Relax MIN_STANDARD_TX_NONWITNESS_SIZE to 65 non-witness bytes

Since the original fix was set to be a "reasonable" transaction
to reduce allocations and the true motivation later revealed,
it makes sense to relax this check to something more principled.

There are more exotic transaction patterns that could take advantage
of a relaxed requirement, such as 1 input, 1 output OP_RETURN to burn
a utxo to fees for CPFP purposes when change isn't practical.

Two changes could be accomplished:

1) Anything not 64 bytes could be allowed

2) Anything above 64 bytes could be allowed

In the Great Consensus Cleanup, suggestion (2) was the route taken.
It would not allow an "empty" OP_RETURN
but would reduce the required padding from 22 bytes to 5.

The functional test is also modified to test the actual case
we care about: 64 bytes

3 files changed, 54 insertions, 13 deletions

diff --git a/test/functional/data/invalid_txs.py b/test/functional/data/invalid_txs.py
index 3747b2a98d..7892410826 100644
--- a/test/functional/data/invalid_txs.py
+++ b/test/functional/data/invalid_txs.py
@@ -46,18 +46,19 @@ from test_framework.script import (
     OP_MOD,
     OP_MUL,
     OP_OR,
+    OP_RETURN,
     OP_RIGHT,
     OP_RSHIFT,
     OP_SUBSTR,
-    OP_TRUE,
     OP_XOR,
 )
 from test_framework.script_util import (
+    MIN_PADDING,
+    MIN_STANDARD_TX_NONWITNESS_SIZE,
     script_to_p2sh_script,
 )
 basic_p2sh = script_to_p2sh_script(CScript([OP_0]))
 
-
 class BadTxTemplate:
     """Allows simple construction of a certain kind of invalid tx. Base class to be subclassed."""
     __metaclass__ = abc.ABCMeta
@@ -122,7 +123,9 @@ class SizeTooSmall(BadTxTemplate):
     def get_tx(self):
         tx = CTransaction()
         tx.vin.append(self.valid_txin)
-        tx.vout.append(CTxOut(0, CScript([OP_TRUE])))
+        tx.vout.append(CTxOut(0, CScript([OP_RETURN] + ([OP_0] * (MIN_PADDING - 2)))))
+        assert len(tx.serialize_without_witness()) == 64
+        assert MIN_STANDARD_TX_NONWITNESS_SIZE - 1 == 64
         tx.calc_sha256()
         return tx
 
diff --git a/test/functional/mempool_accept.py b/test/functional/mempool_accept.py
index 02ec18140c..7ec3fbe7cf 100755
--- a/test/functional/mempool_accept.py
+++ b/test/functional/mempool_accept.py
@@ -14,7 +14,9 @@ from test_framework.messages import (
     MAX_BIP125_RBF_SEQUENCE,
     COIN,
     COutPoint,
+    CTransaction,
     CTxIn,
+    CTxInWitness,
     CTxOut,
     MAX_BLOCK_WEIGHT,
     MAX_MONEY,
@@ -26,13 +28,19 @@ from test_framework.script import (
     OP_0,
     OP_HASH160,
     OP_RETURN,
+    OP_TRUE,
 )
 from test_framework.script_util import (
+    DUMMY_MIN_OP_RETURN_SCRIPT,
     keys_to_multisig_script,
+    MIN_PADDING,
+    MIN_STANDARD_TX_NONWITNESS_SIZE,
     script_to_p2sh_script,
+    script_to_p2wsh_script,
 )
 from test_framework.util import (
     assert_equal,
+    assert_greater_than,
     assert_raises_rpc_error,
 )
 from test_framework.wallet import MiniWallet
@@ -333,6 +341,35 @@ class MempoolAcceptanceTest(BitcoinTestFramework):
             maxfeerate=0,
         )
 
+        # Prep for tiny-tx tests with wsh(OP_TRUE) output
+        seed_tx = self.wallet.send_to(from_node=node, scriptPubKey=script_to_p2wsh_script(CScript([OP_TRUE])), amount=COIN)
+        self.generate(node, 1)
+
+        self.log.info('A tiny transaction(in non-witness bytes) that is disallowed')
+        tx = CTransaction()
+        tx.vin.append(CTxIn(COutPoint(int(seed_tx[0], 16), seed_tx[1]), b"", SEQUENCE_FINAL))
+        tx.wit.vtxinwit = [CTxInWitness()]
+        tx.wit.vtxinwit[0].scriptWitness.stack = [CScript([OP_TRUE])]
+        tx.vout.append(CTxOut(0, CScript([OP_RETURN] + ([OP_0] * (MIN_PADDING - 2)))))
+        # Note it's only non-witness size that matters!
+        assert_equal(len(tx.serialize_without_witness()), 64)
+        assert_equal(MIN_STANDARD_TX_NONWITNESS_SIZE - 1, 64)
+        assert_greater_than(len(tx.serialize()), 64)
+
+        self.check_mempool_result(
+            result_expected=[{'txid': tx.rehash(), 'allowed': False, 'reject-reason': 'tx-size-small'}],
+            rawtxs=[tx.serialize().hex()],
+            maxfeerate=0,
+        )
+
+        self.log.info('Minimally-small transaction(in non-witness bytes) that is allowed')
+        tx.vout[0] = CTxOut(COIN - 1000, DUMMY_MIN_OP_RETURN_SCRIPT)
+        assert_equal(len(tx.serialize_without_witness()), MIN_STANDARD_TX_NONWITNESS_SIZE)
+        self.check_mempool_result(
+            result_expected=[{'txid': tx.rehash(), 'allowed': True, 'vsize': tx.get_vsize(), 'fees': { 'base': Decimal('0.00001000')}}],
+            rawtxs=[tx.serialize().hex()],
+            maxfeerate=0,
+        )
 
 if __name__ == '__main__':
     MempoolAcceptanceTest().main()
diff --git a/test/functional/test_framework/script_util.py b/test/functional/test_framework/script_util.py
index b114002145..39dcb418b7 100755
--- a/test/functional/test_framework/script_util.py
+++ b/test/functional/test_framework/script_util.py
@@ -13,12 +13,13 @@ from test_framework.script import (
     OP_EQUAL,
     OP_EQUALVERIFY,
     OP_HASH160,
+    OP_RETURN,
     hash160,
     sha256,
 )
 
 # 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
+# non-witness size of at least 65 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:
@@ -28,15 +29,15 @@ from test_framework.script import (
 # 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])
-DUMMY_2_P2WPKH_SCRIPT = CScript([b'b' * 21])
-
+# least 5 bytes.
+MIN_STANDARD_TX_NONWITNESS_SIZE = 65
+MIN_PADDING = MIN_STANDARD_TX_NONWITNESS_SIZE - 10 - 41 - 9
+assert MIN_PADDING == 5
+
+# This script cannot be spent, allowing dust output values under
+# standardness checks
+DUMMY_MIN_OP_RETURN_SCRIPT = CScript([OP_RETURN] + ([OP_0] * (MIN_PADDING - 1)))
+assert len(DUMMY_MIN_OP_RETURN_SCRIPT) == MIN_PADDING
 
 def key_to_p2pk_script(key):
     key = check_key(key)