[test] mempool coins disappearing mid-package evaluation

Test for scenario(s) outlined in PR 28251.
Test what happens when a package transaction spends a mempool coin which
is fetched and then disappears mid-package evaluation due to eviction or
replacement.

test/functional/mempool_limit.py

@@ -78,13 +78,172 @@ class MempoolLimitTest(BitcoinTestFramework):
         assert_equal(node.getmempoolinfo()['minrelaytxfee'], Decimal('0.00001000'))
         assert_greater_than(node.getmempoolinfo()['mempoolminfee'], Decimal('0.00001000'))
 
+    def test_mid_package_eviction(self):
+        node = self.nodes[0]
+        self.log.info("Check a package where each parent passes the current mempoolminfee but would cause eviction before package submission terminates")
+
+        self.restart_node(0, extra_args=self.extra_args[0])
+
+        # Restarting the node resets mempool minimum feerate
+        assert_equal(node.getmempoolinfo()['minrelaytxfee'], Decimal('0.00001000'))
+        assert_equal(node.getmempoolinfo()['mempoolminfee'], Decimal('0.00001000'))
+
+        self.fill_mempool()
+        current_info = node.getmempoolinfo()
+        mempoolmin_feerate = current_info["mempoolminfee"]
+
+        package_hex = []
+        # UTXOs to be spent by the ultimate child transaction
+        parent_utxos = []
+
+        evicted_weight = 8000
+        # Mempool transaction which is evicted due to being at the "bottom" of the mempool when the
+        # mempool overflows and evicts by descendant score. It's important that the eviction doesn't
+        # happen in the middle of package evaluation, as it can invalidate the coins cache.
+        mempool_evicted_tx = self.wallet.send_self_transfer(
+            from_node=node,
+            fee=(mempoolmin_feerate / 1000) * (evicted_weight // 4) + Decimal('0.000001'),
+            target_weight=evicted_weight,
+            confirmed_only=True
+        )
+        # Already in mempool when package is submitted.
+        assert mempool_evicted_tx["txid"] in node.getrawmempool()
+
+        # This parent spends the above mempool transaction that exists when its inputs are first
+        # looked up, but disappears later. It is rejected for being too low fee (but eligible for
+        # reconsideration), and its inputs are cached. When the mempool transaction is evicted, its
+        # coin is no longer available, but the cache could still contains the tx.
+        cpfp_parent = self.wallet.create_self_transfer(
+            utxo_to_spend=mempool_evicted_tx["new_utxo"],
+            fee_rate=mempoolmin_feerate - Decimal('0.00001'),
+            confirmed_only=True)
+        package_hex.append(cpfp_parent["hex"])
+        parent_utxos.append(cpfp_parent["new_utxo"])
+        assert_equal(node.testmempoolaccept([cpfp_parent["hex"]])[0]["reject-reason"], "mempool min fee not met")
+
+        self.wallet.rescan_utxos()
+
+        # Series of parents that don't need CPFP and are submitted individually. Each one is large and
+        # high feerate, which means they should trigger eviction but not be evicted.
+        parent_weight = 100000
+        num_big_parents = 3
+        assert_greater_than(parent_weight * num_big_parents, current_info["maxmempool"] - current_info["bytes"])
+        parent_fee = (100 * mempoolmin_feerate / 1000) * (parent_weight // 4)
+
+        big_parent_txids = []
+        for i in range(num_big_parents):
+            parent = self.wallet.create_self_transfer(fee=parent_fee, target_weight=parent_weight, confirmed_only=True)
+            parent_utxos.append(parent["new_utxo"])
+            package_hex.append(parent["hex"])
+            big_parent_txids.append(parent["txid"])
+            # There is room for each of these transactions independently
+            assert node.testmempoolaccept([parent["hex"]])[0]["allowed"]
+
+        # Create a child spending everything, bumping cpfp_parent just above mempool minimum
+        # feerate. It's important not to bump too much as otherwise mempool_evicted_tx would not be
+        # evicted, making this test much less meaningful.
+        approx_child_vsize = self.wallet.create_self_transfer_multi(utxos_to_spend=parent_utxos)["tx"].get_vsize()
+        cpfp_fee = (mempoolmin_feerate / 1000) * (cpfp_parent["tx"].get_vsize() + approx_child_vsize) - cpfp_parent["fee"]
+        # Specific number of satoshis to fit within a small window. The parent_cpfp + child package needs to be
+        # - When there is mid-package eviction, high enough feerate to meet the new mempoolminfee
+        # - When there is no mid-package eviction, low enough feerate to be evicted immediately after submission.
+        magic_satoshis = 1200
+        cpfp_satoshis = int(cpfp_fee * COIN) + magic_satoshis
+
+        child = self.wallet.create_self_transfer_multi(utxos_to_spend=parent_utxos, fee_per_output=cpfp_satoshis)
+        package_hex.append(child["hex"])
+
+        # Package should be submitted, temporarily exceeding maxmempool, and then evicted.
+        with node.assert_debug_log(expected_msgs=["rolling minimum fee bumped"]):
+            assert_raises_rpc_error(-26, "mempool full", node.submitpackage, package_hex)
+
+        # Maximum size must never be exceeded.
+        assert_greater_than(node.getmempoolinfo()["maxmempool"], node.getmempoolinfo()["bytes"])
+
+        # Evicted transaction and its descendants must not be in mempool.
+        resulting_mempool_txids = node.getrawmempool()
+        assert mempool_evicted_tx["txid"] not in resulting_mempool_txids
+        assert cpfp_parent["txid"] not in resulting_mempool_txids
+        assert child["txid"] not in resulting_mempool_txids
+        for txid in big_parent_txids:
+            assert txid in resulting_mempool_txids
+
+    def test_mid_package_replacement(self):
+        node = self.nodes[0]
+        self.log.info("Check a package where an early tx depends on a later-replaced mempool tx")
+
+        self.restart_node(0, extra_args=self.extra_args[0])
+
+        # Restarting the node resets mempool minimum feerate
+        assert_equal(node.getmempoolinfo()['minrelaytxfee'], Decimal('0.00001000'))
+        assert_equal(node.getmempoolinfo()['mempoolminfee'], Decimal('0.00001000'))
+
+        self.fill_mempool()
+        current_info = node.getmempoolinfo()
+        mempoolmin_feerate = current_info["mempoolminfee"]
+
+        # Mempool transaction which is evicted due to being at the "bottom" of the mempool when the
+        # mempool overflows and evicts by descendant score. It's important that the eviction doesn't
+        # happen in the middle of package evaluation, as it can invalidate the coins cache.
+        double_spent_utxo = self.wallet.get_utxo(confirmed_only=True)
+        replaced_tx = self.wallet.send_self_transfer(
+            from_node=node,
+            utxo_to_spend=double_spent_utxo,
+            fee_rate=mempoolmin_feerate,
+            confirmed_only=True
+        )
+        # Already in mempool when package is submitted.
+        assert replaced_tx["txid"] in node.getrawmempool()
+
+        # This parent spends the above mempool transaction that exists when its inputs are first
+        # looked up, but disappears later. It is rejected for being too low fee (but eligible for
+        # reconsideration), and its inputs are cached. When the mempool transaction is evicted, its
+        # coin is no longer available, but the cache could still contain the tx.
+        cpfp_parent = self.wallet.create_self_transfer(
+            utxo_to_spend=replaced_tx["new_utxo"],
+            fee_rate=mempoolmin_feerate - Decimal('0.00001'),
+            confirmed_only=True)
+
+        self.wallet.rescan_utxos()
+
+        # Parent that replaces the parent of cpfp_parent.
+        replacement_tx = self.wallet.create_self_transfer(
+            utxo_to_spend=double_spent_utxo,
+            fee_rate=10*mempoolmin_feerate,
+            confirmed_only=True
+        )
+        parent_utxos = [cpfp_parent["new_utxo"], replacement_tx["new_utxo"]]
+
+        # Create a child spending everything, CPFPing the low-feerate parent.
+        approx_child_vsize = self.wallet.create_self_transfer_multi(utxos_to_spend=parent_utxos)["tx"].get_vsize()
+        cpfp_fee = (2 * mempoolmin_feerate / 1000) * (cpfp_parent["tx"].get_vsize() + approx_child_vsize) - cpfp_parent["fee"]
+        child = self.wallet.create_self_transfer_multi(utxos_to_spend=parent_utxos, fee_per_output=int(cpfp_fee * COIN))
+        # It's very important that the cpfp_parent is before replacement_tx so that its input (from
+        # replaced_tx) is first looked up *before* replacement_tx is submitted.
+        package_hex = [cpfp_parent["hex"], replacement_tx["hex"], child["hex"]]
+
+        # Package should be submitted, temporarily exceeding maxmempool, and then evicted.
+        assert_raises_rpc_error(-26, "bad-txns-inputs-missingorspent", node.submitpackage, package_hex)
+
+        # Maximum size must never be exceeded.
+        assert_greater_than(node.getmempoolinfo()["maxmempool"], node.getmempoolinfo()["bytes"])
+
+        resulting_mempool_txids = node.getrawmempool()
+        # The replacement should be successful.
+        assert replacement_tx["txid"] in resulting_mempool_txids
+        # The replaced tx and all of its descendants must not be in mempool.
+        assert replaced_tx["txid"] not in resulting_mempool_txids
+        assert cpfp_parent["txid"] not in resulting_mempool_txids
+        assert child["txid"] not in resulting_mempool_txids
+
+
     def run_test(self):
         node = self.nodes[0]
         self.wallet = MiniWallet(node)
         miniwallet = self.wallet
 
         # Generate coins needed to create transactions in the subtests (excluding coins used in fill_mempool).
-        self.generate(miniwallet, 10)
+        self.generate(miniwallet, 20)
 
         relayfee = node.getnetworkinfo()['relayfee']
         self.log.info('Check that mempoolminfee is minrelaytxfee')
@@ -163,6 +322,9 @@ class MempoolLimitTest(BitcoinTestFramework):
         self.stop_node(0)
         self.nodes[0].assert_start_raises_init_error(["-maxmempool=4"], "Error: -maxmempool must be at least 5 MB")
 
+        self.test_mid_package_replacement()
+        self.test_mid_package_eviction()
+
 
 if __name__ == '__main__':
     MempoolLimitTest().main()