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

Author: glozow

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;
+}

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

src/validation.cpp

@@ -798,32 +798,13 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         }
 
         // Check if it's economically rational to mine this transaction rather
-        // than the ones it replaces.
+        // than the ones it replaces. Enforce Rules #3 and #4.
         for (CTxMemPool::txiter it : allConflicting) {
             nConflictingFees += it->GetModifiedFee();
             nConflictingSize += it->GetTxSize();
         }
-
-        // 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 state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "insufficient fee",
-                    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 state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "insufficient fee",
-                    strprintf("rejecting replacement %s, not enough additional fees to relay; %s < %s",
-                        hash.ToString(),
-                        FormatMoney(nDeltaFees),
-                        FormatMoney(::incrementalRelayFee.GetFee(nSize))));
+        if (const auto err_string{PaysForRBF(nConflictingFees, nModifiedFees, nSize, hash)}) {
+            return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "insufficient fee", *err_string);
         }
     }
     return true;