Merge bitcoin/bitcoin#30272: doc: use TRUC instead of v3 and add release note

926b8e39dcbc0a3a8a75ef0a29bdca2bf738d746 [doc] add release note for TRUC (glozow)
19a9b90617419f68d0f1c90ee115b5220be99a16 use version=3 instead of v3 in debug strings (glozow)
881fac8e609be17eb71bd9a54c0284b304e2e2e2 scripted-diff: change names from V3 to TRUC (glozow)
a573dd261748d2a80560f73db08f7dca788c7fcf [doc] replace mentions of v3 with TRUC (glozow)
089b5757dff39a9a06cdb625aaced9beeb72958d rename mempool_accept_v3.py to mempool_truc.py (glozow)
f543852a89d93441645250c40c3980aeb0c3b664 rename policy/v3_policy.* to policy/truc_policy.* (glozow)

Pull request description:

  Adds a release note for TRUC policy which will be live in v28.0.

  For clarity, replaces mentions of "v3" with "TRUC" in most places. Suggested in
  - https://github.com/bitcoin/bitcoin/pull/29496#discussion_r1629749583
  - https://github.com/bitcoin/bitcoin/pull/29496#discussion_r1624500904

  I changed error strings from "v3-violation" to "TRUC-violation" but left v3 in the debug strings because I think it might be clearer for somebody who is debugging. Similarly, I left some variables unchanged because I think they're more descriptive this way, e.g. `tx_v3_from_v2_and_v3`. I'm happy to debate places that should or shouldn't be documented differently in this PR, whatever is clearest to everyone.

ACKs for top commit:
  instagibbs:
    reACK https://github.com/bitcoin/bitcoin/pull/30272/commits/926b8e39dcbc0a3a8a75ef0a29bdca2bf738d746
  achow101:
    ACK 926b8e39dcbc0a3a8a75ef0a29bdca2bf738d746
  ismaelsadeeq:
    Code review ACK 926b8e39dcbc0a3a8a75ef0a29bdca2bf738d746

Tree-SHA512: 16c88add0a29dc6d1236c4d45f34a17b850f6727b231953cbd52eb9f7268d1d802563eadfc8b7928c94ed3d7a615275dd103e57e81439ebf3ba2b12efa1e42af

1 files changed, 243 insertions, 0 deletions

diff --git a/src/policy/truc_policy.cpp b/src/policy/truc_policy.cpp
new file mode 100644
index 0000000000..69e8d5ed1d
--- /dev/null
+++ b/src/policy/truc_policy.cpp
@@ -0,0 +1,243 @@
+// 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/truc_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 PackageTRUCChecks: 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 PackageTRUCChecks, 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;
+    /** version used to check inheritance of TRUC and non-TRUC */
+    decltype(CTransaction::version) 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::version) 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> PackageTRUCChecks(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(TRUC_ANCESTOR_LIMIT == 2);
+    static_assert(TRUC_DESCENDANT_LIMIT == 2);
+
+    const auto in_package_parents{FindInPackageParents(package, ptx)};
+
+    // Now we have all ancestors, so we can start checking TRUC rules.
+    if (ptx->version == TRUC_VERSION) {
+        // SingleTRUCChecks should have checked this already.
+        if (!Assume(vsize <= TRUC_MAX_VSIZE)) {
+            return strprintf("version=3 tx %s (wtxid=%s) is too big: %u > %u virtual bytes",
+                             ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(), vsize, TRUC_MAX_VSIZE);
+        }
+
+        if (mempool_ancestors.size() + in_package_parents.size() + 1 > TRUC_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 TRUC child cannot be too large.
+            if (vsize > TRUC_CHILD_MAX_VSIZE) {
+                return strprintf("version=3 child tx %s (wtxid=%s) is too big: %u > %u virtual bytes",
+                                 ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(),
+                                 vsize, TRUC_CHILD_MAX_VSIZE);
+            }
+
+            // Exactly 1 parent exists, either in mempool or package. Find it.
+            const auto parent_info = [&] {
+                if (mempool_ancestors.size() > 0) {
+                    auto& mempool_parent = *mempool_ancestors.begin();
+                    return ParentInfo{mempool_parent->GetTx().GetHash(),
+                                      mempool_parent->GetTx().GetWitnessHash(),
+                                      mempool_parent->GetTx().version,
+                                      /*has_mempool_descendant=*/mempool_parent->GetCountWithDescendants() > 1};
+                } else {
+                    auto& parent_index = in_package_parents.front();
+                    auto& package_parent = package.at(parent_index);
+                    return ParentInfo{package_parent->GetHash(),
+                                      package_parent->GetWitnessHash(),
+                                      package_parent->version,
+                                      /*has_mempool_descendant=*/false};
+                }
+            }();
+
+            // If there is a parent, it must have the right version.
+            if (parent_info.m_version != TRUC_VERSION) {
+                return strprintf("version=3 tx %s (wtxid=%s) cannot spend from non-version=3 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 SingleTRUCChecks) 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());
+                    }
+                }
+            }
+
+            if (parent_info.m_has_mempool_descendant) {
+                return strprintf("tx %s (wtxid=%s) would exceed descendant count limit",
+                                parent_info.m_txid.ToString(), parent_info.m_wtxid.ToString());
+            }
+        }
+    } else {
+        // Non-TRUC transactions cannot have TRUC parents.
+        for (auto it : mempool_ancestors) {
+            if (it->GetTx().version == TRUC_VERSION) {
+                return strprintf("non-version=3 tx %s (wtxid=%s) cannot spend from version=3 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)->version == TRUC_VERSION) {
+                return strprintf("non-version=3 tx %s (wtxid=%s) cannot spend from version=3 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::pair<std::string, CTransactionRef>> SingleTRUCChecks(const CTransactionRef& ptx,
+                                          const CTxMemPool::setEntries& mempool_ancestors,
+                                          const std::set<Txid>& direct_conflicts,
+                                          int64_t vsize)
+{
+    // Check TRUC and non-TRUC inheritance.
+    for (const auto& entry : mempool_ancestors) {
+        if (ptx->version != TRUC_VERSION && entry->GetTx().version == TRUC_VERSION) {
+            return std::make_pair(strprintf("non-version=3 tx %s (wtxid=%s) cannot spend from version=3 tx %s (wtxid=%s)",
+                             ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(),
+                             entry->GetSharedTx()->GetHash().ToString(), entry->GetSharedTx()->GetWitnessHash().ToString()),
+                nullptr);
+        } else if (ptx->version == TRUC_VERSION && entry->GetTx().version != TRUC_VERSION) {
+            return std::make_pair(strprintf("version=3 tx %s (wtxid=%s) cannot spend from non-version=3 tx %s (wtxid=%s)",
+                             ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(),
+                             entry->GetSharedTx()->GetHash().ToString(), entry->GetSharedTx()->GetWitnessHash().ToString()),
+                nullptr);
+        }
+    }
+
+    // This function is specialized for these limits, and must be reimplemented if they ever change.
+    static_assert(TRUC_ANCESTOR_LIMIT == 2);
+    static_assert(TRUC_DESCENDANT_LIMIT == 2);
+
+    // The rest of the rules only apply to transactions with version=3.
+    if (ptx->version != TRUC_VERSION) return std::nullopt;
+
+    if (vsize > TRUC_MAX_VSIZE) {
+        return std::make_pair(strprintf("version=3 tx %s (wtxid=%s) is too big: %u > %u virtual bytes",
+                         ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(), vsize, TRUC_MAX_VSIZE),
+            nullptr);
+    }
+
+    // Check that TRUC_ANCESTOR_LIMIT would not be violated.
+    if (mempool_ancestors.size() + 1 > TRUC_ANCESTOR_LIMIT) {
+        return std::make_pair(strprintf("tx %s (wtxid=%s) would have too many ancestors",
+                         ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString()),
+            nullptr);
+    }
+
+    // Remaining checks only pertain to transactions with unconfirmed ancestors.
+    if (mempool_ancestors.size() > 0) {
+        // If this transaction spends TRUC parents, it cannot be too large.
+        if (vsize > TRUC_CHILD_MAX_VSIZE) {
+            return std::make_pair(strprintf("version=3 child tx %s (wtxid=%s) is too big: %u > %u virtual bytes",
+                             ptx->GetHash().ToString(), ptx->GetWitnessHash().ToString(), vsize, TRUC_CHILD_MAX_VSIZE),
+                nullptr);
+        }
+
+        // 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 TRUC transaction is a direct child, which makes sense because a
+        // TRUC 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 > TRUC_DESCENDANT_LIMIT && !child_will_be_replaced) {
+            // Allow sibling eviction for TRUC transaction: if another child already exists, even if
+            // we don't conflict inputs with it, consider evicting it under RBF rules. We rely on TRUC rules
+            // only permitting 1 descendant, as otherwise we would need to have logic for deciding
+            // which descendant to evict. Skip if this isn't true, e.g. if the transaction has
+            // multiple children or the sibling also has descendants due to a reorg.
+            const bool consider_sibling_eviction{parent_entry->GetCountWithDescendants() == 2 &&
+                children.begin()->get().GetCountWithAncestors() == 2};
+
+            // Return the sibling if its eviction can be considered. Provide the "descendant count
+            // limit" string either way, as the caller may decide not to do sibling eviction.
+            return std::make_pair(strprintf("tx %u (wtxid=%s) would exceed descendant count limit",
+                                            parent_entry->GetSharedTx()->GetHash().ToString(),
+                                            parent_entry->GetSharedTx()->GetWitnessHash().ToString()),
+                                  consider_sibling_eviction ?  children.begin()->get().GetSharedTx() : nullptr);
+        }
+    }
+    return std::nullopt;
+}