Modify fee validation in mempool

Transaction fee is validated before accepting transaction to mempool

Sources/Stratis.Bitcoin.Features.MemoryPool/DeStreamMempoolValidator.cs

-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.Extensions.Logging;
 using NBitcoin;
@@ -186,5 +188,21 @@ namespace Stratis.Bitcoin.Features.MemoryPool
                 context.SetConflicts.Add(ptxConflicting.GetHash());
             }
         }
+        
+        /// <summary>
+        /// Validates the transaction fee is valid.
+        /// </summary>
+        /// <param name="context">Current validation context.</param>
+        protected override void CheckFee(MempoolValidationContext context)
+        {
+            long expectedFee = Convert.ToInt64(context.Transaction.Outputs
+                                                   .Where(p => !context.Transaction.Inputs.RemoveChangePointer()
+                                                       .Select(q => context.View.Set.GetOutputFor(q).ScriptPubKey)
+                                                       .Contains(p.ScriptPubKey))
+                                                   .Sum(p => p.Value) * this.network.FeeRate);
+            
+            if (context.ModifiedFees < expectedFee)
+                context.State.Fail(MempoolErrors.InsufficientFee, $" {context.Fees} < {expectedFee}").Throw();
+        }
     }
 }
\ No newline at end of file

Sources/Stratis.Bitcoin.Features.MemoryPool/MempoolValidator.cs

@@ -153,7 +153,7 @@ namespace Stratis.Bitcoin.Features.MemoryPool
         //  public long LastTime;
         //}
 
-        private Network network;
+        protected Network network;
 
         /// <summary>
         /// Constructs a memory pool validator object.
@@ -697,7 +697,7 @@ namespace Stratis.Bitcoin.Features.MemoryPool
         /// and absurdly high fees.
         /// </summary>
         /// <param name="context">Current validation context.</param>
-        protected void CheckFee(MempoolValidationContext context)
+        protected virtual void CheckFee(MempoolValidationContext context)
         {
             Money mempoolRejectFee = this.memPool.GetMinFee(this.mempoolSettings.MaxMempool * 1000000).GetFee(context.EntrySize);
             if (mempoolRejectFee > 0 && context.ModifiedFees < mempoolRejectFee)