CValidationInterface, mempool: add new callback to CValidationInterface

This commit adds a new callback `MempoolTransactionsRemovedForBlock` which notify
its listeners of the transactions that are removed from the mempool because a new
block is connected, along with the block height the transactions were removed.
The transactions are in `RemovedMempoolTransactionInfo` format.

`CTransactionRef`, base fee, virtual size, and height which the transaction was added
to the mempool are all members of the struct called `RemovedMempoolTransactionInfo`.

A struct `NewMempoolTransactionInfo`, which has fields similar to `RemovedMempoolTransactionInfo`,
will be added in a later commit, create a struct `TransactionInfo` with all similar fields.
They can both have a member with type `TransactionInfo`.

src/kernel/mempool_entry.h

@@ -178,4 +178,34 @@ public:
 
 using CTxMemPoolEntryRef = CTxMemPoolEntry::CTxMemPoolEntryRef;
 
+struct TransactionInfo {
+    const CTransactionRef m_tx;
+    /* The fee the transaction paid */
+    const CAmount m_fee;
+    /**
+     * The virtual transaction size.
+     *
+     * This is a policy field which considers the sigop cost of the
+     * transaction as well as its weight, and reinterprets it as bytes.
+     *
+     * It is the primary metric by which the mining algorithm selects
+     * transactions.
+     */
+    const int64_t m_virtual_transaction_size;
+    /* The block height the transaction entered the mempool */
+    const unsigned int txHeight;
+
+    TransactionInfo(const CTransactionRef& tx, const CAmount& fee, const int64_t vsize, const unsigned int height)
+        : m_tx{tx},
+          m_fee{fee},
+          m_virtual_transaction_size{vsize},
+          txHeight{height} {}
+};
+
+struct RemovedMempoolTransactionInfo {
+    TransactionInfo info;
+    explicit RemovedMempoolTransactionInfo(const CTxMemPoolEntry& entry)
+        : info{entry.GetSharedTx(), entry.GetFee(), entry.GetTxSize(), entry.GetHeight()} {}
+};
+
 #endif // BITCOIN_KERNEL_MEMPOOL_ENTRY_H

src/txmempool.cpp

@@ -654,17 +654,21 @@ void CTxMemPool::removeForBlock(const std::vector<CTransactionRef>& vtx, unsigne
     }
     // Before the txs in the new block have been removed from the mempool, update policy estimates
     if (minerPolicyEstimator) {minerPolicyEstimator->processBlock(nBlockHeight, entries);}
+    std::vector<RemovedMempoolTransactionInfo> txs_removed_for_block;
+    txs_removed_for_block.reserve(vtx.size());
     for (const auto& tx : vtx)
     {
         txiter it = mapTx.find(tx->GetHash());
         if (it != mapTx.end()) {
             setEntries stage;
             stage.insert(it);
+            txs_removed_for_block.emplace_back(*it);
             RemoveStaged(stage, true, MemPoolRemovalReason::BLOCK);
         }
         removeConflicts(*tx);
         ClearPrioritisation(tx->GetHash());
     }
+    GetMainSignals().MempoolTransactionsRemovedForBlock(txs_removed_for_block, nBlockHeight);
     lastRollingFeeUpdate = GetTime();
     blockSinceLastRollingFeeBump = true;
 }

src/validationinterface.cpp

@@ -9,6 +9,7 @@
 #include <chain.h>
 #include <consensus/validation.h>
 #include <kernel/chain.h>
+#include <kernel/mempool_entry.h>
 #include <logging.h>
 #include <primitives/block.h>
 #include <primitives/transaction.h>
@@ -233,6 +234,16 @@ void CMainSignals::BlockConnected(ChainstateRole role, const std::shared_ptr<con
                           pindex->nHeight);
 }
 
+void CMainSignals::MempoolTransactionsRemovedForBlock(const std::vector<RemovedMempoolTransactionInfo>& txs_removed_for_block, unsigned int nBlockHeight)
+{
+    auto event = [txs_removed_for_block, nBlockHeight, this] {
+        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.MempoolTransactionsRemovedForBlock(txs_removed_for_block, nBlockHeight); });
+    };
+    ENQUEUE_AND_LOG_EVENT(event, "%s: block height=%s txs removed=%s", __func__,
+                          nBlockHeight,
+                          txs_removed_for_block.size());
+}
+
 void CMainSignals::BlockDisconnected(const std::shared_ptr<const CBlock>& pblock, const CBlockIndex* pindex)
 {
     auto event = [pblock, pindex, this] {

src/validationinterface.h

@@ -21,6 +21,7 @@ struct CBlockLocator;
 class CValidationInterface;
 class CScheduler;
 enum class MemPoolRemovalReason;
+struct RemovedMempoolTransactionInfo;
 
 /** Register subscriber */
 void RegisterValidationInterface(CValidationInterface* callbacks);
@@ -60,10 +61,10 @@ void CallFunctionInValidationInterfaceQueue(std::function<void ()> func);
 void SyncWithValidationInterfaceQueue() LOCKS_EXCLUDED(cs_main);
 
 /**
- * Implement this to subscribe to events generated in validation
+ * Implement this to subscribe to events generated in validation and mempool
  *
  * Each CValidationInterface() subscriber will receive event callbacks
- * in the order in which the events were generated by validation.
+ * in the order in which the events were generated by validation and mempool.
  * Furthermore, each ValidationInterface() subscriber may assume that
  * callbacks effectively run in a single thread with single-threaded
  * memory consistency. That is, for a given ValidationInterface()
@@ -113,7 +114,7 @@ protected:
      * This does not fire for transactions that are removed from the mempool
      * because they have been included in a block. Any client that is interested
      * in transactions removed from the mempool for inclusion in a block can learn
-     * about those transactions from the BlockConnected notification.
+     * about those transactions from the MempoolTransactionsRemovedForBlock notification.
      *
      * Transactions that are removed from the mempool because they conflict
      * with a transaction in the new block will have
@@ -131,6 +132,14 @@ protected:
      * Called on a background thread.
      */
     virtual void TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason, uint64_t mempool_sequence) {}
+    /*
+     * Notifies listeners of transactions removed from the mempool as
+     * as a result of new block being connected.
+     * MempoolTransactionsRemovedForBlock will be fired before BlockConnected.
+     *
+     * Called on a background thread.
+     */
+    virtual void MempoolTransactionsRemovedForBlock(const std::vector<RemovedMempoolTransactionInfo>& txs_removed_for_block, unsigned int nBlockHeight) {}
     /**
      * Notifies listeners of a block being connected.
      * Provides a vector of transactions evicted from the mempool as a result.
@@ -140,6 +149,7 @@ protected:
     virtual void BlockConnected(ChainstateRole role, const std::shared_ptr<const CBlock> &block, const CBlockIndex *pindex) {}
     /**
      * Notifies listeners of a block being disconnected
+     *  Provides the block that was connected.
      *
      * Called on a background thread. Only called for the active chainstate, since
      * background chainstates should never disconnect blocks.
@@ -202,6 +212,7 @@ public:
     void UpdatedBlockTip(const CBlockIndex *, const CBlockIndex *, bool fInitialDownload);
     void TransactionAddedToMempool(const CTransactionRef&, uint64_t mempool_sequence);
     void TransactionRemovedFromMempool(const CTransactionRef&, MemPoolRemovalReason, uint64_t mempool_sequence);
+    void MempoolTransactionsRemovedForBlock(const std::vector<RemovedMempoolTransactionInfo>&, unsigned int nBlockHeight);
     void BlockConnected(ChainstateRole, const std::shared_ptr<const CBlock> &, const CBlockIndex *pindex);
     void BlockDisconnected(const std::shared_ptr<const CBlock> &, const CBlockIndex* pindex);
     void ChainStateFlushed(ChainstateRole, const CBlockLocator &);