[Mempool] Fix mempool limiting and replace-by-fee for PrioritiseTransaction

1) Fix mempool limiting for PrioritiseTransaction

Redo the feerate index to be based on mining score, rather than fee.

Update mempool_packages.py to test prioritisetransaction's effect on
package scores.

2) Update replace-by-fee logic to use fee deltas

3) Use fee deltas for determining mempool acceptance

4) Remove GetMinRelayFee

One test in AcceptToMemoryPool was to compare a transaction's fee
agains the value returned by GetMinRelayFee. This value was zero for
all small transactions.  For larger transactions (between
DEFAULT_BLOCK_PRIORITY_SIZE and MAX_STANDARD_TX_SIZE), this function
was preventing low fee transactions from ever being accepted.

With this function removed, we will now allow transactions in that range
with fees (including modifications via PrioritiseTransaction) below
the minRelayTxFee, provided that they have sufficient priority.

Github-Pull: #7062
Rebased-From: eb306664e786ae43d539fde66f0fbe2a3e89d910 9ef2a25603c9ec4e44c4f45c6a5d4e4386ec86d3 27fae3484cdb21b0d24face833b966fce5926be5 901b01d674031f9aca717deeb372bafa160a24af

5 files changed, 69 insertions, 90 deletions

diff --git a/src/main.cpp b/src/main.cpp
index 9e29fad978..7bc025308b 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -800,32 +800,6 @@ void LimitMempoolSize(CTxMemPool& pool, size_t limit, unsigned long age) {
         pcoinsTip->Uncache(removed);
 }
 
-CAmount GetMinRelayFee(const CTransaction& tx, const CTxMemPool& pool, unsigned int nBytes, bool fAllowFree)
-{
-    uint256 hash = tx.GetHash();
-    double dPriorityDelta = 0;
-    CAmount nFeeDelta = 0;
-    pool.ApplyDeltas(hash, dPriorityDelta, nFeeDelta);
-    if (dPriorityDelta > 0 || nFeeDelta > 0)
-        return 0;
-
-    CAmount nMinFee = ::minRelayTxFee.GetFee(nBytes);
-
-    if (fAllowFree)
-    {
-        // There is a free transaction area in blocks created by most miners,
-        // * If we are relaying we allow transactions up to DEFAULT_BLOCK_PRIORITY_SIZE - 1000
-        //   to be considered to fall into this category. We don't want to encourage sending
-        //   multiple transactions instead of one big transaction to avoid fees.
-        if (nBytes < (DEFAULT_BLOCK_PRIORITY_SIZE - 1000))
-            nMinFee = 0;
-    }
-
-    if (!MoneyRange(nMinFee))
-        nMinFee = MAX_MONEY;
-    return nMinFee;
-}
-
 /** Convert CValidationState to a human-readable message for logging */
 std::string FormatStateMessage(const CValidationState &state)
 {
@@ -968,6 +942,11 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState &state, const C
 
         CAmount nValueOut = tx.GetValueOut();
         CAmount nFees = nValueIn-nValueOut;
+        // nModifiedFees includes any fee deltas from PrioritiseTransaction
+        CAmount nModifiedFees = nFees;
+        double nPriorityDummy = 0;
+        pool.ApplyDeltas(hash, nPriorityDummy, nModifiedFees);
+
         CAmount inChainInputValue;
         double dPriority = view.GetPriority(tx, chainActive.Height(), inChainInputValue);
 
@@ -985,16 +964,10 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState &state, const C
         CTxMemPoolEntry entry(tx, nFees, GetTime(), dPriority, chainActive.Height(), pool.HasNoInputsOf(tx), inChainInputValue, fSpendsCoinbase, nSigOps);
         unsigned int nSize = entry.GetTxSize();
 
-        // Don't accept it if it can't get into a block
-        CAmount txMinFee = GetMinRelayFee(tx, pool, nSize, true);
-        if (fLimitFree && nFees < txMinFee)
-            return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "insufficient fee", false,
-                strprintf("%d < %d", nFees, txMinFee));
-
         CAmount mempoolRejectFee = pool.GetMinFee(GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000).GetFee(nSize);
-        if (mempoolRejectFee > 0 && nFees < mempoolRejectFee) {
+        if (mempoolRejectFee > 0 && nModifiedFees < mempoolRejectFee) {
             return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "mempool min fee not met", false, strprintf("%d < %d", nFees, mempoolRejectFee));
-        } else if (GetBoolArg("-relaypriority", DEFAULT_RELAYPRIORITY) && nFees < ::minRelayTxFee.GetFee(nSize) && !AllowFree(entry.GetPriority(chainActive.Height() + 1))) {
+        } else if (GetBoolArg("-relaypriority", DEFAULT_RELAYPRIORITY) && nModifiedFees < ::minRelayTxFee.GetFee(nSize) && !AllowFree(entry.GetPriority(chainActive.Height() + 1))) {
             // Require that free transactions have sufficient priority to be mined in the next block.
             return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "insufficient priority");
         }
@@ -1002,7 +975,7 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState &state, const C
         // Continuously rate-limit free (really, very-low-fee) transactions
         // This mitigates 'penny-flooding' -- sending thousands of free transactions just to
         // be annoying or make others' transactions take longer to confirm.
-        if (fLimitFree && nFees < ::minRelayTxFee.GetFee(nSize))
+        if (fLimitFree && nModifiedFees < ::minRelayTxFee.GetFee(nSize))
         {
             static CCriticalSection csFreeLimiter;
             static double dFreeCount;
@@ -1067,7 +1040,7 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState &state, const C
         LOCK(pool.cs);
         if (setConflicts.size())
         {
-            CFeeRate newFeeRate(nFees, nSize);
+            CFeeRate newFeeRate(nModifiedFees, nSize);
             set<uint256> setConflictsParents;
             const int maxDescendantsToVisit = 100;
             CTxMemPool::setEntries setIterConflicting;
@@ -1110,7 +1083,7 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState &state, const C
                 // ignored when deciding whether or not to replace, we do
                 // require the replacement to pay more overall fees too,
                 // mitigating most cases.
-                CFeeRate oldFeeRate(mi->GetFee(), mi->GetTxSize());
+                CFeeRate oldFeeRate(mi->GetModifiedFee(), mi->GetTxSize());
                 if (newFeeRate <= oldFeeRate)
                 {
                     return state.DoS(0,
@@ -1138,7 +1111,7 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState &state, const C
                     pool.CalculateDescendants(it, allConflicting);
                 }
                 BOOST_FOREACH(CTxMemPool::txiter it, allConflicting) {
-                    nConflictingFees += it->GetFee();
+                    nConflictingFees += it->GetModifiedFee();
                     nConflictingSize += it->GetTxSize();
                 }
             } else {
@@ -1171,16 +1144,16 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState &state, const C
             // 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 (nFees < nConflictingFees)
+            if (nModifiedFees < nConflictingFees)
             {
                 return state.DoS(0, error("AcceptToMemoryPool: rejecting replacement %s, less fees than conflicting txs; %s < %s",
-                                          hash.ToString(), FormatMoney(nFees), FormatMoney(nConflictingFees)),
+                                          hash.ToString(), FormatMoney(nModifiedFees), FormatMoney(nConflictingFees)),
                                  REJECT_INSUFFICIENTFEE, "insufficient fee");
             }
 
             // Finally in addition to paying more fees than the conflicts the
             // new transaction must pay for its own bandwidth.
-            CAmount nDeltaFees = nFees - nConflictingFees;
+            CAmount nDeltaFees = nModifiedFees - nConflictingFees;
             if (nDeltaFees < ::minRelayTxFee.GetFee(nSize))
             {
                 return state.DoS(0,
@@ -1218,7 +1191,7 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState &state, const C
             LogPrint("mempool", "replacing tx %s with %s for %s BTC additional fees, %d delta bytes\n",
                     it->GetTx().GetHash().ToString(),
                     hash.ToString(),
-                    FormatMoney(nFees - nConflictingFees),
+                    FormatMoney(nModifiedFees - nConflictingFees),
                     (int)nSize - (int)nConflictingSize);
         }
         pool.RemoveStaged(allConflicting);
diff --git a/src/main.h b/src/main.h
index 25a0063873..7ae4893e07 100644
--- a/src/main.h
+++ b/src/main.h
@@ -300,8 +300,6 @@ struct CDiskTxPos : public CDiskBlockPos
 };
 
 
-CAmount GetMinRelayFee(const CTransaction& tx, unsigned int nBytes, bool fAllowFree);
-
 /** 
  * Count ECDSA signature operations the old-fashioned (pre-0.6) way
  * @return number of sigops this transaction's outputs will produce when spent
diff --git a/src/rpcblockchain.cpp b/src/rpcblockchain.cpp
index ee04636ce8..73e6f8029b 100644
--- a/src/rpcblockchain.cpp
+++ b/src/rpcblockchain.cpp
@@ -197,7 +197,7 @@ UniValue mempoolToJSON(bool fVerbose = false)
             info.push_back(Pair("currentpriority", e.GetPriority(chainActive.Height())));
             info.push_back(Pair("descendantcount", e.GetCountWithDescendants()));
             info.push_back(Pair("descendantsize", e.GetSizeWithDescendants()));
-            info.push_back(Pair("descendantfees", e.GetFeesWithDescendants()));
+            info.push_back(Pair("descendantfees", e.GetModFeesWithDescendants()));
             const CTransaction& tx = e.GetTx();
             set<string> setDepends;
             BOOST_FOREACH(const CTxIn& txin, tx.vin)
@@ -255,7 +255,7 @@ UniValue getrawmempool(const UniValue& params, bool fHelp)
             "    \"currentpriority\" : n,  (numeric) transaction priority now\n"
             "    \"descendantcount\" : n,  (numeric) number of in-mempool descendant transactions (including this one)\n"
             "    \"descendantsize\" : n,   (numeric) size of in-mempool descendants (including this one)\n"
-            "    \"descendantfees\" : n,   (numeric) fees of in-mempool descendants (including this one)\n"
+            "    \"descendantfees\" : n,   (numeric) modified fees (see above) of in-mempool descendants (including this one)\n"
             "    \"depends\" : [           (array) unconfirmed transactions used as inputs for this transaction\n"
             "        \"transactionid\",    (string) parent transaction id\n"
             "       ... ]\n"
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index fea5da8029..c72a1e8c19 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -33,7 +33,7 @@ CTxMemPoolEntry::CTxMemPoolEntry(const CTransaction& _tx, const CAmount& _nFee,
 
     nCountWithDescendants = 1;
     nSizeWithDescendants = nTxSize;
-    nFeesWithDescendants = nFee;
+    nModFeesWithDescendants = nFee;
     CAmount nValueIn = tx.GetValueOut()+nFee;
     assert(inChainInputValue <= nValueIn);
 
@@ -57,6 +57,7 @@ CTxMemPoolEntry::GetPriority(unsigned int currentHeight) const
 
 void CTxMemPoolEntry::UpdateFeeDelta(int64_t newFeeDelta)
 {
+    nModFeesWithDescendants += newFeeDelta - feeDelta;
     feeDelta = newFeeDelta;
 }
 
@@ -114,7 +115,7 @@ bool CTxMemPool::UpdateForDescendants(txiter updateIt, int maxDescendantsToVisit
     BOOST_FOREACH(txiter cit, setAllDescendants) {
         if (!setExclude.count(cit->GetTx().GetHash())) {
             modifySize += cit->GetTxSize();
-            modifyFee += cit->GetFee();
+            modifyFee += cit->GetModifiedFee();
             modifyCount++;
             cachedDescendants[updateIt].insert(cit);
         }
@@ -244,7 +245,7 @@ void CTxMemPool::UpdateAncestorsOf(bool add, txiter it, setEntries &setAncestors
     }
     const int64_t updateCount = (add ? 1 : -1);
     const int64_t updateSize = updateCount * it->GetTxSize();
-    const CAmount updateFee = updateCount * it->GetFee();
+    const CAmount updateFee = updateCount * it->GetModifiedFee();
     BOOST_FOREACH(txiter ancestorIt, setAncestors) {
         mapTx.modify(ancestorIt, update_descendant_state(updateSize, updateFee, updateCount));
     }
@@ -304,7 +305,7 @@ void CTxMemPoolEntry::SetDirty()
 {
     nCountWithDescendants = 0;
     nSizeWithDescendants = nTxSize;
-    nFeesWithDescendants = nFee;
+    nModFeesWithDescendants = GetModifiedFee();
 }
 
 void CTxMemPoolEntry::UpdateState(int64_t modifySize, CAmount modifyFee, int64_t modifyCount)
@@ -312,8 +313,7 @@ void CTxMemPoolEntry::UpdateState(int64_t modifySize, CAmount modifyFee, int64_t
     if (!IsDirty()) {
         nSizeWithDescendants += modifySize;
         assert(int64_t(nSizeWithDescendants) > 0);
-        nFeesWithDescendants += modifyFee;
-        assert(nFeesWithDescendants >= 0);
+        nModFeesWithDescendants += modifyFee;
         nCountWithDescendants += modifyCount;
         assert(int64_t(nCountWithDescendants) > 0);
     }
@@ -372,6 +372,17 @@ bool CTxMemPool::addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry,
     indexed_transaction_set::iterator newit = mapTx.insert(entry).first;
     mapLinks.insert(make_pair(newit, TxLinks()));
 
+    // Update transaction for any feeDelta created by PrioritiseTransaction
+    // TODO: refactor so that the fee delta is calculated before inserting
+    // into mapTx.
+    std::map<uint256, std::pair<double, CAmount> >::const_iterator pos = mapDeltas.find(hash);
+    if (pos != mapDeltas.end()) {
+        const std::pair<double, CAmount> &deltas = pos->second;
+        if (deltas.second) {
+            mapTx.modify(newit, update_fee_delta(deltas.second));
+        }
+    }
+
     // Update cachedInnerUsage to include contained transaction's usage.
     // (When we update the entry for in-mempool parents, memory usage will be
     // further updated.)
@@ -399,15 +410,6 @@ bool CTxMemPool::addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry,
     }
     UpdateAncestorsOf(true, newit, setAncestors);
 
-    // Update transaction's score for any feeDelta created by PrioritiseTransaction
-    std::map<uint256, std::pair<double, CAmount> >::const_iterator pos = mapDeltas.find(hash);
-    if (pos != mapDeltas.end()) {
-        const std::pair<double, CAmount> &deltas = pos->second;
-        if (deltas.second) {
-            mapTx.modify(newit, update_fee_delta(deltas.second));
-        }
-    }
-
     nTransactionsUpdated++;
     totalTxSize += entry.GetTxSize();
     minerPolicyEstimator->processTransaction(entry, fCurrentEstimate);
@@ -644,27 +646,24 @@ void CTxMemPool::check(const CCoinsViewCache *pcoins) const
         CTxMemPool::setEntries setChildrenCheck;
         std::map<COutPoint, CInPoint>::const_iterator iter = mapNextTx.lower_bound(COutPoint(it->GetTx().GetHash(), 0));
         int64_t childSizes = 0;
-        CAmount childFees = 0;
+        CAmount childModFee = 0;
         for (; iter != mapNextTx.end() && iter->first.hash == it->GetTx().GetHash(); ++iter) {
             txiter childit = mapTx.find(iter->second.ptx->GetHash());
             assert(childit != mapTx.end()); // mapNextTx points to in-mempool transactions
             if (setChildrenCheck.insert(childit).second) {
                 childSizes += childit->GetTxSize();
-                childFees += childit->GetFee();
+                childModFee += childit->GetModifiedFee();
             }
         }
         assert(setChildrenCheck == GetMemPoolChildren(it));
-        // Also check to make sure size/fees is greater than sum with immediate children.
+        // Also check to make sure size is greater than sum with immediate children.
         // just a sanity check, not definitive that this calc is correct...
-        // also check that the size is less than the size of the entire mempool.
         if (!it->IsDirty()) {
             assert(it->GetSizeWithDescendants() >= childSizes + it->GetTxSize());
-            assert(it->GetFeesWithDescendants() >= childFees + it->GetFee());
         } else {
             assert(it->GetSizeWithDescendants() == it->GetTxSize());
-            assert(it->GetFeesWithDescendants() == it->GetFee());
+            assert(it->GetModFeesWithDescendants() == it->GetModifiedFee());
         }
-        assert(it->GetFeesWithDescendants() >= 0);
 
         if (fDependsWait)
             waitingOnDependants.push_back(&(*it));
@@ -788,6 +787,14 @@ void CTxMemPool::PrioritiseTransaction(const uint256 hash, const string strHash,
         txiter it = mapTx.find(hash);
         if (it != mapTx.end()) {
             mapTx.modify(it, update_fee_delta(deltas.second));
+            // Now update all ancestors' modified fees with descendants
+            setEntries setAncestors;
+            uint64_t nNoLimit = std::numeric_limits<uint64_t>::max();
+            std::string dummy;
+            CalculateMemPoolAncestors(*it, setAncestors, nNoLimit, nNoLimit, nNoLimit, nNoLimit, dummy, false);
+            BOOST_FOREACH(txiter ancestorIt, setAncestors) {
+                mapTx.modify(ancestorIt, update_descendant_state(0, nFeeDelta, 0));
+            }
         }
     }
     LogPrintf("PrioritiseTransaction: %s priority += %f, fee += %d\n", strHash, dPriorityDelta, FormatMoney(nFeeDelta));
@@ -956,7 +963,7 @@ void CTxMemPool::TrimToSize(size_t sizelimit, std::vector<uint256>* pvNoSpendsRe
         // "minimum reasonable fee rate" (ie some value under which we consider txn
         // to have 0 fee). This way, we don't allow txn to enter mempool with feerate
         // equal to txn which were removed with no block in between.
-        CFeeRate removed(it->GetFeesWithDescendants(), it->GetSizeWithDescendants());
+        CFeeRate removed(it->GetModFeesWithDescendants(), it->GetSizeWithDescendants());
         removed += minReasonableRelayFee;
         trackPackageRemoved(removed);
         maxFeeRateRemoved = std::max(maxFeeRateRemoved, removed);
diff --git a/src/txmempool.h b/src/txmempool.h
index 9203171868..4b726cc902 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -44,12 +44,12 @@ class CTxMemPool;
  * ("descendant" transactions).
  *
  * When a new entry is added to the mempool, we update the descendant state
- * (nCountWithDescendants, nSizeWithDescendants, and nFeesWithDescendants) for
+ * (nCountWithDescendants, nSizeWithDescendants, and nModFeesWithDescendants) for
  * all ancestors of the newly added transaction.
  *
  * If updating the descendant state is skipped, we can mark the entry as
- * "dirty", and set nSizeWithDescendants/nFeesWithDescendants to equal nTxSize/
- * nTxFee. (This can potentially happen during a reorg, where we limit the
+ * "dirty", and set nSizeWithDescendants/nModFeesWithDescendants to equal nTxSize/
+ * nFee+feeDelta. (This can potentially happen during a reorg, where we limit the
  * amount of work we're willing to do to avoid consuming too much CPU.)
  *
  */
@@ -74,11 +74,11 @@ private:
     // Information about descendants of this transaction that are in the
     // mempool; if we remove this transaction we must remove all of these
     // descendants as well.  if nCountWithDescendants is 0, treat this entry as
-    // dirty, and nSizeWithDescendants and nFeesWithDescendants will not be
+    // dirty, and nSizeWithDescendants and nModFeesWithDescendants will not be
     // correct.
     uint64_t nCountWithDescendants; //! number of descendant transactions
     uint64_t nSizeWithDescendants;  //! ... and size
-    CAmount nFeesWithDescendants;  //! ... and total fees (all including us)
+    CAmount nModFeesWithDescendants;  //! ... and total fees (all including us)
 
 public:
     CTxMemPoolEntry(const CTransaction& _tx, const CAmount& _nFee,
@@ -104,7 +104,8 @@ public:
 
     // Adjusts the descendant state, if this entry is not dirty.
     void UpdateState(int64_t modifySize, CAmount modifyFee, int64_t modifyCount);
-    // Updates the fee delta used for mining priority score
+    // Updates the fee delta used for mining priority score, and the
+    // modified fees with descendants.
     void UpdateFeeDelta(int64_t feeDelta);
 
     /** We can set the entry to be dirty if doing the full calculation of in-
@@ -116,7 +117,7 @@ public:
 
     uint64_t GetCountWithDescendants() const { return nCountWithDescendants; }
     uint64_t GetSizeWithDescendants() const { return nSizeWithDescendants; }
-    CAmount GetFeesWithDescendants() const { return nFeesWithDescendants; }
+    CAmount GetModFeesWithDescendants() const { return nModFeesWithDescendants; }
 
     bool GetSpendsCoinbase() const { return spendsCoinbase; }
 };
@@ -163,27 +164,27 @@ struct mempoolentry_txid
     }
 };
 
-/** \class CompareTxMemPoolEntryByFee
+/** \class CompareTxMemPoolEntryByDescendantScore
  *
- *  Sort an entry by max(feerate of entry's tx, feerate with all descendants).
+ *  Sort an entry by max(score/size of entry's tx, score/size with all descendants).
  */
-class CompareTxMemPoolEntryByFee
+class CompareTxMemPoolEntryByDescendantScore
 {
 public:
     bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b)
     {
-        bool fUseADescendants = UseDescendantFeeRate(a);
-        bool fUseBDescendants = UseDescendantFeeRate(b);
+        bool fUseADescendants = UseDescendantScore(a);
+        bool fUseBDescendants = UseDescendantScore(b);
 
-        double aFees = fUseADescendants ? a.GetFeesWithDescendants() : a.GetFee();
+        double aModFee = fUseADescendants ? a.GetModFeesWithDescendants() : a.GetModifiedFee();
         double aSize = fUseADescendants ? a.GetSizeWithDescendants() : a.GetTxSize();
 
-        double bFees = fUseBDescendants ? b.GetFeesWithDescendants() : b.GetFee();
+        double bModFee = fUseBDescendants ? b.GetModFeesWithDescendants() : b.GetModifiedFee();
         double bSize = fUseBDescendants ? b.GetSizeWithDescendants() : b.GetTxSize();
 
         // Avoid division by rewriting (a/b > c/d) as (a*d > c*b).
-        double f1 = aFees * bSize;
-        double f2 = aSize * bFees;
+        double f1 = aModFee * bSize;
+        double f2 = aSize * bModFee;
 
         if (f1 == f2) {
             return a.GetTime() >= b.GetTime();
@@ -191,11 +192,11 @@ public:
         return f1 < f2;
     }
 
-    // Calculate which feerate to use for an entry (avoiding division).
-    bool UseDescendantFeeRate(const CTxMemPoolEntry &a)
+    // Calculate which score to use for an entry (avoiding division).
+    bool UseDescendantScore(const CTxMemPoolEntry &a)
     {
-        double f1 = (double)a.GetFee() * a.GetSizeWithDescendants();
-        double f2 = (double)a.GetFeesWithDescendants() * a.GetTxSize();
+        double f1 = (double)a.GetModifiedFee() * a.GetSizeWithDescendants();
+        double f2 = (double)a.GetModFeesWithDescendants() * a.GetTxSize();
         return f2 > f1;
     }
 };
@@ -350,7 +351,7 @@ public:
             // sorted by fee rate
             boost::multi_index::ordered_non_unique<
                 boost::multi_index::identity<CTxMemPoolEntry>,
-                CompareTxMemPoolEntryByFee
+                CompareTxMemPoolEntryByDescendantScore
             >,
             // sorted by entry time
             boost::multi_index::ordered_non_unique<