txpool: Make nTransactionsUpdated atomic

src/rpc/mining.cpp

@@ -480,6 +480,7 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
                 if (g_best_block_cv.wait_until(lock, checktxtime) == std::cv_status::timeout)
                 {
                     // Timeout: Check transactions for update
+                    // without holding ::mempool.cs to avoid deadlocks
                     if (mempool.GetTransactionsUpdated() != nTransactionsUpdatedLastLP)
                         break;
                     checktxtime += std::chrono::seconds(10);

src/txmempool.cpp

@@ -322,8 +322,8 @@ void CTxMemPoolEntry::UpdateAncestorState(int64_t modifySize, CAmount modifyFee,
     assert(int(nSigOpCostWithAncestors) >= 0);
 }
 
-CTxMemPool::CTxMemPool(CBlockPolicyEstimator* estimator) :
-    nTransactionsUpdated(0), minerPolicyEstimator(estimator)
+CTxMemPool::CTxMemPool(CBlockPolicyEstimator* estimator)
+    : nTransactionsUpdated(0), minerPolicyEstimator(estimator)
 {
     _clear(); //lock free clear
 
@@ -341,13 +341,11 @@ bool CTxMemPool::isSpent(const COutPoint& outpoint) const
 
 unsigned int CTxMemPool::GetTransactionsUpdated() const
 {
-    LOCK(cs);
     return nTransactionsUpdated;
 }
 
 void CTxMemPool::AddTransactionsUpdated(unsigned int n)
 {
-    LOCK(cs);
     nTransactionsUpdated += n;
 }
 

src/txmempool.h

@@ -6,12 +6,13 @@
 #ifndef BITCOIN_TXMEMPOOL_H
 #define BITCOIN_TXMEMPOOL_H
 
+#include <atomic>
+#include <map>
 #include <memory>
 #include <set>
-#include <map>
-#include <vector>
-#include <utility>
 #include <string>
+#include <utility>
+#include <vector>
 
 #include <amount.h>
 #include <coins.h>
@@ -443,7 +444,7 @@ class CTxMemPool
 {
 private:
     uint32_t nCheckFrequency GUARDED_BY(cs); //!< Value n means that n times in 2^32 we check.
-    unsigned int nTransactionsUpdated; //!< Used by getblocktemplate to trigger CreateNewBlock() invocation
+    std::atomic<unsigned int> nTransactionsUpdated; //!< Used by getblocktemplate to trigger CreateNewBlock() invocation
     CBlockPolicyEstimator* minerPolicyEstimator;
 
     uint64_t totalTxSize;      //!< sum of all mempool tx's virtual sizes. Differs from serialized tx size since witness data is discounted. Defined in BIP 141.