Make sure rate-limiting code is thread-safe

main.cpp

@@ -739,11 +739,17 @@ bool CTransaction::AcceptToMemoryPool(CTxDB& txdb, bool fCheckInputs, bool* pfMi
             return error("AcceptToMemoryPool() : not enough fees");
 
         // Continuously rate-limit free transactions
+        // This mitigates 'penny-flooding' -- sending thousands of free transactions just to
+        // be annoying or make other's transactions take longer to confirm.
         if (nFees < CENT)
         {
+            static CCriticalSection cs;
             static double dFreeCount;
             static int64 nLastTime;
             int64 nNow = GetTime();
+
+            CRITICAL_BLOCK(cs)
+            {
                 // Use an exponentially decaying ~10-minute window:
                 dFreeCount *= pow(1.0 - 1.0/600.0, (double)(nNow - nLastTime));
                 nLastTime = nNow;
@@ -756,6 +762,7 @@ bool CTransaction::AcceptToMemoryPool(CTxDB& txdb, bool fCheckInputs, bool* pfMi
                 dFreeCount += nSize;
             }
         }
+    }
 
     // Store transaction in memory
     CRITICAL_BLOCK(cs_mapTransactions)