relax child-with-unconfirmed-parents rule

This rule was originally introduced along with a very early proposal for
package relay as a way to verify that the "correct"
child-with-unconfirmed-parents package was provided for a transaction,
where correctness was defined as all of the transactions unconfirmed
parents. However, we are not planning to introduce a protocol where
peers would be asked to send these packages.

This rule has downsides: if a transaction has multiple parents but only
1 that requires package CPFP to be accepted, the current rule prevents
us from accepting that package. Even if the other parents are already in
mempool, the p2p logic will only submit the 1p1c package, which fails
this check. See the test in p2p_1p1c_network.py

Author: glozow

doc/policy/packages.md

@@ -8,11 +8,9 @@ Graph (a directed edge exists between a transaction that spends the output of an
 For every transaction `t` in a **topologically sorted** package, if any of its parents are present
 in the package, they appear somewhere in the list before `t`.
 
-A **child-with-unconfirmed-parents** package is a topologically sorted package that consists of
-exactly one child and all of its unconfirmed parents (no other transactions may be present).
-The last transaction in the package is the child, and its package can be canonically defined based
-on the current state: each of its inputs must be available in the UTXO set as of the current chain
-tip or some preceding transaction in the package.
+A **child-with-parents** package is a topologically sorted package that consists of exactly one child and at least one
+of its unconfirmed parents.  Not all unconfirmed parents need to be present but no other transactions may be present; the
+parent of a parent should not be in this package (unless this "grandparent" is also a direct parent of the child).
 
 ## Package Mempool Acceptance Rules
 
@@ -73,7 +71,7 @@ The following rules are enforced for all packages:
 The following rules are only enforced for packages to be submitted to the mempool (not
 enforced for test accepts):
 
-* Packages must be child-with-unconfirmed-parents packages. This also means packages must contain at
+* Packages must be child-with-parents packages. This also means packages must contain at
   least 1 transaction. (#31096)
 
    - *Rationale*: This allows for fee-bumping by CPFP. Allowing multiple parents makes it possible

src/policy/packages.h

@@ -26,7 +26,7 @@ static_assert(MAX_PACKAGE_WEIGHT >= MAX_STANDARD_TX_WEIGHT);
 
 // If a package is to be evaluated, it must be at least as large as the mempool's ancestor/descendant limits,
 // otherwise transactions that would be individually accepted may be rejected in a package erroneously.
-// Since a submitted package must be child-with-unconfirmed-parents (all of the transactions are an ancestor
+// Since a submitted package must be child-with-parents (all of the transactions are a parent
 // of the child), package limits are ultimately bounded by mempool package limits. Ensure that the
 // defaults reflect this constraint.
 static_assert(DEFAULT_DESCENDANT_LIMIT >= MAX_PACKAGE_COUNT);

src/test/txpackage_tests.cpp

@@ -441,20 +441,6 @@ BOOST_AUTO_TEST_CASE(package_submission_tests)
         BOOST_CHECK_EQUAL(result_quit_early.m_state.GetResult(), PackageValidationResult::PCKG_TX);
     }
 
-    // Child with missing parent.
-    mtx_child.vin.emplace_back(COutPoint(package_unrelated[0]->GetHash(), 0));
-    Package package_missing_parent;
-    package_missing_parent.push_back(tx_parent);
-    package_missing_parent.push_back(MakeTransactionRef(mtx_child));
-    {
-        const auto result_missing_parent = ProcessNewPackage(m_node.chainman->ActiveChainstate(), *m_node.mempool,
-                                                             package_missing_parent, /*test_accept=*/false, /*client_maxfeerate=*/{});
-        BOOST_CHECK(result_missing_parent.m_state.IsInvalid());
-        BOOST_CHECK_EQUAL(result_missing_parent.m_state.GetResult(), PackageValidationResult::PCKG_POLICY);
-        BOOST_CHECK_EQUAL(result_missing_parent.m_state.GetRejectReason(), "package-not-child-with-unconfirmed-parents");
-        BOOST_CHECK_EQUAL(m_node.mempool->size(), expected_pool_size);
-    }
-
     // Submit package with parent + child.
     {
         const auto submit_parent_child = ProcessNewPackage(m_node.chainman->ActiveChainstate(), *m_node.mempool,

src/validation.cpp

@@ -522,7 +522,7 @@ class MemPoolAccept
             };
         }
 
-        /** Parameters for child-with-unconfirmed-parents package validation. */
+        /** Parameters for child-with-parents package validation. */
         static ATMPArgs PackageChildWithParents(const CChainParams& chainparams, int64_t accept_time,
                                                 std::vector<COutPoint>& coins_to_uncache, const std::optional<CFeeRate>& client_maxfeerate) {
             return ATMPArgs{/* m_chainparams */ chainparams,
@@ -1694,7 +1694,7 @@ PackageMempoolAcceptResult MemPoolAccept::AcceptPackage(const Package& package,
 
     // There are two topologies we are able to handle through this function:
     // (1) A single transaction
-    // (2) A child-with-unconfirmed-parents package.
+    // (2) A child-with-parents package.
     // Check that the package is well-formed. If it isn't, we won't try to validate any of the
     // transactions and thus won't return any MempoolAcceptResults, just a package-wide error.
 
@@ -1703,49 +1703,11 @@ PackageMempoolAcceptResult MemPoolAccept::AcceptPackage(const Package& package,
         return PackageMempoolAcceptResult(package_state_quit_early, {});
     }
 
-    if (package.size() > 1) {
+    if (package.size() > 1 && !IsChildWithParents(package)) {
         // All transactions in the package must be a parent of the last transaction. This is just an
         // opportunity for us to fail fast on a context-free check without taking the mempool lock.
-        if (!IsChildWithParents(package)) {
-            package_state_quit_early.Invalid(PackageValidationResult::PCKG_POLICY, "package-not-child-with-parents");
-            return PackageMempoolAcceptResult(package_state_quit_early, {});
-        }
-
-        // IsChildWithParents() guarantees the package is > 1 transactions.
-        assert(package.size() > 1);
-        // The package must be 1 child with all of its unconfirmed parents. The package is expected to
-        // be sorted, so the last transaction is the child.
-        const auto& child = package.back();
-        std::unordered_set<uint256, SaltedTxidHasher> unconfirmed_parent_txids;
-        std::transform(package.cbegin(), package.cend() - 1,
-                       std::inserter(unconfirmed_parent_txids, unconfirmed_parent_txids.end()),
-                       [](const auto& tx) { return tx->GetHash(); });
-
-        // All child inputs must refer to a preceding package transaction or a confirmed UTXO. The only
-        // way to verify this is to look up the child's inputs in our current coins view (not including
-        // mempool), and enforce that all parents not present in the package be available at chain tip.
-        // Since this check can bring new coins into the coins cache, keep track of these coins and
-        // uncache them if we don't end up submitting this package to the mempool.
-        const CCoinsViewCache& coins_tip_cache = m_active_chainstate.CoinsTip();
-        for (const auto& input : child->vin) {
-            if (!coins_tip_cache.HaveCoinInCache(input.prevout)) {
-                args.m_coins_to_uncache.push_back(input.prevout);
-            }
-        }
-        // Using the MemPoolAccept m_view cache allows us to look up these same coins faster later.
-        // This should be connecting directly to CoinsTip, not to m_viewmempool, because we specifically
-        // require inputs to be confirmed if they aren't in the package.
-        m_view.SetBackend(m_active_chainstate.CoinsTip());
-        const auto package_or_confirmed = [this, &unconfirmed_parent_txids](const auto& input) {
-             return unconfirmed_parent_txids.count(input.prevout.hash) > 0 || m_view.HaveCoin(input.prevout);
-        };
-        if (!std::all_of(child->vin.cbegin(), child->vin.cend(), package_or_confirmed)) {
-            package_state_quit_early.Invalid(PackageValidationResult::PCKG_POLICY, "package-not-child-with-unconfirmed-parents");
-            return PackageMempoolAcceptResult(package_state_quit_early, {});
-        }
-        // Protect against bugs where we pull more inputs from disk that miss being added to
-        // coins_to_uncache. The backend will be connected again when needed in PreChecks.
-        m_view.SetBackend(m_dummy);
+        package_state_quit_early.Invalid(PackageValidationResult::PCKG_POLICY, "package-not-child-with-parents");
+        return PackageMempoolAcceptResult(package_state_quit_early, {});
     }
 
     LOCK(m_pool.cs);

test/functional/p2p_1p1c_network.py

@@ -101,7 +101,7 @@ def create_packages(self):
         package_hex_2, parent_2, child_2 = self.create_basic_1p1c(self.wallet_nonsegwit)
 
         # 3: 2-parent-1-child package. Both parents are above mempool min feerate. No package submission happens.
-        # We require packages to be child-with-unconfirmed-parents and only allow 1-parent-1-child packages.
+        # We require packages to be child-with-parents and only allow 1-parent-1-child packages.
         package_hex_3, parent_31, _parent_32, child_3 = self.create_package_2p1c(self.wallet)
 
         # 4: parent + child package where the child spends 2 different outputs from the parent.

test/functional/p2p_opportunistic_1p1c.py

@@ -78,14 +78,14 @@ def set_test_params(self):
             "-maxmempool=5",
         ]]
 
-    def create_tx_below_mempoolminfee(self, wallet):
+    def create_tx_below_mempoolminfee(self, wallet, utxo_to_spend=None):
         """Create a 1-input 1sat/vB transaction using a confirmed UTXO. Decrement and use
         self.sequence so that subsequent calls to this function result in unique transactions."""
 
         self.sequence -= 1
         assert_greater_than(self.nodes[0].getmempoolinfo()["mempoolminfee"], FEERATE_1SAT_VB)
 
-        return wallet.create_self_transfer(fee_rate=FEERATE_1SAT_VB, sequence=self.sequence, confirmed_only=True)
+        return wallet.create_self_transfer(fee_rate=FEERATE_1SAT_VB, sequence=self.sequence, utxo_to_spend=utxo_to_spend, confirmed_only=True)
 
     @cleanup
     def test_basic_child_then_parent(self):
@@ -353,11 +353,14 @@ def test_multiple_parents(self):
 
     @cleanup
     def test_other_parent_in_mempool(self):
-        self.log.info("Check opportunistic 1p1c fails if child already has another parent in mempool")
+        self.log.info("Check opportunistic 1p1c works even if child already has another parent in mempool")
         node = self.nodes[0]
 
+        # Grandparent will enter mempool by itself
+        grandparent_high = self.wallet.create_self_transfer(fee_rate=FEERATE_1SAT_VB*10, confirmed_only=True)
+
         # This parent needs CPFP
-        parent_low = self.create_tx_below_mempoolminfee(self.wallet)
+        parent_low = self.create_tx_below_mempoolminfee(self.wallet, utxo_to_spend=grandparent_high["new_utxo"])
         # This parent does not need CPFP and can be submitted alone ahead of time
         parent_high = self.wallet.create_self_transfer(fee_rate=FEERATE_1SAT_VB*10, confirmed_only=True)
         child = self.wallet.create_self_transfer_multi(
@@ -367,27 +370,27 @@ def test_other_parent_in_mempool(self):
 
         peer_sender = node.add_outbound_p2p_connection(P2PInterface(), p2p_idx=1, connection_type="outbound-full-relay")
 
-        # 1. Send first parent which will be accepted.
+        # 1. Send grandparent which is accepted
+        peer_sender.send_and_ping(msg_tx(grandparent_high["tx"]))
+        assert grandparent_high["txid"] in node.getrawmempool()
+
+        # 2. Send first parent which is accepted.
         peer_sender.send_and_ping(msg_tx(parent_high["tx"]))
         assert parent_high["txid"] in node.getrawmempool()
 
-        # 2. Send child.
+        # 3. Send child which is handled as an orphan.
         peer_sender.send_and_ping(msg_tx(child["tx"]))
 
-        # 3. Node requests parent_low. However, 1p1c fails because package-not-child-with-unconfirmed-parents
+        # 4. Node requests parent_low.
         parent_low_txid_int = int(parent_low["txid"], 16)
         peer_sender.wait_for_getdata([parent_low_txid_int])
         peer_sender.send_and_ping(msg_tx(parent_low["tx"]))
 
         node_mempool = node.getrawmempool()
+        assert grandparent_high["txid"] in node_mempool
         assert parent_high["txid"] in node_mempool
-        assert parent_low["txid"] not in node_mempool
-        assert child["txid"] not in node_mempool
-
-        # Same error if submitted through submitpackage without parent_high
-        package_hex_missing_parent = [parent_low["hex"], child["hex"]]
-        result_missing_parent = node.submitpackage(package_hex_missing_parent)
-        assert_equal(result_missing_parent["package_msg"], "package-not-child-with-unconfirmed-parents")
+        assert parent_low["txid"] in node_mempool
+        assert child["txid"] in node_mempool
 
     def create_small_orphan(self):
         """Create small orphan transaction"""

test/functional/rpc_packages.py

@@ -271,9 +271,11 @@ def test_conflicting(self):
         assert_equal(submitres, {'package_msg': 'conflict-in-package', 'tx-results': {}, 'replaced-transactions': []})
         assert tx_child["txid"] not in node.getrawmempool()
 
-        # ... and without the in-mempool ancestor tx1 included in the call
+        # without the in-mempool ancestor tx1 included in the call, tx2 can be submitted, but
+        # tx_child is missing an input.
         submitres = node.submitpackage([tx2["hex"], tx_child["hex"]])
-        assert_equal(submitres, {'package_msg': 'package-not-child-with-unconfirmed-parents', 'tx-results': {}, 'replaced-transactions': []})
+        assert_equal(submitres["tx-results"][tx_child["wtxid"]], {"txid": tx_child["txid"], "error": "bad-txns-inputs-missingorspent"})
+        assert tx2["txid"] in node.getrawmempool()
 
         # Regardless of error type, the child can never enter the mempool
         assert tx_child["txid"] not in node.getrawmempool()