mempool: add notification for added/removed entries

Add notification signals to make it possible to subscribe to mempool
changes:

- NotifyEntryAdded(CTransactionRef)>
- NotifyEntryRemoved(CTransactionRef, MemPoolRemovalReason)>

Also add a mempool removal reason enumeration, which is passed to the
removed notification based on why the transaction was removed from
the mempool.

src/txmempool.cpp

@@ -393,6 +393,7 @@ void CTxMemPool::AddTransactionsUpdated(unsigned int n)
 
 bool CTxMemPool::addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry, setEntries &setAncestors, bool validFeeEstimate)
 {
+    NotifyEntryAdded(entry.GetSharedTx());
     // Add to memory pool without checking anything.
     // Used by main.cpp AcceptToMemoryPool(), which DOES do
     // all the appropriate checks.
@@ -449,8 +450,9 @@ bool CTxMemPool::addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry,
     return true;
 }
 
-void CTxMemPool::removeUnchecked(txiter it)
+void CTxMemPool::removeUnchecked(txiter it, MemPoolRemovalReason reason)
 {
+    NotifyEntryRemoved(it->GetSharedTx(), reason);
     const uint256 hash = it->GetTx().GetHash();
     BOOST_FOREACH(const CTxIn& txin, it->GetTx().vin)
         mapNextTx.erase(txin.prevout);
@@ -502,7 +504,7 @@ void CTxMemPool::CalculateDescendants(txiter entryit, setEntries &setDescendants
     }
 }
 
-void CTxMemPool::removeRecursive(const CTransaction &origTx)
+void CTxMemPool::removeRecursive(const CTransaction &origTx, MemPoolRemovalReason reason)
 {
     // Remove transaction from memory pool
     {
@@ -529,7 +531,8 @@ void CTxMemPool::removeRecursive(const CTransaction &origTx)
         BOOST_FOREACH(txiter it, txToRemove) {
             CalculateDescendants(it, setAllRemoves);
         }
-        RemoveStaged(setAllRemoves, false);
+
+        RemoveStaged(setAllRemoves, false, reason);
     }
 }
 
@@ -567,7 +570,7 @@ void CTxMemPool::removeForReorg(const CCoinsViewCache *pcoins, unsigned int nMem
     for (txiter it : txToRemove) {
         CalculateDescendants(it, setAllRemoves);
     }
-    RemoveStaged(setAllRemoves, false);
+    RemoveStaged(setAllRemoves, false, MemPoolRemovalReason::REORG);
 }
 
 void CTxMemPool::removeConflicts(const CTransaction &tx)
@@ -581,7 +584,7 @@ void CTxMemPool::removeConflicts(const CTransaction &tx)
             if (txConflict != tx)
             {
                 ClearPrioritisation(txConflict.GetHash());
-                removeRecursive(txConflict);
+                removeRecursive(txConflict, MemPoolRemovalReason::CONFLICT);
             }
         }
     }
@@ -610,7 +613,7 @@ void CTxMemPool::removeForBlock(const std::vector<CTransactionRef>& vtx, unsigne
         if (it != mapTx.end()) {
             setEntries stage;
             stage.insert(it);
-            RemoveStaged(stage, true);
+            RemoveStaged(stage, true, MemPoolRemovalReason::BLOCK);
         }
         removeConflicts(*tx);
         ClearPrioritisation(tx->GetHash());
@@ -989,11 +992,11 @@ size_t CTxMemPool::DynamicMemoryUsage() const {
     return memusage::MallocUsage(sizeof(CTxMemPoolEntry) + 15 * sizeof(void*)) * mapTx.size() + memusage::DynamicUsage(mapNextTx) + memusage::DynamicUsage(mapDeltas) + memusage::DynamicUsage(mapLinks) + memusage::DynamicUsage(vTxHashes) + cachedInnerUsage;
 }
 
-void CTxMemPool::RemoveStaged(setEntries &stage, bool updateDescendants) {
+void CTxMemPool::RemoveStaged(setEntries &stage, bool updateDescendants, MemPoolRemovalReason reason) {
     AssertLockHeld(cs);
     UpdateForRemoveFromMempool(stage, updateDescendants);
     BOOST_FOREACH(const txiter& it, stage) {
-        removeUnchecked(it);
+        removeUnchecked(it, reason);
     }
 }
 
@@ -1009,7 +1012,7 @@ int CTxMemPool::Expire(int64_t time) {
     BOOST_FOREACH(txiter removeit, toremove) {
         CalculateDescendants(removeit, stage);
     }
-    RemoveStaged(stage, false);
+    RemoveStaged(stage, false, MemPoolRemovalReason::EXPIRY);
     return stage.size();
 }
 
@@ -1118,7 +1121,7 @@ void CTxMemPool::TrimToSize(size_t sizelimit, std::vector<uint256>* pvNoSpendsRe
             BOOST_FOREACH(txiter iter, stage)
                 txn.push_back(iter->GetTx());
         }
-        RemoveStaged(stage, false);
+        RemoveStaged(stage, false, MemPoolRemovalReason::SIZELIMIT);
         if (pvNoSpendsRemaining) {
             BOOST_FOREACH(const CTransaction& tx, txn) {
                 BOOST_FOREACH(const CTxIn& txin, tx.vin) {

src/txmempool.h

@@ -25,6 +25,8 @@
 #include "boost/multi_index/ordered_index.hpp"
 #include "boost/multi_index/hashed_index.hpp"
 
+#include <boost/signals2/signal.hpp>
+
 class CAutoFile;
 class CBlockIndex;
 
@@ -333,6 +335,19 @@ struct TxMempoolInfo
     int64_t nFeeDelta;
 };
 
+/** Reason why a transaction was removed from the mempool,
+ * this is passed to the notification signal.
+ */
+enum class MemPoolRemovalReason {
+    UNKNOWN = 0, //! Manually removed or unknown reason
+    EXPIRY,      //! Expired from mempool
+    SIZELIMIT,   //! Removed in size limiting
+    REORG,       //! Removed for reorganization
+    BLOCK,       //! Removed for block
+    CONFLICT,    //! Removed for conflict with in-block transaction
+    REPLACED     //! Removed for replacement
+};
+
 /**
  * CTxMemPool stores valid-according-to-the-current-best-chain transactions
  * that may be included in the next block.
@@ -521,10 +536,11 @@ public:
     bool addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry, bool validFeeEstimate = true);
     bool addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry, setEntries &setAncestors, bool validFeeEstimate = true);
 
-    void removeRecursive(const CTransaction &tx);
+    void removeRecursive(const CTransaction &tx, MemPoolRemovalReason reason = MemPoolRemovalReason::UNKNOWN);
     void removeForReorg(const CCoinsViewCache *pcoins, unsigned int nMemPoolHeight, int flags);
     void removeConflicts(const CTransaction &tx);
     void removeForBlock(const std::vector<CTransactionRef>& vtx, unsigned int nBlockHeight);
+
     void clear();
     void _clear(); //lock free
     bool CompareDepthAndScore(const uint256& hasha, const uint256& hashb);
@@ -551,7 +567,7 @@ public:
      *  Set updateDescendants to true when removing a tx that was in a block, so
      *  that any in-mempool descendants have their ancestor state updated.
      */
-    void RemoveStaged(setEntries &stage, bool updateDescendants);
+    void RemoveStaged(setEntries &stage, bool updateDescendants, MemPoolRemovalReason reason = MemPoolRemovalReason::UNKNOWN);
 
     /** When adding transactions from a disconnected block back to the mempool,
      *  new mempool entries may have children in the mempool (which is generally
@@ -647,6 +663,9 @@ public:
 
     size_t DynamicMemoryUsage() const;
 
+    boost::signals2::signal<void (CTransactionRef)> NotifyEntryAdded;
+    boost::signals2::signal<void (CTransactionRef, MemPoolRemovalReason)> NotifyEntryRemoved;
+
 private:
     /** UpdateForDescendants is used by UpdateTransactionsFromBlock to update
      *  the descendants for a single transaction that has been added to the
@@ -683,7 +702,7 @@ private:
      *  transactions in a chain before we've updated all the state for the
      *  removal.
      */
-    void removeUnchecked(txiter entry);
+    void removeUnchecked(txiter entry, MemPoolRemovalReason reason = MemPoolRemovalReason::UNKNOWN);
 };
 
 /** 

src/validation.cpp

@@ -956,7 +956,7 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState& state, const C
             if (plTxnReplaced)
                 plTxnReplaced->push_back(it->GetSharedTx());
         }
-        pool.RemoveStaged(allConflicting, false);
+        pool.RemoveStaged(allConflicting, false, MemPoolRemovalReason::REPLACED);
 
         // This transaction should only count for fee estimation if
         // the node is not behind and it is not dependent on any other
@@ -2166,7 +2166,7 @@ bool static DisconnectTip(CValidationState& state, const CChainParams& chainpara
             // ignore validation errors in resurrected transactions
             CValidationState stateDummy;
             if (tx.IsCoinBase() || !AcceptToMemoryPool(mempool, stateDummy, it, false, NULL, NULL, true)) {
-                mempool.removeRecursive(tx);
+                mempool.removeRecursive(tx, MemPoolRemovalReason::REORG);
             } else if (mempool.exists(tx.GetHash())) {
                 vHashUpdate.push_back(tx.GetHash());
             }