partial bitcoin#14926: test: consensus: Check that final transactions…

… are valid

Does not include commit (fae3617) that makes a change to code introduced here:
	330b0f3 [qa] p2p segwit tests (Merge bitcoin#8149: Segregated witness rebased)
which I don't think is in Dash

aaaa8eb test: consensus: Check that final transactions are valid (MarcoFalke)
fae3617 test: Correctly deserialize without witness (MarcoFalke)

Pull request description:

  There is no check that checks that final transactions are valid, i.e. the consensus rules could be changed (accidentally) with none of the tests failing.

Tree-SHA512: 48f4c24bfcc525ddbc1bfe8c37131953b464823428c1f7a278ba6d98b98827b6b84a8eb2b33396bfb5b8cc4012b7cc1cd771637f405ea20beddae001c22aa290

test/functional/mempool_accept.py

@@ -6,6 +6,7 @@
 
 from io import BytesIO
 import math
+
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.messages import (
     BIP125_SEQUENCE_NUMBER,
@@ -69,6 +70,7 @@ def run_test(self):
         ))['hex']
         txid_in_block = node.sendrawtransaction(hexstring=raw_tx_in_block, allowhighfees=True)
         node.generate(1)
+        self.mempool_size = 0
         self.check_mempool_result(
             result_expected=[{'txid': txid_in_block, 'allowed': False, 'reject-reason': '18: txn-already-known'}],
             rawtxs=[raw_tx_in_block],
@@ -88,9 +90,25 @@ def run_test(self):
             rawtxs=[raw_tx_0],
         )
 
+        self.log.info('A final transaction not in the mempool')
+        coin = node.listunspent()[0]  # Pick a random coin(base) to spend
+        raw_tx_final = node.signrawtransactionwithwallet(node.createrawtransaction(
+            inputs=[{'txid': coin['txid'], 'vout': coin['vout'], "sequence": 0xffffffff}],  # SEQUENCE_FINAL
+            outputs=[{node.getnewaddress(): 0.025}],
+            locktime=node.getblockcount() + 2000,  # Can be anything
+        ))['hex']
+        tx.deserialize(BytesIO(hex_str_to_bytes(raw_tx_final)))
+        self.check_mempool_result(
+            result_expected=[{'txid': tx.rehash(), 'allowed': True}],
+            rawtxs=[bytes_to_hex_str(tx.serialize())],
+            allowhighfees=True,
+        )
+        node.sendrawtransaction(hexstring=raw_tx_final, allowhighfees=True)
+        self.mempool_size += 1
+
         self.log.info('A transaction in the mempool')
         node.sendrawtransaction(hexstring=raw_tx_0)
-        self.mempool_size = 1
+        self.mempool_size += 1
         self.check_mempool_result(
             result_expected=[{'txid': txid_0, 'allowed': False, 'reject-reason': '18: txn-already-in-mempool'}],
             rawtxs=[raw_tx_0],