Merge bitcoin/bitcoin#22408: test: add tests for bad-txns-prevout-null reject reason

1f449586a9e39bc4fb53cb5c7a31362e47aea19b test: add `bad-txns-prevout-null` test to mempool_accept.py (Sebastian Falbesoner)
aa0a5bb70d77739d43d5a9ceae78fb0c6fafd435 test: add `bad-txns-prevout-null` test case to invalid_txs.py (Sebastian Falbesoner)

Pull request description:

  This simple PR adds missing tests for the reject reason `bad-txns-prevout-null`, which is thrown in the function `CheckTransaction()`: a62fc35a15/src/consensus/tx_check.cpp (L52-L54)

  Basically this condition is met for non-coinbase transactions (the code snippet above only hits if `!tx.IsCoinBase()`) with coinbase-like outpoints, i.e. hash=0, n=0xffffffff.

  Can be tested by running the functional tests `feature_block.py`, `p2p_invalid_tx.py` and `mempool_accept.py`. Not sure if the redundancy in the tests is desired (I guess it would make sense if the mempool acceptance test also makes use of the invalid_txs templates?).

ACKs for top commit:
  rajarshimaitra:
    tACK 1f449586a9
  brunoerg:
    tACK 1f449586a9e39bc4fb53cb5c7a31362e47aea19b
  kristapsk:
    ACK 1f449586a9e39bc4fb53cb5c7a31362e47aea19b, code looks correct and all tests pass.

Tree-SHA512: 2d4f940a6ac8e0d80d2670c9e1111cbf43ae6ac62809a2ccf17cffee9a41d387ea4d889ee300eb4a407c055b13bfa5d37102a32ed59964a9b6950bd907ba7204

test/functional/data/invalid_txs.py

@@ -151,6 +151,19 @@ class DuplicateInput(BadTxTemplate):
         return tx
 
 
+class PrevoutNullInput(BadTxTemplate):
+    reject_reason = 'bad-txns-prevout-null'
+    expect_disconnect = True
+
+    def get_tx(self):
+        tx = CTransaction()
+        tx.vin.append(self.valid_txin)
+        tx.vin.append(CTxIn(COutPoint(hash=0, n=0xffffffff)))
+        tx.vout.append(CTxOut(1, basic_p2sh))
+        tx.calc_sha256()
+        return tx
+
+
 class NonexistentInput(BadTxTemplate):
     reject_reason = None  # Added as an orphan tx.
     expect_disconnect = False

test/functional/mempool_accept.py

@@ -13,6 +13,7 @@ from test_framework.messages import (
     BIP125_SEQUENCE_NUMBER,
     COIN,
     COutPoint,
+    CTxIn,
     CTxOut,
     MAX_BLOCK_BASE_SIZE,
     MAX_MONEY,
@@ -247,6 +248,14 @@ class MempoolAcceptanceTest(BitcoinTestFramework):
             rawtxs=[tx.serialize().hex()],
         )
 
+        self.log.info('A non-coinbase transaction with coinbase-like outpoint')
+        tx = tx_from_hex(raw_tx_reference)
+        tx.vin.append(CTxIn(COutPoint(hash=0, n=0xffffffff)))
+        self.check_mempool_result(
+            result_expected=[{'txid': tx.rehash(), 'allowed': False, 'reject-reason': 'bad-txns-prevout-null'}],
+            rawtxs=[tx.serialize().hex()],
+        )
+
         self.log.info('A coinbase transaction')
         # Pick the input of the first tx we signed, so it has to be a coinbase tx
         raw_tx_coinbase_spent = node.getrawtransaction(txid=node.decoderawtransaction(hexstring=raw_tx_in_block)['vin'][0]['txid'])