MOVEONLY: fee checks (Rules 3 and 4) to policy/rbf

2 files changed, 40 insertions, 0 deletions

diff --git a/src/policy/rbf.cpp b/src/policy/rbf.cpp
index 5d1db6b58d..1e03e2331a 100644
--- a/src/policy/rbf.cpp
+++ b/src/policy/rbf.cpp
@@ -161,3 +161,29 @@ std::optional<std::string> PaysMoreThanConflicts(const CTxMemPool::setEntries& s
     return std::nullopt;
 }
 
+std::optional<std::string> PaysForRBF(CAmount nConflictingFees,
+                                      CAmount nModifiedFees,
+                                      size_t nSize,
+                                      const uint256& hash)
+{
+    // The replacement must pay greater fees than the transactions it
+    // replaces - if we did the bandwidth used by those conflicting
+    // transactions would not be paid for.
+    if (nModifiedFees < nConflictingFees)
+    {
+        return strprintf("rejecting replacement %s, less fees than conflicting txs; %s < %s",
+                         hash.ToString(), FormatMoney(nModifiedFees), FormatMoney(nConflictingFees));
+    }
+
+    // Finally in addition to paying more fees than the conflicts the
+    // new transaction must pay for its own bandwidth.
+    CAmount nDeltaFees = nModifiedFees - nConflictingFees;
+    if (nDeltaFees < ::incrementalRelayFee.GetFee(nSize))
+    {
+        return strprintf("rejecting replacement %s, not enough additional fees to relay; %s < %s",
+                         hash.ToString(),
+                         FormatMoney(nDeltaFees),
+                         FormatMoney(::incrementalRelayFee.GetFee(nSize)));
+    }
+    return std::nullopt;
+}
diff --git a/src/policy/rbf.h b/src/policy/rbf.h
index 2c548152b5..d32ba010fb 100644
--- a/src/policy/rbf.h
+++ b/src/policy/rbf.h
@@ -78,4 +78,18 @@ std::optional<std::string> EntriesAndTxidsDisjoint(const CTxMemPool::setEntries&
 std::optional<std::string> PaysMoreThanConflicts(const CTxMemPool::setEntries& setIterConflicting,
                                                  CFeeRate newFeeRate, const uint256& hash);
 
+/** Enforce BIP125 Rule #3 "The replacement transaction pays an absolute fee of at least the sum
+ * paid by the original transactions." Enforce BIP125 Rule #4 "The replacement transaction must also
+ * pay for its own bandwidth at or above the rate set by the node's minimum relay fee setting."
+ * @param[in]   nConflictingFees    Total modified fees of original transaction(s).
+ * @param[in]   nModifiedFees       Total modified fees of replacement transaction(s).
+ * @param[in]   nSize               Total virtual size of replacement transaction(s).
+ * @param[in]   hash                Transaction ID, included in the error message if violation occurs.
+ * @returns error string if fees are insufficient, otherwise std::nullopt.
+ */
+std::optional<std::string> PaysForRBF(CAmount nConflictingFees,
+                                      CAmount nModifiedFees,
+                                      size_t nSize,
+                                      const uint256& hash);
+
 #endif // BITCOIN_POLICY_RBF_H