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.

1 files changed, 22 insertions, 3 deletions

diff --git a/src/txmempool.h b/src/txmempool.h
index ffb1c1309b..f842a07dd6 100644
--- a/src/txmempool.h
+++ b/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);
 };
 
 /**