refactor: Replace feeDelta by m_modified_fee

* feeDelta tracked the delta (to be applied on top of the actual fee)
* m_modified_fee tracks the actual fee with the delta included
* Instead of passing in the new total delta to the Updater, pass in by
  how much the total delta should be modified.

This is needed for the next commit, but makes sense on its own because
the same is done by UpdateDescendantState and UpdateAncestorState.

src/txmempool.cpp

@@ -14,6 +14,7 @@
 #include <policy/policy.h>
 #include <policy/settings.h>
 #include <reverse_iterator.h>
+#include <util/check.h>
 #include <util/moneystr.h>
 #include <util/system.h>
 #include <util/time.h>
@@ -82,6 +83,7 @@ CTxMemPoolEntry::CTxMemPoolEntry(const CTransactionRef& tx, CAmount fee,
       entryHeight{entry_height},
       spendsCoinbase{spends_coinbase},
       sigOpCost{sigops_cost},
+      m_modified_fee{nFee},
       lockPoints{lp},
       nSizeWithDescendants{GetTxSize()},
       nModFeesWithDescendants{nFee},
@@ -89,11 +91,11 @@ CTxMemPoolEntry::CTxMemPoolEntry(const CTransactionRef& tx, CAmount fee,
       nModFeesWithAncestors{nFee},
       nSigOpCostWithAncestors{sigOpCost} {}
 
-void CTxMemPoolEntry::UpdateFeeDelta(CAmount newFeeDelta)
+void CTxMemPoolEntry::UpdateModifiedFee(CAmount fee_diff)
 {
-    nModFeesWithDescendants += newFeeDelta - feeDelta;
+    nModFeesWithDescendants += fee_diff;
-    nModFeesWithAncestors += newFeeDelta - feeDelta;
+    nModFeesWithAncestors += fee_diff;
-    feeDelta = newFeeDelta;
+    m_modified_fee += fee_diff;
 }
 
 void CTxMemPoolEntry::UpdateLockPoints(const LockPoints& lp)
@@ -483,8 +485,10 @@ void CTxMemPool::addUnchecked(const CTxMemPoolEntry &entry, setEntries &setAnces
     // Update transaction for any feeDelta created by PrioritiseTransaction
     CAmount delta{0};
     ApplyDelta(entry.GetTx().GetHash(), delta);
+    // The following call to UpdateModifiedFee assumes no previous fee modifications
+    Assume(entry.GetFee() == entry.GetModifiedFee());
     if (delta) {
-        mapTx.modify(newit, [&delta](CTxMemPoolEntry& e) { e.UpdateFeeDelta(delta); });
+        mapTx.modify(newit, [&delta](CTxMemPoolEntry& e) { e.UpdateModifiedFee(delta); });
     }
 
     // Update cachedInnerUsage to include contained transaction's usage.
@@ -920,7 +924,7 @@ void CTxMemPool::PrioritiseTransaction(const uint256& hash, const CAmount& nFeeD
         delta += nFeeDelta;
         txiter it = mapTx.find(hash);
         if (it != mapTx.end()) {
-            mapTx.modify(it, [&delta](CTxMemPoolEntry& e) { e.UpdateFeeDelta(delta); });
+            mapTx.modify(it, [&nFeeDelta](CTxMemPoolEntry& e) { e.UpdateModifiedFee(nFeeDelta); });
             // Now update all ancestors' modified fees with descendants
             setEntries setAncestors;
             uint64_t nNoLimit = std::numeric_limits<uint64_t>::max();

src/txmempool.h

@@ -101,7 +101,7 @@ private:
     const unsigned int entryHeight; //!< Chain height when entering the mempool
     const bool spendsCoinbase;      //!< keep track of transactions that spend a coinbase
     const int64_t sigOpCost;        //!< Total sigop cost
-    CAmount feeDelta{0};            //!< Used for determining the priority of the transaction for mining in a block
+    CAmount m_modified_fee;         //!< Used for determining the priority of the transaction for mining in a block
     LockPoints lockPoints;     //!< Track the height and time at which tx was final
 
     // Information about descendants of this transaction that are in the
@@ -131,7 +131,7 @@ public:
     std::chrono::seconds GetTime() const { return std::chrono::seconds{nTime}; }
     unsigned int GetHeight() const { return entryHeight; }
     int64_t GetSigOpCost() const { return sigOpCost; }
-    CAmount GetModifiedFee() const { return nFee + feeDelta; }
+    CAmount GetModifiedFee() const { return m_modified_fee; }
     size_t DynamicMemoryUsage() const { return nUsageSize; }
     const LockPoints& GetLockPoints() const { return lockPoints; }
 
@@ -139,9 +139,8 @@ public:
     void UpdateDescendantState(int64_t modifySize, CAmount modifyFee, int64_t modifyCount);
     // Adjusts the ancestor state
     void UpdateAncestorState(int64_t modifySize, CAmount modifyFee, int64_t modifyCount, int64_t modifySigOps);
-    // Updates the fee delta used for mining priority score, and the
+    // Updates the modified fees with descendants/ancestors.
-    // modified fees with descendants/ancestors.
+    void UpdateModifiedFee(CAmount fee_diff);
-    void UpdateFeeDelta(CAmount newFeeDelta);
     // Update the LockPoints after a reorg
     void UpdateLockPoints(const LockPoints& lp);