[policy] add v3 policy rules

Co-authored-by: Suhas Daftuar <sdaftuar@gmail.com>

4 files changed, 596 insertions, 0 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index df56ac7090..f9c6e502ae 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -238,6 +238,7 @@ BITCOIN_CORE_H = \
   node/validation_cache_args.h \
   noui.h \
   outputtype.h \
+  policy/v3_policy.h \
   policy/feerate.h \
   policy/fees.h \
   policy/fees_args.h \
@@ -439,6 +440,7 @@ libbitcoin_node_a_SOURCES = \
   node/utxo_snapshot.cpp \
   node/validation_cache_args.cpp \
   noui.cpp \
+  policy/v3_policy.cpp \
   policy/fees.cpp \
   policy/fees_args.cpp \
   policy/packages.cpp \
@@ -694,6 +696,7 @@ libbitcoin_common_a_SOURCES = \
   netbase.cpp \
   net_permissions.cpp \
   outputtype.cpp \
+  policy/v3_policy.cpp \
   policy/feerate.cpp \
   policy/policy.cpp \
   protocol.cpp \
@@ -953,6 +956,7 @@ libbitcoinkernel_la_SOURCES = \
   node/blockstorage.cpp \
   node/chainstate.cpp \
   node/utxo_snapshot.cpp \
+  policy/v3_policy.cpp \
   policy/feerate.cpp \
   policy/packages.cpp \
   policy/policy.cpp \
diff --git a/src/policy/v3_policy.cpp b/src/policy/v3_policy.cpp
new file mode 100644
index 0000000000..158881aeb9
--- /dev/null
+++ b/src/policy/v3_policy.cpp
@@ -0,0 +1,220 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <policy/v3_policy.h>
+
+#include <coins.h>
+#include <consensus/amount.h>
+#include <logging.h>
+#include <tinyformat.h>
+#include <util/check.h>
+
+#include <algorithm>
+#include <numeric>
+#include <vector>
+
+/** Helper for PackageV3Checks: Returns a vector containing the indices of transactions (within
+ * package) that are direct parents of ptx. */
+std::vector<size_t> FindInPackageParents(const Package& package, const CTransactionRef& ptx)
+{
+    std::vector<size_t> in_package_parents;
+
+    std::set<Txid> possible_parents;
+    for (auto &input : ptx->vin) {
+        possible_parents.insert(input.prevout.hash);
+    }
+
+    for (size_t i{0}; i < package.size(); ++i) {
+        const auto& tx = package.at(i);
+        // We assume the package is sorted, so that we don't need to continue
+        // looking past the transaction itself.
+        if (&(*tx) == &(*ptx)) break;
+        if (possible_parents.count(tx->GetHash())) {
+            in_package_parents.push_back(i);
+        }
+    }
+    return in_package_parents;
+}
+
+/** Helper for PackageV3Checks, storing info for a mempool or package parent. */
+struct ParentInfo {
+    /** Txid used to identify this parent by prevout */
+    const Txid& m_txid;
+    /** Wtxid used for debug string */
+    const Wtxid& m_wtxid;
+    /** nVersion used to check inheritance of v3 and non-v3 */
+    decltype(CTransaction::nVersion) m_version;
+    /** If parent is in mempool, whether it has any descendants in mempool. */
+    bool m_has_mempool_descendant;
+
+    ParentInfo() = delete;
+    ParentInfo(const Txid& txid, const Wtxid& wtxid, decltype(CTransaction::nVersion) version, bool has_mempool_descendant) :
+        m_txid{txid}, m_wtxid{wtxid}, m_version{version},
+        m_has_mempool_descendant{has_mempool_descendant}
+    {}
+};
+
+std::optional<std::string> PackageV3Checks(const CTransactionRef& ptx, int64_t vsize,
+                                           const Package& package,
+                                           const CTxMemPool::setEntries& mempool_ancestors)
+{
+    // This function is specialized for these limits, and must be reimplemented if they ever change.
+    static_assert(V3_ANCESTOR_LIMIT == 2);
+    static_assert(V3_DESCENDANT_LIMIT == 2);
+
+    const auto in_package_parents{FindInPackageParents(package, ptx)};
+
+    // Now we have all ancestors, so we can start checking v3 rules.
+    if (ptx->nVersion == 3) {
+        if (mempool_ancestors.size() + in_package_parents.size() + 1 > V3_ANCESTOR_LIMIT) {
+            return strprintf("tx %s (wtxid=%s) would have too many ancestors",
+                             ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString());
+        }
+
+        const bool has_parent{mempool_ancestors.size() + in_package_parents.size() > 0};
+        if (has_parent) {
+            // A v3 child cannot be too large.
+            if (vsize > V3_CHILD_MAX_VSIZE) {
+                return strprintf("v3 child tx %s (wtxid=%s) is too big: %u > %u virtual bytes",
+                                 ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(),
+                                 vsize, V3_CHILD_MAX_VSIZE);
+            }
+
+            const auto parent_info = [&] {
+                if (mempool_ancestors.size() > 0) {
+                    // There's a parent in the mempool.
+                    auto& mempool_parent = *mempool_ancestors.begin();
+                    Assume(mempool_parent->GetCountWithDescendants() == 1);
+                    return ParentInfo{mempool_parent->GetTx().GetHash(),
+                                      mempool_parent->GetTx().GetWitnessHash(),
+                                      mempool_parent->GetTx().nVersion,
+                                      /*has_mempool_descendant=*/mempool_parent->GetCountWithDescendants() > 1};
+                } else {
+                    // Ancestor must be in the package. Find it.
+                    auto& parent_index = in_package_parents.front();
+                    auto& package_parent = package.at(parent_index);
+                    return ParentInfo{package_parent->GetHash(),
+                                      package_parent->GetWitnessHash(),
+                                      package_parent->nVersion,
+                                      /*has_mempool_descendant=*/false};
+                }
+            }();
+
+            // If there is a parent, it must have the right version.
+            if (parent_info.m_version != 3) {
+                return strprintf("v3 tx %s (wtxid=%s) cannot spend from non-v3 tx %s (wtxid=%s)",
+                                 ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(),
+                                 parent_info.m_txid.ToString(), parent_info.m_wtxid.ToString());
+            }
+
+            for (const auto& package_tx : package) {
+                // Skip same tx.
+                if (&(*package_tx) == &(*ptx)) continue;
+
+                for (auto& input : package_tx->vin) {
+                    // Fail if we find another tx with the same parent. We don't check whether the
+                    // sibling is to-be-replaced (done in SingleV3Checks) because these transactions
+                    // are within the same package.
+                    if (input.prevout.hash == parent_info.m_txid) {
+                        return strprintf("tx %s (wtxid=%s) would exceed descendant count limit",
+                                         parent_info.m_txid.ToString(),
+                                         parent_info.m_wtxid.ToString());
+                    }
+
+                    // This tx can't have both a parent and an in-package child.
+                    if (input.prevout.hash == ptx->GetHash()) {
+                        return strprintf("tx %s (wtxid=%s) would have too many ancestors",
+                                         package_tx->GetHash().ToString(), package_tx->GetWitnessHash().ToString());
+                    }
+                }
+            }
+
+            // It shouldn't be possible to have any mempool siblings at this point. SingleV3Checks
+            // catches mempool siblings. Also, if the package consists of connected transactions,
+            // any tx having a mempool ancestor would mean the package exceeds ancestor limits.
+            if (!Assume(!parent_info.m_has_mempool_descendant)) {
+                return strprintf("tx %u would exceed descendant count limit", parent_info.m_wtxid.ToString());
+            }
+        }
+    } else {
+        // Non-v3 transactions cannot have v3 parents.
+        for (auto it : mempool_ancestors) {
+            if (it->GetTx().nVersion == 3) {
+                return strprintf("non-v3 tx %s (wtxid=%s) cannot spend from v3 tx %s (wtxid=%s)",
+                                 ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(),
+                                 it->GetSharedTx()->GetHash().ToString(), it->GetSharedTx()->GetWitnessHash().ToString());
+            }
+        }
+        for (const auto& index: in_package_parents) {
+            if (package.at(index)->nVersion == 3) {
+                return strprintf("non-v3 tx %s (wtxid=%s) cannot spend from v3 tx %s (wtxid=%s)",
+                                 ptx->GetHash().ToString(),
+                                 ptx->GetWitnessHash().ToString(),
+                                 package.at(index)->GetHash().ToString(),
+                                 package.at(index)->GetWitnessHash().ToString());
+            }
+        }
+    }
+    return std::nullopt;
+}
+
+std::optional<std::string> SingleV3Checks(const CTransactionRef& ptx,
+                                          const CTxMemPool::setEntries& mempool_ancestors,
+                                          const std::set<Txid>& direct_conflicts,
+                                          int64_t vsize)
+{
+    // Check v3 and non-v3 inheritance.
+    for (const auto& entry : mempool_ancestors) {
+        if (ptx->nVersion != 3 && entry->GetTx().nVersion == 3) {
+            return strprintf("non-v3 tx %s (wtxid=%s) cannot spend from v3 tx %s (wtxid=%s)",
+                             ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(),
+                             entry->GetSharedTx()->GetHash().ToString(), entry->GetSharedTx()->GetWitnessHash().ToString());
+        } else if (ptx->nVersion == 3 && entry->GetTx().nVersion != 3) {
+            return strprintf("v3 tx %s (wtxid=%s) cannot spend from non-v3 tx %s (wtxid=%s)",
+                             ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(),
+                             entry->GetSharedTx()->GetHash().ToString(), entry->GetSharedTx()->GetWitnessHash().ToString());
+        }
+    }
+
+    // This function is specialized for these limits, and must be reimplemented if they ever change.
+    static_assert(V3_ANCESTOR_LIMIT == 2);
+    static_assert(V3_DESCENDANT_LIMIT == 2);
+
+    // The rest of the rules only apply to transactions with nVersion=3.
+    if (ptx->nVersion != 3) return std::nullopt;
+
+    // Check that V3_ANCESTOR_LIMIT would not be violated, including both in-package and in-mempool.
+    if (mempool_ancestors.size() + 1 > V3_ANCESTOR_LIMIT) {
+        return strprintf("tx %s (wtxid=%s) would have too many ancestors",
+                         ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString());
+    }
+
+    // Remaining checks only pertain to transactions with unconfirmed ancestors.
+    if (mempool_ancestors.size() > 0) {
+        // If this transaction spends V3 parents, it cannot be too large.
+        if (vsize > V3_CHILD_MAX_VSIZE) {
+            return strprintf("v3 child tx %s (wtxid=%s) is too big: %u > %u virtual bytes",
+                             ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(), vsize, V3_CHILD_MAX_VSIZE);
+        }
+
+        // Check the descendant counts of in-mempool ancestors.
+        const auto& parent_entry = *mempool_ancestors.begin();
+        // If there are any ancestors, this is the only child allowed. The parent cannot have any
+        // other descendants. We handle the possibility of multiple children as that case is
+        // possible through a reorg.
+        const auto& children = parent_entry->GetMemPoolChildrenConst();
+        // Don't double-count a transaction that is going to be replaced. This logic assumes that
+        // any descendant of the V3 transaction is a direct child, which makes sense because a V3
+        // transaction can only have 1 descendant.
+        const bool child_will_be_replaced = !children.empty() &&
+            std::any_of(children.cbegin(), children.cend(),
+                [&direct_conflicts](const CTxMemPoolEntry& child){return direct_conflicts.count(child.GetTx().GetHash()) > 0;});
+        if (parent_entry->GetCountWithDescendants() + 1 > V3_DESCENDANT_LIMIT && !child_will_be_replaced) {
+            return strprintf("tx %u (wtxid=%s) would exceed descendant count limit",
+                             parent_entry->GetSharedTx()->GetHash().ToString(),
+                             parent_entry->GetSharedTx()->GetWitnessHash().ToString());
+        }
+    }
+    return std::nullopt;
+}
diff --git a/src/policy/v3_policy.h b/src/policy/v3_policy.h
new file mode 100644
index 0000000000..9e871915e5
--- /dev/null
+++ b/src/policy/v3_policy.h
@@ -0,0 +1,83 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_POLICY_V3_POLICY_H
+#define BITCOIN_POLICY_V3_POLICY_H
+
+#include <consensus/amount.h>
+#include <policy/packages.h>
+#include <policy/policy.h>
+#include <primitives/transaction.h>
+#include <txmempool.h>
+#include <util/result.h>
+
+#include <set>
+#include <string>
+
+// This module enforces rules for transactions with nVersion=3 ("v3 transactions") which help make
+// RBF abilities more robust.
+
+// v3 only allows 1 parent and 1 child when unconfirmed.
+/** Maximum number of transactions including an unconfirmed tx and its descendants. */
+static constexpr unsigned int V3_DESCENDANT_LIMIT{2};
+/** Maximum number of transactions including a V3 tx and all its mempool ancestors. */
+static constexpr unsigned int V3_ANCESTOR_LIMIT{2};
+
+/** Maximum sigop-adjusted virtual size of a tx which spends from an unconfirmed v3 transaction. */
+static constexpr int64_t V3_CHILD_MAX_VSIZE{1000};
+// These limits are within the default ancestor/descendant limits.
+static_assert(V3_CHILD_MAX_VSIZE + MAX_STANDARD_TX_WEIGHT / WITNESS_SCALE_FACTOR <= DEFAULT_ANCESTOR_SIZE_LIMIT_KVB * 1000);
+static_assert(V3_CHILD_MAX_VSIZE + MAX_STANDARD_TX_WEIGHT / WITNESS_SCALE_FACTOR <= DEFAULT_DESCENDANT_SIZE_LIMIT_KVB * 1000);
+
+/** Must be called for every transaction, even if not v3. Not strictly necessary for transactions
+ * accepted through AcceptMultipleTransactions.
+ *
+ * Checks the following rules:
+ * 1. A v3 tx must only have v3 unconfirmed ancestors.
+ * 2. A non-v3 tx must only have non-v3 unconfirmed ancestors.
+ * 3. A v3's ancestor set, including itself, must be within V3_ANCESTOR_LIMIT.
+ * 4. A v3's descendant set, including itself, must be within V3_DESCENDANT_LIMIT.
+ * 5. If a v3 tx has any unconfirmed ancestors, the tx's sigop-adjusted vsize must be within
+ * V3_CHILD_MAX_VSIZE.
+ *
+ *
+ * @param[in]   mempool_ancestors       The in-mempool ancestors of ptx.
+ * @param[in]   direct_conflicts        In-mempool transactions this tx conflicts with. These conflicts
+ *                                      are used to more accurately calculate the resulting descendant
+ *                                      count of in-mempool ancestors.
+ * @param[in]   vsize                   The sigop-adjusted virtual size of ptx.
+ *
+ * @returns debug string if an error occurs, std::nullopt otherwise.
+ */
+std::optional<std::string> SingleV3Checks(const CTransactionRef& ptx,
+                                          const CTxMemPool::setEntries& mempool_ancestors,
+                                          const std::set<Txid>& direct_conflicts,
+                                          int64_t vsize);
+
+/** Must be called for every transaction that is submitted within a package, even if not v3.
+ *
+ * For each transaction in a package:
+ * If it's not a v3 transaction, verify it has no direct v3 parents in the mempool or the package.
+
+ * If it is a v3 transaction, verify that any direct parents in the mempool or the package are v3.
+ * If such a parent exists, verify that parent has no other children in the package or the mempool,
+ * and that the transaction itself has no children in the package.
+ *
+ * If any v3 violations in the package exist, this test will fail for one of them:
+ * - if a v3 transaction T has a parent in the mempool and a child in the package, then PV3C(T) will fail
+ * - if a v3 transaction T has a parent in the package and a child in the package, then PV3C(T) will fail
+ * - if a v3 transaction T and a v3 (sibling) transaction U have some parent in the mempool,
+ *   then PV3C(T) and PV3C(U) will fail
+ * - if a v3 transaction T and a v3 (sibling) transaction U have some parent in the package,
+ *   then PV3C(T) and PV3C(U) will fail
+ * - if a v3 transaction T has a parent P and a grandparent G in the package, then
+ *   PV3C(P) will fail (though PV3C(G) and PV3C(T) might succeed).
+ *
+ * @returns debug string if an error occurs, std::nullopt otherwise.
+ * */
+std::optional<std::string> PackageV3Checks(const CTransactionRef& ptx, int64_t vsize,
+                                           const Package& package,
+                                           const CTxMemPool::setEntries& mempool_ancestors);
+
+#endif // BITCOIN_POLICY_V3_POLICY_H
diff --git a/src/test/txvalidation_tests.cpp b/src/test/txvalidation_tests.cpp
index ecf0889711..98d5e892f9 100644
--- a/src/test/txvalidation_tests.cpp
+++ b/src/test/txvalidation_tests.cpp
@@ -4,11 +4,14 @@
 
 #include <consensus/validation.h>
 #include <key_io.h>
+#include <policy/v3_policy.h>
 #include <policy/packages.h>
 #include <policy/policy.h>
 #include <primitives/transaction.h>
+#include <random.h>
 #include <script/script.h>
 #include <test/util/setup_common.h>
+#include <test/util/txmempool.h>
 #include <validation.h>
 
 #include <boost/test/unit_test.hpp>
@@ -48,4 +51,290 @@ BOOST_FIXTURE_TEST_CASE(tx_mempool_reject_coinbase, TestChain100Setup)
     BOOST_CHECK_EQUAL(result.m_state.GetRejectReason(), "coinbase");
     BOOST_CHECK(result.m_state.GetResult() == TxValidationResult::TX_CONSENSUS);
 }
+
+// Generate a number of random, nonexistent outpoints.
+static inline std::vector<COutPoint> random_outpoints(size_t num_outpoints) {
+    std::vector<COutPoint> outpoints;
+    for (size_t i{0}; i < num_outpoints; ++i) {
+        outpoints.emplace_back(Txid::FromUint256(GetRandHash()), 0);
+    }
+    return outpoints;
+}
+
+static inline std::vector<CPubKey> random_keys(size_t num_keys) {
+    std::vector<CPubKey> keys;
+    keys.reserve(num_keys);
+    for (size_t i{0}; i < num_keys; ++i) {
+        CKey key;
+        key.MakeNewKey(true);
+        keys.emplace_back(key.GetPubKey());
+    }
+    return keys;
+}
+
+// Creates a placeholder tx (not valid) with 25 outputs. Specify the nVersion and the inputs.
+static inline CTransactionRef make_tx(const std::vector<COutPoint>& inputs, int32_t version)
+{
+    CMutableTransaction mtx = CMutableTransaction{};
+    mtx.nVersion = version;
+    mtx.vin.resize(inputs.size());
+    mtx.vout.resize(25);
+    for (size_t i{0}; i < inputs.size(); ++i) {
+        mtx.vin[i].prevout = inputs[i];
+    }
+    for (auto i{0}; i < 25; ++i) {
+        mtx.vout[i].scriptPubKey = CScript() << OP_TRUE;
+        mtx.vout[i].nValue = 10000;
+    }
+    return MakeTransactionRef(mtx);
+}
+
+BOOST_FIXTURE_TEST_CASE(version3_tests, RegTestingSetup)
+{
+    // Test V3 policy helper functions
+    CTxMemPool& pool = *Assert(m_node.mempool);
+    LOCK2(cs_main, pool.cs);
+    TestMemPoolEntryHelper entry;
+    std::set<Txid> empty_conflicts_set;
+    CTxMemPool::setEntries empty_ancestors;
+
+    auto mempool_tx_v3 = make_tx(random_outpoints(1), /*version=*/3);
+    pool.addUnchecked(entry.FromTx(mempool_tx_v3));
+    auto mempool_tx_v2 = make_tx(random_outpoints(1), /*version=*/2);
+    pool.addUnchecked(entry.FromTx(mempool_tx_v2));
+    // Default values.
+    CTxMemPool::Limits m_limits{};
+
+    // Cannot spend from an unconfirmed v3 transaction unless this tx is also v3.
+    {
+        // mempool_tx_v3
+        //      ^
+        // tx_v2_from_v3
+        auto tx_v2_from_v3 = make_tx({COutPoint{mempool_tx_v3->GetHash(), 0}}, /*version=*/2);
+        auto ancestors_v2_from_v3{pool.CalculateMemPoolAncestors(entry.FromTx(tx_v2_from_v3), m_limits)};
+        const auto expected_error_str{strprintf("non-v3 tx %s (wtxid=%s) cannot spend from v3 tx %s (wtxid=%s)",
+            tx_v2_from_v3->GetHash().ToString(), tx_v2_from_v3->GetWitnessHash().ToString(),
+            mempool_tx_v3->GetHash().ToString(), mempool_tx_v3->GetWitnessHash().ToString())};
+        BOOST_CHECK(*SingleV3Checks(tx_v2_from_v3, *ancestors_v2_from_v3, empty_conflicts_set, GetVirtualTransactionSize(*tx_v2_from_v3)) == expected_error_str);
+
+        Package package_v3_v2{mempool_tx_v3, tx_v2_from_v3};
+        BOOST_CHECK_EQUAL(*PackageV3Checks(tx_v2_from_v3, GetVirtualTransactionSize(*tx_v2_from_v3), package_v3_v2, empty_ancestors), expected_error_str);
+
+        // mempool_tx_v3  mempool_tx_v2
+        //            ^    ^
+        //    tx_v2_from_v2_and_v3
+        auto tx_v2_from_v2_and_v3 = make_tx({COutPoint{mempool_tx_v3->GetHash(), 0}, COutPoint{mempool_tx_v2->GetHash(), 0}}, /*version=*/2);
+        auto ancestors_v2_from_both{pool.CalculateMemPoolAncestors(entry.FromTx(tx_v2_from_v2_and_v3), m_limits)};
+        const auto expected_error_str_2{strprintf("non-v3 tx %s (wtxid=%s) cannot spend from v3 tx %s (wtxid=%s)",
+            tx_v2_from_v2_and_v3->GetHash().ToString(), tx_v2_from_v2_and_v3->GetWitnessHash().ToString(),
+            mempool_tx_v3->GetHash().ToString(), mempool_tx_v3->GetWitnessHash().ToString())};
+        BOOST_CHECK(*SingleV3Checks(tx_v2_from_v2_and_v3, *ancestors_v2_from_both, empty_conflicts_set, GetVirtualTransactionSize(*tx_v2_from_v2_and_v3))
+                    == expected_error_str_2);
+
+        Package package_v3_v2_v2{mempool_tx_v3, mempool_tx_v2, tx_v2_from_v2_and_v3};
+        BOOST_CHECK_EQUAL(*PackageV3Checks(tx_v2_from_v2_and_v3, GetVirtualTransactionSize(*tx_v2_from_v2_and_v3), package_v3_v2_v2, empty_ancestors), expected_error_str_2);
+    }
+
+    // V3 cannot spend from an unconfirmed non-v3 transaction.
+    {
+        // mempool_tx_v2
+        //      ^
+        // tx_v3_from_v2
+        auto tx_v3_from_v2 = make_tx({COutPoint{mempool_tx_v2->GetHash(), 0}}, /*version=*/3);
+        auto ancestors_v3_from_v2{pool.CalculateMemPoolAncestors(entry.FromTx(tx_v3_from_v2), m_limits)};
+        const auto expected_error_str{strprintf("v3 tx %s (wtxid=%s) cannot spend from non-v3 tx %s (wtxid=%s)",
+            tx_v3_from_v2->GetHash().ToString(), tx_v3_from_v2->GetWitnessHash().ToString(),
+            mempool_tx_v2->GetHash().ToString(), mempool_tx_v2->GetWitnessHash().ToString())};
+        BOOST_CHECK(*SingleV3Checks(tx_v3_from_v2, *ancestors_v3_from_v2,  empty_conflicts_set, GetVirtualTransactionSize(*tx_v3_from_v2)) == expected_error_str);
+
+        Package package_v2_v3{mempool_tx_v2, tx_v3_from_v2};
+        BOOST_CHECK_EQUAL(*PackageV3Checks(tx_v3_from_v2, GetVirtualTransactionSize(*tx_v3_from_v2), package_v2_v3, empty_ancestors), expected_error_str);
+
+        // mempool_tx_v3  mempool_tx_v2
+        //            ^    ^
+        //    tx_v3_from_v2_and_v3
+        auto tx_v3_from_v2_and_v3 = make_tx({COutPoint{mempool_tx_v3->GetHash(), 0}, COutPoint{mempool_tx_v2->GetHash(), 0}}, /*version=*/3);
+        auto ancestors_v3_from_both{pool.CalculateMemPoolAncestors(entry.FromTx(tx_v3_from_v2_and_v3), m_limits)};
+        const auto expected_error_str_2{strprintf("v3 tx %s (wtxid=%s) cannot spend from non-v3 tx %s (wtxid=%s)",
+            tx_v3_from_v2_and_v3->GetHash().ToString(), tx_v3_from_v2_and_v3->GetWitnessHash().ToString(),
+            mempool_tx_v2->GetHash().ToString(), mempool_tx_v2->GetWitnessHash().ToString())};
+        BOOST_CHECK(*SingleV3Checks(tx_v3_from_v2_and_v3, *ancestors_v3_from_both, empty_conflicts_set, GetVirtualTransactionSize(*tx_v3_from_v2_and_v3))
+                    == expected_error_str_2);
+
+        // tx_v3_from_v2_and_v3 also violates V3_ANCESTOR_LIMIT.
+        const auto expected_error_str_3{strprintf("tx %s (wtxid=%s) would have too many ancestors",
+            tx_v3_from_v2_and_v3->GetHash().ToString(), tx_v3_from_v2_and_v3->GetWitnessHash().ToString())};
+        Package package_v3_v2_v3{mempool_tx_v3, mempool_tx_v2, tx_v3_from_v2_and_v3};
+        BOOST_CHECK_EQUAL(*PackageV3Checks(tx_v3_from_v2_and_v3, GetVirtualTransactionSize(*tx_v3_from_v2_and_v3), package_v3_v2_v3, empty_ancestors), expected_error_str_3);
+    }
+    // V3 from V3 is ok, and non-V3 from non-V3 is ok.
+    {
+        // mempool_tx_v3
+        //      ^
+        // tx_v3_from_v3
+        auto tx_v3_from_v3 = make_tx({COutPoint{mempool_tx_v3->GetHash(), 0}}, /*version=*/3);
+        auto ancestors_v3{pool.CalculateMemPoolAncestors(entry.FromTx(tx_v3_from_v3), m_limits)};
+        BOOST_CHECK(SingleV3Checks(tx_v3_from_v3, *ancestors_v3, empty_conflicts_set, GetVirtualTransactionSize(*tx_v3_from_v3))
+                    == std::nullopt);
+
+        Package package_v3_v3{mempool_tx_v3, tx_v3_from_v3};
+        BOOST_CHECK(PackageV3Checks(tx_v3_from_v3, GetVirtualTransactionSize(*tx_v3_from_v3), package_v3_v3, empty_ancestors) == std::nullopt);
+
+        // mempool_tx_v2
+        //      ^
+        // tx_v2_from_v2
+        auto tx_v2_from_v2 = make_tx({COutPoint{mempool_tx_v2->GetHash(), 0}}, /*version=*/2);
+        auto ancestors_v2{pool.CalculateMemPoolAncestors(entry.FromTx(tx_v2_from_v2), m_limits)};
+        BOOST_CHECK(SingleV3Checks(tx_v2_from_v2, *ancestors_v2, empty_conflicts_set, GetVirtualTransactionSize(*tx_v2_from_v2))
+                    == std::nullopt);
+
+        Package package_v2_v2{mempool_tx_v2, tx_v2_from_v2};
+        BOOST_CHECK(PackageV3Checks(tx_v2_from_v2, GetVirtualTransactionSize(*tx_v2_from_v2), package_v2_v2, empty_ancestors) == std::nullopt);
+    }
+
+    // Tx spending v3 cannot have too many mempool ancestors
+    // Configuration where the tx has multiple direct parents.
+    {
+        Package package_multi_parents;
+        std::vector<COutPoint> mempool_outpoints;
+        mempool_outpoints.emplace_back(mempool_tx_v3->GetHash(), 0);
+        package_multi_parents.emplace_back(mempool_tx_v3);
+        for (size_t i{0}; i < 2; ++i) {
+            auto mempool_tx = make_tx(random_outpoints(i + 1), /*version=*/3);
+            pool.addUnchecked(entry.FromTx(mempool_tx));
+            mempool_outpoints.emplace_back(mempool_tx->GetHash(), 0);
+            package_multi_parents.emplace_back(mempool_tx);
+        }
+        auto tx_v3_multi_parent = make_tx(mempool_outpoints, /*version=*/3);
+        package_multi_parents.emplace_back(tx_v3_multi_parent);
+        auto ancestors{pool.CalculateMemPoolAncestors(entry.FromTx(tx_v3_multi_parent), m_limits)};
+        BOOST_CHECK_EQUAL(ancestors->size(), 3);
+        const auto expected_error_str{strprintf("tx %s (wtxid=%s) would have too many ancestors",
+            tx_v3_multi_parent->GetHash().ToString(), tx_v3_multi_parent->GetWitnessHash().ToString())};
+        BOOST_CHECK_EQUAL(*SingleV3Checks(tx_v3_multi_parent, *ancestors, empty_conflicts_set, GetVirtualTransactionSize(*tx_v3_multi_parent)),
+                          expected_error_str);
+
+        BOOST_CHECK_EQUAL(*PackageV3Checks(tx_v3_multi_parent, GetVirtualTransactionSize(*tx_v3_multi_parent), package_multi_parents, empty_ancestors),
+                          expected_error_str);
+    }
+
+    // Configuration where the tx is in a multi-generation chain.
+    {
+        Package package_multi_gen;
+        CTransactionRef middle_tx;
+        auto last_outpoint{random_outpoints(1)[0]};
+        for (size_t i{0}; i < 2; ++i) {
+            auto mempool_tx = make_tx({last_outpoint}, /*version=*/3);
+            pool.addUnchecked(entry.FromTx(mempool_tx));
+            last_outpoint = COutPoint{mempool_tx->GetHash(), 0};
+            package_multi_gen.emplace_back(mempool_tx);
+            if (i == 1) middle_tx = mempool_tx;
+        }
+        auto tx_v3_multi_gen = make_tx({last_outpoint}, /*version=*/3);
+        package_multi_gen.emplace_back(tx_v3_multi_gen);
+        auto ancestors{pool.CalculateMemPoolAncestors(entry.FromTx(tx_v3_multi_gen), m_limits)};
+        const auto expected_error_str{strprintf("tx %s (wtxid=%s) would have too many ancestors",
+            tx_v3_multi_gen->GetHash().ToString(), tx_v3_multi_gen->GetWitnessHash().ToString())};
+        BOOST_CHECK_EQUAL(*SingleV3Checks(tx_v3_multi_gen, *ancestors, empty_conflicts_set, GetVirtualTransactionSize(*tx_v3_multi_gen)),
+                          expected_error_str);
+
+        // Middle tx is what triggers a failure for the grandchild:
+        BOOST_CHECK_EQUAL(*PackageV3Checks(middle_tx, GetVirtualTransactionSize(*middle_tx), package_multi_gen, empty_ancestors), expected_error_str);
+        BOOST_CHECK(PackageV3Checks(tx_v3_multi_gen, GetVirtualTransactionSize(*tx_v3_multi_gen), package_multi_gen, empty_ancestors) == std::nullopt);
+    }
+
+    // Tx spending v3 cannot be too large in virtual size.
+    auto many_inputs{random_outpoints(100)};
+    many_inputs.emplace_back(mempool_tx_v3->GetHash(), 0);
+    {
+        auto tx_v3_child_big = make_tx(many_inputs, /*version=*/3);
+        const auto vsize{GetVirtualTransactionSize(*tx_v3_child_big)};
+        auto ancestors{pool.CalculateMemPoolAncestors(entry.FromTx(tx_v3_child_big), m_limits)};
+        const auto expected_error_str{strprintf("v3 child tx %s (wtxid=%s) is too big: %u > %u virtual bytes",
+            tx_v3_child_big->GetHash().ToString(), tx_v3_child_big->GetWitnessHash().ToString(), vsize, V3_CHILD_MAX_VSIZE)};
+        BOOST_CHECK_EQUAL(*SingleV3Checks(tx_v3_child_big, *ancestors, empty_conflicts_set, GetVirtualTransactionSize(*tx_v3_child_big)),
+                          expected_error_str);
+
+        Package package_child_big{mempool_tx_v3, tx_v3_child_big};
+        BOOST_CHECK_EQUAL(*PackageV3Checks(tx_v3_child_big, GetVirtualTransactionSize(*tx_v3_child_big), package_child_big, empty_ancestors),
+                          expected_error_str);
+    }
+
+    // Tx spending v3 cannot have too many sigops.
+    // This child has 10 P2WSH multisig inputs.
+    auto multisig_outpoints{random_outpoints(10)};
+    multisig_outpoints.emplace_back(mempool_tx_v3->GetHash(), 0);
+    auto keys{random_keys(2)};
+    CScript script_multisig;
+    script_multisig << OP_1;
+    for (const auto& key : keys) {
+        script_multisig << ToByteVector(key);
+    }
+    script_multisig << OP_2 << OP_CHECKMULTISIG;
+    {
+        CMutableTransaction mtx_many_sigops = CMutableTransaction{};
+        mtx_many_sigops.nVersion = 3;
+        for (const auto& outpoint : multisig_outpoints) {
+            mtx_many_sigops.vin.emplace_back(outpoint);
+            mtx_many_sigops.vin.back().scriptWitness.stack.emplace_back(script_multisig.begin(), script_multisig.end());
+        }
+        mtx_many_sigops.vout.resize(1);
+        mtx_many_sigops.vout.back().scriptPubKey = CScript() << OP_TRUE;
+        mtx_many_sigops.vout.back().nValue = 10000;
+        auto tx_many_sigops{MakeTransactionRef(mtx_many_sigops)};
+
+        auto ancestors{pool.CalculateMemPoolAncestors(entry.FromTx(tx_many_sigops), m_limits)};
+        // legacy uses fAccurate = false, and the maximum number of multisig keys is used
+        const int64_t total_sigops{static_cast<int64_t>(tx_many_sigops->vin.size()) * static_cast<int64_t>(script_multisig.GetSigOpCount(/*fAccurate=*/false))};
+        BOOST_CHECK_EQUAL(total_sigops, tx_many_sigops->vin.size() * MAX_PUBKEYS_PER_MULTISIG);
+        const int64_t bip141_vsize{GetVirtualTransactionSize(*tx_many_sigops)};
+        // Weight limit is not reached...
+        BOOST_CHECK(SingleV3Checks(tx_many_sigops, *ancestors, empty_conflicts_set, bip141_vsize) == std::nullopt);
+        // ...but sigop limit is.
+        const auto expected_error_str{strprintf("v3 child tx %s (wtxid=%s) is too big: %u > %u virtual bytes",
+            tx_many_sigops->GetHash().ToString(), tx_many_sigops->GetWitnessHash().ToString(),
+            total_sigops * DEFAULT_BYTES_PER_SIGOP / WITNESS_SCALE_FACTOR, V3_CHILD_MAX_VSIZE)};
+        BOOST_CHECK_EQUAL(*SingleV3Checks(tx_many_sigops, *ancestors, empty_conflicts_set,
+                                        GetVirtualTransactionSize(*tx_many_sigops, /*nSigOpCost=*/total_sigops, /*bytes_per_sigop=*/ DEFAULT_BYTES_PER_SIGOP)),
+                    expected_error_str);
+
+        Package package_child_sigops{mempool_tx_v3, tx_many_sigops};
+        BOOST_CHECK_EQUAL(*PackageV3Checks(tx_many_sigops, total_sigops * DEFAULT_BYTES_PER_SIGOP / WITNESS_SCALE_FACTOR, package_child_sigops, empty_ancestors),
+                          expected_error_str);
+    }
+
+    // Parent + child with v3 in the mempool. Child is allowed as long as it is under V3_CHILD_MAX_VSIZE.
+    auto tx_mempool_v3_child = make_tx({COutPoint{mempool_tx_v3->GetHash(), 0}}, /*version=*/3);
+    {
+        BOOST_CHECK(GetTransactionWeight(*tx_mempool_v3_child) <= V3_CHILD_MAX_VSIZE * WITNESS_SCALE_FACTOR);
+        auto ancestors{pool.CalculateMemPoolAncestors(entry.FromTx(tx_mempool_v3_child), m_limits)};
+        BOOST_CHECK(SingleV3Checks(tx_mempool_v3_child, *ancestors, empty_conflicts_set, GetVirtualTransactionSize(*tx_mempool_v3_child)) == std::nullopt);
+        pool.addUnchecked(entry.FromTx(tx_mempool_v3_child));
+
+        Package package_v3_1p1c{mempool_tx_v3, tx_mempool_v3_child};
+        BOOST_CHECK(PackageV3Checks(tx_mempool_v3_child, GetVirtualTransactionSize(*tx_mempool_v3_child), package_v3_1p1c, empty_ancestors) == std::nullopt);
+    }
+
+    // A v3 transaction cannot have more than 1 descendant.
+    // Configuration where tx has multiple direct children.
+    {
+        auto tx_v3_child2 = make_tx({COutPoint{mempool_tx_v3->GetHash(), 1}}, /*version=*/3);
+        auto ancestors{pool.CalculateMemPoolAncestors(entry.FromTx(tx_v3_child2), m_limits)};
+        const auto expected_error_str{strprintf("tx %s (wtxid=%s) would exceed descendant count limit",
+            mempool_tx_v3->GetHash().ToString(), mempool_tx_v3->GetWitnessHash().ToString())};
+        BOOST_CHECK_EQUAL(*SingleV3Checks(tx_v3_child2, *ancestors, empty_conflicts_set, GetVirtualTransactionSize(*tx_v3_child2)),
+                          expected_error_str);
+        // If replacing the child, make sure there is no double-counting.
+        BOOST_CHECK(SingleV3Checks(tx_v3_child2, *ancestors, {tx_mempool_v3_child->GetHash()}, GetVirtualTransactionSize(*tx_v3_child2))
+                    == std::nullopt);
+
+        Package package_v3_1p2c{mempool_tx_v3, tx_mempool_v3_child, tx_v3_child2};
+        BOOST_CHECK_EQUAL(*PackageV3Checks(tx_v3_child2, GetVirtualTransactionSize(*tx_v3_child2), package_v3_1p2c, empty_ancestors),
+                          expected_error_str);
+    }
+
+    // Configuration where tx has multiple generations of descendants is not tested because that is
+    // equivalent to the tx with multiple generations of ancestors.
+}
+
 BOOST_AUTO_TEST_SUITE_END()