test: incorrect rbf status when max replacement limit exceeded

Highlight that transactions may signal they are replaceable even
though they are not due to BIP125 RBF (Rule bitcoin#5).

test/functional/feature_rbf.py

@@ -91,8 +91,8 @@ def run_test(self):
         self.log.info("Running test prechecks overestimates replacements...")
         self.test_prechecks_overestimates_replacements()
 
-        self.log.info("Running test reorged inherited signaling...")
-        self.test_reorged_inherited_signaling()
+        self.log.info("Running test reorged inherited signaling and descendant limit...")
+        self.test_reorged_inherited_signaling_and_descendant_limit()
 
         self.log.info("Passed")
 
@@ -705,7 +705,7 @@ def test_prechecks_overestimates_replacements(self):
         self.wallet.get_utxo(txid=tx['txid'])
         self.wallet.get_utxo(txid=replacement_tx['txid'])
 
-    def test_reorged_inherited_signaling(self):
+    def test_reorged_inherited_signaling_and_descendant_limit(self):
         confirmed_utxos = [self.wallet.get_utxo(), self.wallet.get_utxo()]
 
         # One opt-in parent transaction, one opt-out
@@ -738,5 +738,34 @@ def test_reorged_inherited_signaling(self):
         self.nodes[0].invalidateblock(hash)
         assert_equal(True, self.nodes[0].getmempoolentry(joined_tx_txid)['bip125-replaceable'])
 
+        # Create a chain the size of `MAX_REPLACEMENT_LIMIT` spending `joined_tx` - the BIP125 Rule #5 imposed limit
+        tx = None
+        for _ in range(MAX_REPLACEMENT_LIMIT - 1):
+            tx = self.wallet.send_self_transfer(
+                from_node=self.nodes[0],
+                utxo_to_spend=joined_utxo if tx is None else self.wallet.get_utxo(txid=tx['txid']),  # a straight line of descendants
+                sequence=0xffffffff,
+                fee_rate=Decimal('0.0001'),
+            )
+            assert_equal(True, self.nodes[0].getmempoolentry(tx['txid'])['bip125-replaceable'])  # inherited
+        # Now we have a chain of: `optin_parent_tx`, `joined_tx`, and 99 txs. The last tx in the loop exceeded `MAX_REPLACEMENT_LIMIT`
+
+        # Attempting to replace the opt-in parent transaction will now result in more than `MAX_REPLACEMENT_LIMIT`
+        # conflicting txns being evicted from the mempool. However, it (and all of its descendants) are still signaling replaceability.
+        # We would've expected that once `MAX_REPLACEMENT_LIMIT` is exceeded, the opt-in parent txn stops signaling
+        # replaceability, along with _all_ of its descendants.
+        entry = self.nodes[0].getmempoolentry(optin_parent_tx["txid"])
+        assert_greater_than(entry['descendantcount'], MAX_REPLACEMENT_LIMIT)
+        assert_equal(True, entry['bip125-replaceable'])
+        assert_equal(True, self.nodes[0].getmempoolentry(tx["txid"])['bip125-replaceable'])
+
+        # Case in point, we can't actually replace `optin_parent_tx` once it has `MAX_REPLACEMENT_LIMIT` descendants
+        self.wallet.create_self_transfer(
+            from_node=self.nodes[0],
+            utxo_to_spend=confirmed_utxos[0],
+            fee_rate=Decimal('0.01'),
+            mempool_valid=False,
+        )
+
 if __name__ == '__main__':
     ReplaceByFeeTest().main()