interfaces: Add Chain::requestMempoolTransactions

Github-Pull: #15652
Rebased-From: 57908a739c

src/interfaces/chain.cpp

@@ -8,6 +8,7 @@
 #include <chainparams.h>
 #include <primitives/block.h>
 #include <sync.h>
+#include <txmempool.h>
 #include <uint256.h>
 #include <util/system.h>
 #include <validation.h>
@@ -177,6 +178,13 @@ public:
         LOCK(cs_main);
         return GuessVerificationProgress(Params().TxData(), LookupBlockIndex(block_hash));
     }
+    void requestMempoolTransactions(std::function<void(const CTransactionRef&)> fn) override
+    {
+        LOCK2(::cs_main, ::mempool.cs);
+        for (const CTxMemPoolEntry& entry : ::mempool.mapTx) {
+            fn(entry.GetSharedTx());
+        }
+    }
 };
 
 } // namespace

src/interfaces/chain.h

@@ -16,6 +16,9 @@ class CBlock;
 class CScheduler;
 class uint256;
 struct CBlockLocator;
+class CTransaction;
+
+using CTransactionRef = std::shared_ptr<const CTransaction>;
 
 namespace interfaces {
 
@@ -127,6 +130,16 @@ public:
     //! Estimate fraction of total transactions verified if blocks up to
     //! the specified block hash are verified.
     virtual double guessVerificationProgress(const uint256& block_hash) = 0;
+
+    //! Synchronously send TransactionAddedToMempool notifications about all
+    //! current mempool transactions to the specified handler and return after
+    //! the last one is sent. These notifications aren't coordinated with async
+    //! notifications sent by handleNotifications, so out of date async
+    //! notifications from handleNotifications can arrive during and after
+    //! synchronous notifications from requestMempoolTransactions. Clients need
+    //! to be prepared to handle this by ignoring notifications about unknown
+    //! removed transactions and already added new transactions.
+    virtual void requestMempoolTransactions(std::function<void(const CTransactionRef&)> fn) = 0;
 };
 
 //! Interface to let node manage chain clients (wallets, or maybe tools for