Implement ImprovesFeerateDiagram

This new function takes the populated sets of
direct and all conflicts computed in the current
mempool, assuming the replacements are a single
chunk, and computes a diagram check.

The diagram check only works against cluster
sizes of 2 or less, and fails if it encounters
a different topology.

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

2 files changed, 49 insertions, 0 deletions

diff --git a/src/policy/rbf.cpp b/src/policy/rbf.cpp
index f0830d8f22..a2c6990657 100644
--- a/src/policy/rbf.cpp
+++ b/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;
+}
diff --git a/src/policy/rbf.h b/src/policy/rbf.h
index 5a33ed64a3..252fbec8e3 100644
--- a/src/policy/rbf.h
+++ b/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