Merge bitcoin/bitcoin#29242: Mempool util: Add RBF diagram checks for single chunks against clusters of size 2

7295986778 Unit tests for CalculateFeerateDiagramsForRBF (Greg Sanders)
b767e6bd47 test: unit test for ImprovesFeerateDiagram (Greg Sanders)
7e89b659e1 Add fuzz test for FeeFrac (Greg Sanders)
4d6528a3d6 fuzz: fuzz diagram creation and comparison (Greg Sanders)
e9c5aeb11d test: Add tests for CompareFeerateDiagram and CheckConflictTopology (Greg Sanders)
588a98dccc fuzz: Add fuzz target for ImprovesFeerateDiagram (Greg Sanders)
2079b80854 Implement ImprovesFeerateDiagram (Greg Sanders)
66d966dcfa Add FeeFrac unit tests (Greg Sanders)
ce8e22542e Add FeeFrac utils (Greg Sanders)

Pull request description:

  This is a smaller piece of https://github.com/bitcoin/bitcoin/pull/28984 broken off for easier review.

  Up to date explanation of diagram checks are here: https://delvingbitcoin.org/t/mempool-incentive-compatibility/553

  This infrastructure has two near term applications prior to cluster mempool:
  1) Limited Package RBF(https://github.com/bitcoin/bitcoin/pull/28984): We want to allow package RBF only when we know it improves the mempool. This narrowly scoped functionality allows use with v3-like topologies, and will be expanded at some point post-cluster mempool when diagram checks can be done efficiently against bounded cluster sizes.
  2) Replacement for single tx RBF(in a cluster size of up to two) against conflicts of up to cluster size two. `ImprovesFeerateDiagram` interface will have to change for this use-case, which is a future direction to solve certain pins and improve mempool incentive compatibility: https://delvingbitcoin.org/t/ephemeral-anchors-and-mev/383#diagram-checks-fix-this-3

  And longer-term, this would be the proposed way we would compute incentive compatibility for all conflicts, post-cluster mempool.

ACKs for top commit:
  sipa:
    utACK 7295986778
  glozow:
    code review ACK 7295986778
  murchandamus:
    utACK 7295986778
  ismaelsadeeq:
    Re-ACK 7295986778
  willcl-ark:
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  sdaftuar:
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Tree-SHA512: 79593e5a087801c06f06cc8b73aa3e7b96ab938d3b90f5d229c4e4bfca887a77b447605c49aa5eb7ddcead85706c534ac5eb6146ae2396af678f4beaaa5bea8e

src/policy/rbf.cpp

@@ -19,6 +19,8 @@
 #include <limits>
 #include <vector>
 
+#include <compare>
+
 RBFTransactionState IsRBFOptIn(const CTransaction& tx, const CTxMemPool& pool)
 {
     AssertLockHeld(pool.cs);
@@ -181,3 +183,24 @@ std::optional<std::string> PaysForRBF(CAmount original_fees,
     }
     return std::nullopt;
 }
+
+std::optional<std::pair<DiagramCheckError, std::string>> ImprovesFeerateDiagram(CTxMemPool& pool,
+                                                const CTxMemPool::setEntries& direct_conflicts,
+                                                const CTxMemPool::setEntries& all_conflicts,
+                                                CAmount replacement_fees,
+                                                int64_t replacement_vsize)
+{
+    // Require that the replacement strictly improve the mempool's feerate diagram.
+    std::vector<FeeFrac> old_diagram, new_diagram;
+
+    const auto diagram_results{pool.CalculateFeerateDiagramsForRBF(replacement_fees, replacement_vsize, direct_conflicts, all_conflicts)};
+
+    if (!diagram_results.has_value()) {
+        return std::make_pair(DiagramCheckError::UNCALCULABLE, util::ErrorString(diagram_results).original);
+    }
+
+    if (!std::is_gt(CompareFeerateDiagram(diagram_results.value().second, diagram_results.value().first))) {
+        return std::make_pair(DiagramCheckError::FAILURE, "insufficient feerate: does not improve feerate diagram");
+    }
+    return std::nullopt;
+}

src/policy/rbf.h

@@ -9,7 +9,9 @@
 #include <primitives/transaction.h>
 #include <threadsafety.h>
 #include <txmempool.h>
+#include <util/feefrac.h>
 
+#include <compare>
 #include <cstddef>
 #include <cstdint>
 #include <optional>
@@ -33,6 +35,13 @@ enum class RBFTransactionState {
     FINAL,
 };
 
+enum class DiagramCheckError {
+    /** Unable to calculate due to topology or other reason */
+    UNCALCULABLE,
+    /** New diagram wasn't strictly superior  */
+    FAILURE,
+};
+
 /**
  * Determine whether an unconfirmed transaction is signaling opt-in to RBF
  * according to BIP 125
@@ -106,4 +115,21 @@ std::optional<std::string> PaysForRBF(CAmount original_fees,
                                       CFeeRate relay_fee,
                                       const uint256& txid);
 
+/**
+ * The replacement transaction must improve the feerate diagram of the mempool.
+ * @param[in]   pool                The mempool.
+ * @param[in]   direct_conflicts    Set of in-mempool txids corresponding to the direct conflicts i.e.
+ *                                  input double-spends with the proposed transaction
+ * @param[in]   all_conflicts       Set of mempool entries corresponding to all transactions to be evicted
+ * @param[in]   replacement_fees    Fees of proposed replacement package
+ * @param[in]   replacement_vsize   Size of proposed replacement package
+ * @returns error type and string if mempool diagram doesn't improve, otherwise std::nullopt.
+ */
+std::optional<std::pair<DiagramCheckError, std::string>> ImprovesFeerateDiagram(CTxMemPool& pool,
+                                                const CTxMemPool::setEntries& direct_conflicts,
+                                                const CTxMemPool::setEntries& all_conflicts,
+                                                CAmount replacement_fees,
+                                                int64_t replacement_vsize)
+                                                EXCLUSIVE_LOCKS_REQUIRED(pool.cs);
+
 #endif // BITCOIN_POLICY_RBF_H