex(tx))//2 + 120*num_inputs + 50
-            tx.vout.append(CTxOut(int(value-self.relayfee*tx_size*COIN/1000), CScript([b'a'])))
+            tx.vout.append(CTxOut(int(value-self.relayfee*tx_size*COIN/1000), DUMMY_P2WPKH_SCRIPT))
             rawtx = self.nodes[0].signrawtransactionwithwallet(ToHex(tx))["hex"]
 
             if (using_sequence_locks and not should_pass):
@@ -221,7 +220,7 @@ class BIP68Test(BitcoinTestFramework):
         tx2 = CTransaction()
         tx2.nVersion = 2
         tx2.vin = [CTxIn(COutPoint(tx1.sha256, 0), nSequence=0)]
-        tx2.vout = [CTxOut(int(tx1.vout[0].nValue - self.relayfee*COIN), CScript([b'a']))]
+        tx2.vout = [CTxOut(int(tx1.vout[0].nValue - self.relayfee*COIN), DUMMY_P2WPKH_SCRIPT)]
         tx2_raw = self.nodes[0].signrawtransactionwithwallet(ToHex(tx2))["hex"]
         tx2 = FromHex(tx2, tx2_raw)
         tx2.rehash()
@@ -239,7 +238,7 @@ class BIP68Test(BitcoinTestFramework):
             tx = CTransaction()
             tx.nVersion = 2
             tx.vin = [CTxIn(COutPoint(orig_tx.sha256, 0), nSequence=sequence_value)]
-            tx.vout = [CTxOut(int(orig_tx.vout[0].nValue - relayfee * COIN), CScript([b'a' * 35]))]
+            tx.vout = [CTxOut(int(orig_tx.vout[0].nValue - relayfee * COIN), DUMMY_P2WPKH_SCRIPT)]
             tx.rehash()
 
             if (orig_tx.hash in node.getrawmempool()):
@@ -352,7 +351,7 @@ class BIP68Test(BitcoinTestFramework):
         tx2 = CTransaction()
         tx2.nVersion = 1
         tx2.vin = [CTxIn(COutPoint(tx1.sha256, 0), nSequence=0)]
-        tx2.vout = [CTxOut(int(tx1.vout[0].nValue - self.relayfee*COIN), CScript([b'a']))]
+        tx2.vout = [CTxOut(int(tx1.vout[0].nValue - self.relayfee*COIN), DUMMY_P2WPKH_SCRIPT)]
 
         # sign tx2
         tx2_raw = self.nodes[0].signrawtransactionwithwallet(ToHex(tx2))["hex"]
@@ -367,7 +366,7 @@ class BIP68Test(BitcoinTestFramework):
         tx3 = CTransaction()
         tx3.nVersion = 2
         tx3.vin = [CTxIn(COutPoint(tx2.sha256, 0), nSequence=sequence_value)]
-        tx3.vout = [CTxOut(int(tx2.vout[0].nValue - self.relayfee * COIN), CScript([b'a' * 35]))]
+        tx3.vout = [CTxOut(int(tx2.vout[0].nValue - self.relayfee * COIN), DUMMY_P2WPKH_SCRIPT)]
         tx3.rehash()
 
         assert_raises_rpc_error(-26, NOT_FINAL_ERROR, self.nodes[0].sendrawtransaction, ToHex(tx3))
diff --git a/test/functional/feature_rbf.py b/test/functional/feature_rbf.py
index a1d4ce4c73..e7afbd0272 100755
--- a/test/functional/feature_rbf.py
+++ b/test/functional/feature_rbf.py
@@ -10,13 +10,14 @@ from test_framework.messages import COIN, COutPoint, CTransaction, CTxIn, CTxOut
 from test_framework.script import CScript, OP_DROP
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal, assert_raises_rpc_error, satoshi_round
+from test_framework.script_util import DUMMY_P2WPKH_SCRIPT
 
 MAX_REPLACEMENT_LIMIT = 100
 
 def txToHex(tx):
     return tx.serialize().hex()
 
-def make_utxo(node, amount, confirmed=True, scriptPubKey=CScript([1])):
+def make_utxo(node, amount, confirmed=True, scriptPubKey=DUMMY_P2WPKH_SCRIPT):
     """Create a txout with a given amount and scriptPubKey
 
     Mines coins as needed.
@@ -65,7 +66,6 @@ def make_utxo(node, amount, confirmed=True, scriptPubKey=CScript([1])):
 class ReplaceByFeeTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 1
-        # TODO remove output type argument and fix resulting "tx-size-small" errors
         self.extra_args = [
             [
                 "-acceptnonstdtxn=1",
@@ -74,7 +74,6 @@ class ReplaceByFeeTest(BitcoinTestFramework):
                 "-limitancestorsize=101",
                 "-limitdescendantcount=200",
                 "-limitdescendantsize=101",
-                "-addresstype=p2sh-segwit",
             ],
         ]
 
@@ -133,7 +132,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
 
         tx1a = CTransaction()
         tx1a.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-        tx1a.vout = [CTxOut(1 * COIN, CScript([b'a' * 35]))]
+        tx1a.vout = [CTxOut(1 * COIN, DUMMY_P2WPKH_SCRIPT)]
         tx1a_hex = txToHex(tx1a)
         tx1a_txid = self.nodes[0].sendrawtransaction(tx1a_hex, 0)
 
@@ -142,7 +141,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # Should fail because we haven't changed the fee
         tx1b = CTransaction()
         tx1b.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-        tx1b.vout = [CTxOut(1 * COIN, CScript([b'b' * 35]))]
+        tx1b.vout = [CTxOut(1 * COIN, DUMMY_P2WPKH_SCRIPT + b'a')]
         tx1b_hex = txToHex(tx1b)
 
         # This will raise an exception due to insufficient fee
@@ -151,7 +150,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # Extra 0.1 BTC fee
         tx1b = CTransaction()
         tx1b.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-        tx1b.vout = [CTxOut(int(0.9 * COIN), CScript([b'b' * 35]))]
+        tx1b.vout = [CTxOut(int(0.9 * COIN), DUMMY_P2WPKH_SCRIPT)]
         tx1b_hex = txToHex(tx1b)
         # Works when enabled
         tx1b_txid = self.nodes[0].sendrawtransaction(tx1b_hex, 0)
@@ -186,7 +185,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # child fees - 40 BTC - so this attempt is rejected.
         dbl_tx = CTransaction()
         dbl_tx.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-        dbl_tx.vout = [CTxOut(initial_nValue - 30 * COIN, CScript([1] * 35))]
+        dbl_tx.vout = [CTxOut(initial_nValue - 30 * COIN, DUMMY_P2WPKH_SCRIPT)]
         dbl_tx_hex = txToHex(dbl_tx)
 
         # This will raise an exception due to insufficient fee
@@ -195,7 +194,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # Accepted with sufficient fee
         dbl_tx = CTransaction()
         dbl_tx.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-        dbl_tx.vout = [CTxOut(1 * COIN, CScript([1] * 35))]
+        dbl_tx.vout = [CTxOut(1 * COIN, DUMMY_P2WPKH_SCRIPT)]
         dbl_tx_hex = txToHex(dbl_tx)
         self.nodes[0].sendrawtransaction(dbl_tx_hex, 0)
 
@@ -248,7 +247,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # Attempt double-spend, will fail because too little fee paid
         dbl_tx = CTransaction()
         dbl_tx.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-        dbl_tx.vout = [CTxOut(initial_nValue - fee * n, CScript([1] * 35))]
+        dbl_tx.vout = [CTxOut(initial_nValue - fee * n, DUMMY_P2WPKH_SCRIPT)]
         dbl_tx_hex = txToHex(dbl_tx)
         # This will raise an exception due to insufficient fee
         assert_raises_rpc_error(-26, "insufficient fee", self.nodes[0].sendrawtransaction, dbl_tx_hex, 0)
@@ -256,7 +255,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # 1 BTC fee is enough
         dbl_tx = CTransaction()
         dbl_tx.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-        dbl_tx.vout = [CTxOut(initial_nValue - fee * n - 1 * COIN, CScript([1] * 35))]
+        dbl_tx.vout = [CTxOut(initial_nValue - fee * n - 1 * COIN, DUMMY_P2WPKH_SCRIPT)]
         dbl_tx_hex = txToHex(dbl_tx)
         self.nodes[0].sendrawtransaction(dbl_tx_hex, 0)
 
@@ -276,7 +275,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
 
             dbl_tx = CTransaction()
             dbl_tx.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-            dbl_tx.vout = [CTxOut(initial_nValue - 2 * fee * n, CScript([1] * 35))]
+            dbl_tx.vout = [CTxOut(initial_nValue - 2 * fee * n, DUMMY_P2WPKH_SCRIPT)]
             dbl_tx_hex = txToHex(dbl_tx)
             # This will raise an exception
             assert_raises_rpc_error(-26, "too many potential replacements", self.nodes[0].sendrawtransaction, dbl_tx_hex, 0)
@@ -291,7 +290,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
 
         tx1a = CTransaction()
         tx1a.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-        tx1a.vout = [CTxOut(1 * COIN, CScript([b'a' * 35]))]
+        tx1a.vout = [CTxOut(1 * COIN, DUMMY_P2WPKH_SCRIPT)]
         tx1a_hex = txToHex(tx1a)
         self.nodes[0].sendrawtransaction(tx1a_hex, 0)
 
@@ -312,7 +311,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
 
         tx1a = CTransaction()
         tx1a.vin = [CTxIn(utxo1, nSequence=0)]
-        tx1a.vout = [CTxOut(int(1.1 * COIN), CScript([b'a' * 35]))]
+        tx1a.vout = [CTxOut(int(1.1 * COIN), DUMMY_P2WPKH_SCRIPT)]
         tx1a_hex = txToHex(tx1a)
         tx1a_txid = self.nodes[0].sendrawtransaction(tx1a_hex, 0)
 
@@ -331,7 +330,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # Spend tx1a's output to test the indirect case.
         tx1b = CTransaction()
         tx1b.vin = [CTxIn(COutPoint(tx1a_txid, 0), nSequence=0)]
-        tx1b.vout = [CTxOut(1 * COIN, CScript([b'a' * 35]))]
+        tx1b.vout = [CTxOut(1 * COIN, DUMMY_P2WPKH_SCRIPT)]
         tx1b_hex = txToHex(tx1b)
         tx1b_txid = self.nodes[0].sendrawtransaction(tx1b_hex, 0)
         tx1b_txid = int(tx1b_txid, 16)
@@ -352,7 +351,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
 
         tx1 = CTransaction()
         tx1.vin = [CTxIn(confirmed_utxo)]
-        tx1.vout = [CTxOut(1 * COIN, CScript([b'a' * 35]))]
+        tx1.vout = [CTxOut(1 * COIN, DUMMY_P2WPKH_SCRIPT)]
         tx1_hex = txToHex(tx1)
         self.nodes[0].sendrawtransaction(tx1_hex, 0)
 
@@ -391,7 +390,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         for i in range(MAX_REPLACEMENT_LIMIT+1):
             tx_i = CTransaction()
             tx_i.vin = [CTxIn(COutPoint(txid, i), nSequence=0)]
-            tx_i.vout = [CTxOut(split_value - fee, CScript([b'a' * 35]))]
+            tx_i.vout = [CTxOut(split_value - fee, DUMMY_P2WPKH_SCRIPT)]
             tx_i_hex = txToHex(tx_i)
             self.nodes[0].sendrawtransaction(tx_i_hex, 0)
 
@@ -424,7 +423,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # Create a non-opting in transaction
         tx1a = CTransaction()
         tx1a.vin = [CTxIn(tx0_outpoint, nSequence=0xffffffff)]
-        tx1a.vout = [CTxOut(1 * COIN, CScript([b'a' * 35]))]
+        tx1a.vout = [CTxOut(1 * COIN, DUMMY_P2WPKH_SCRIPT)]
         tx1a_hex = txToHex(tx1a)
         tx1a_txid = self.nodes[0].sendrawtransaction(tx1a_hex, 0)
 
@@ -434,7 +433,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # Shouldn't be able to double-spend
         tx1b = CTransaction()
         tx1b.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-        tx1b.vout = [CTxOut(int(0.9 * COIN), CScript([b'b' * 35]))]
+        tx1b.vout = [CTxOut(int(0.9 * COIN), DUMMY_P2WPKH_SCRIPT)]
         tx1b_hex = txToHex(tx1b)
 
         # This will raise an exception
@@ -445,14 +444,14 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # Create a different non-opting in transaction
         tx2a = CTransaction()
         tx2a.vin = [CTxIn(tx1_outpoint, nSequence=0xfffffffe)]
-        tx2a.vout = [CTxOut(1 * COIN, CScript([b'a' * 35]))]
+        tx2a.vout = [CTxOut(1 * COIN, DUMMY_P2WPKH_SCRIPT)]
         tx2a_hex = txToHex(tx2a)
         tx2a_txid = self.nodes[0].sendrawtransaction(tx2a_hex, 0)
 
         # Still shouldn't be able to double-spend
         tx2b = CTransaction()
         tx2b.vin = [CTxIn(tx1_outpoint, nSequence=0)]
-        tx2b.vout = [CTxOut(int(0.9 * COIN), CScript([b'b' * 35]))]
+        tx2b.vout = [CTxOut(int(0.9 * COIN), DUMMY_P2WPKH_SCRIPT)]
         tx2b_hex = txToHex(tx2b)
 
         # This will raise an exception
@@ -478,12 +477,12 @@ class ReplaceByFeeTest(BitcoinTestFramework):
 
         tx3b = CTransaction()
         tx3b.vin = [CTxIn(COutPoint(tx1a_txid, 0), nSequence=0)]
-        tx3b.vout = [CTxOut(int(0.5 * COIN), CScript([b'e' * 35]))]
+        tx3b.vout = [CTxOut(int(0.5 * COIN), DUMMY_P2WPKH_SCRIPT)]
         tx3b_hex = txToHex(tx3b)
 
         tx3c = CTransaction()
         tx3c.vin = [CTxIn(COutPoint(tx2a_txid, 0), nSequence=0)]
-        tx3c.vout = [CTxOut(int(0.5 * COIN), CScript([b'f' * 35]))]
+        tx3c.vout = [CTxOut(int(0.5 * COIN), DUMMY_P2WPKH_SCRIPT)]
         tx3c_hex = txToHex(tx3c)
 
         self.nodes[0].sendrawtransaction(tx3b_hex, 0)
@@ -500,7 +499,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
 
         tx1a = CTransaction()
         tx1a.vin = [CTxIn(tx0_outpoint, nSequence=0)]
-        tx1a.vout = [CTxOut(1 * COIN, CScript([b'a' * 35]))]
+        tx1a.vout = [CTxOut(1 * COIN, DUMMY_P2WPKH_SCRIPT)]
         tx1a_hex = txToHex(tx1a)
         tx1a_txid = self.nodes[0].sendrawtransaction(tx1a_hex, 0)
 
@@ -526,14 +525,14 @@ class ReplaceByFeeTest(BitcoinTestFramework):
 
         tx2a = CTransaction()
         tx2a.vin = [CTxIn(tx1_outpoint, nSequence=0)]
-        tx2a.vout = [CTxOut(1 * COIN, CScript([b'a' * 35]))]
+        tx2a.vout = [CTxOut(1 * COIN, DUMMY_P2WPKH_SCRIPT)]
         tx2a_hex = txToHex(tx2a)
         self.nodes[0].sendrawtransaction(tx2a_hex, 0)
 
         # Lower fee, but we'll prioritise it
         tx2b = CTransaction()
         tx2b.vin = [CTxIn(tx1_outpoint, nSequence=0)]
-        tx2b.vout = [CTxOut(int(1.01 * COIN), CScript([b'a' * 35]))]
+        tx2b.vout = [CTxOut(int(1.01 * COIN), DUMMY_P2WPKH_SCRIPT)]
         tx2b.rehash()
         tx2b_hex = txToHex(tx2b)
 
diff --git a/test/functional/test_framework/script_util.py b/test/functional/test_framework/script_util.py
new file mode 100755
index 0000000000..5ef67226c4
--- /dev/null
+++ b/test/functional/test_framework/script_util.py
@@ -0,0 +1,25 @@
+#!/usr/bin/env python3
+# Copyright (c) 2019 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Useful Script constants and utils."""
+from test_framework.script import CScript
+
+# To prevent a "tx-size-small" policy rule error, a transaction has to have a
+# non-witness size of at least 82 bytes (MIN_STANDARD_TX_NONWITNESS_SIZE in
+# src/policy/policy.h). Considering a Tx with the smallest possible single
+# input (blank, empty scriptSig), and with an output omitting the scriptPubKey,
+# we get to a minimum size of 60 bytes:
+#
+# Tx Skeleton: 4 [Version] + 1 [InCount] + 1 [OutCount] + 4 [LockTime] = 10 bytes
+# Blank Input: 32 [PrevTxHash] + 4 [Index] + 1 [scriptSigLen] + 4 [SeqNo] = 41 bytes
+# Output:      8 [Amount] + 1 [scriptPubKeyLen] = 9 bytes
+#
+# Hence, the scriptPubKey of the single output has to have a size of at
+# least 22 bytes, which corresponds to the size of a P2WPKH scriptPubKey.
+# The following script constant consists of a single push of 21 bytes of 'a':
+#   <PUSH_21> <21-bytes of 'a'>
+# resulting in a 22-byte size. It should be used whenever (small) fake
+# scriptPubKeys are needed, to guarantee that the minimum transaction size is
+# met.
+DUMMY_P2WPKH_SCRIPT = CScript([b'a' * 21])