Add Mempool Expire function to remove old transactions

(note the 9x multiplier on (void*)'s for CTxMemPool::DynamicMemoryUsage
 was accidentally introduced in 5add7a7 but should have waited for this
 commit which adds the extra index)

src/init.cpp

@@ -320,6 +320,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageOpt("-dbcache=<n>", strprintf(_("Set database cache size in megabytes (%d to %d, default: %d)"), nMinDbCache, nMaxDbCache, nDefaultDbCache));
     strUsage += HelpMessageOpt("-loadblock=<file>", _("Imports blocks from external blk000??.dat file") + " " + _("on startup"));
     strUsage += HelpMessageOpt("-maxorphantx=<n>", strprintf(_("Keep at most <n> unconnectable transactions in memory (default: %u)"), DEFAULT_MAX_ORPHAN_TRANSACTIONS));
+    strUsage += HelpMessageOpt("-mempoolexpiry=<n>", strprintf(_("Do not keep transactions in the mempool longer than <n> hours (default: %u)"), DEFAULT_MEMPOOL_EXPIRY));
     strUsage += HelpMessageOpt("-par=<n>", strprintf(_("Set the number of script verification threads (%u to %d, 0 = auto, <0 = leave that many cores free, default: %d)"),
         -GetNumCores(), MAX_SCRIPTCHECK_THREADS, DEFAULT_SCRIPTCHECK_THREADS));
 #ifndef WIN32

src/main.h

@@ -51,6 +51,8 @@ static const unsigned int DEFAULT_ANCESTOR_SIZE_LIMIT = 900;
 static const unsigned int DEFAULT_DESCENDANT_LIMIT = 1000;
 /** Default for -limitdescendantsize, maximum kilobytes of in-mempool descendants */
 static const unsigned int DEFAULT_DESCENDANT_SIZE_LIMIT = 2500;
+/** Default for -mempoolexpiry, expiration time for mempool transactions in hours */
+static const unsigned int DEFAULT_MEMPOOL_EXPIRY = 72;
 /** The maximum size of a blk?????.dat file (since 0.8) */
 static const unsigned int MAX_BLOCKFILE_SIZE = 0x8000000; // 128 MiB
 /** The pre-allocation chunk size for blk?????.dat files (since 0.8) */

src/txmempool.cpp

@@ -792,6 +792,22 @@ void CTxMemPool::RemoveStaged(setEntries &stage) {
     }
 }
 
+int CTxMemPool::Expire(int64_t time) {
+    LOCK(cs);
+    indexed_transaction_set::nth_index<2>::type::iterator it = mapTx.get<2>().begin();
+    setEntries toremove;
+    while (it != mapTx.get<2>().end() && it->GetTime() < time) {
+        toremove.insert(mapTx.project<0>(it));
+        it++;
+    }
+    setEntries stage;
+    BOOST_FOREACH(txiter removeit, toremove) {
+        CalculateDescendants(removeit, stage);
+    }
+    RemoveStaged(stage);
+    return stage.size();
+}
+
 bool CTxMemPool::addUnchecked(const uint256&hash, const CTxMemPoolEntry &entry, bool fCurrentEstimate)
 {
     LOCK(cs);

src/txmempool.h

@@ -211,9 +211,10 @@ public:
  *
  * CTxMemPool::mapTx, and CTxMemPoolEntry bookkeeping:
  *
- * mapTx is a boost::multi_index that sorts the mempool on 2 criteria:
+ * mapTx is a boost::multi_index that sorts the mempool on 3 criteria:
  * - transaction hash
  * - feerate [we use max(feerate of tx, feerate of tx with all descendants)]
+ * - time in mempool
  *
  * Note: the term "descendant" refers to in-mempool transactions that depend on
  * this one, while "ancestor" refers to in-mempool transactions that a given
@@ -294,6 +295,11 @@ public:
             boost::multi_index::ordered_non_unique<
                 boost::multi_index::identity<CTxMemPoolEntry>,
                 CompareTxMemPoolEntryByFee
+            >,
+            // sorted by entry time
+            boost::multi_index::ordered_non_unique<
+                boost::multi_index::identity<CTxMemPoolEntry>,
+                CompareTxMemPoolEntryByEntryTime
             >
         >
     > indexed_transaction_set;
@@ -397,6 +403,9 @@ public:
      */
     bool CalculateMemPoolAncestors(const CTxMemPoolEntry &entry, setEntries &setAncestors, uint64_t limitAncestorCount, uint64_t limitAncestorSize, uint64_t limitDescendantCount, uint64_t limitDescendantSize, std::string &errString, bool fSearchForParents = true);
 
+    /** Expire all transaction (and their dependencies) in the mempool older than time. Return the number of removed transactions. */
+    int Expire(int64_t time);
+
     unsigned long size()
     {
         LOCK(cs);