Fix memory leak on TaskManager due to continuous growth of knownHashes (3.x)

neo/IO/Caching/FIFOSet.cs

@@ -1,9 +1,12 @@
 using System;
+using System.Collections;
+using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.Linq;
 
 namespace Neo.IO.Caching
 {
-    internal class FIFOSet<T> where T : IEquatable<T>
+    internal class FIFOSet<T> : IEnumerable<T> where T : IEquatable<T>
     {
         private readonly int maxCapacity;
         private readonly int removeCount;
@@ -37,5 +40,21 @@ public bool Add(T item)
             dictionary.Add(item, null);
             return true;
         }
+
+        public void ExceptWith(IEnumerable<UInt256> hashes)
+        {
+            foreach (var hash in hashes)
+            {
+                dictionary.Remove(hash);
+            }
+        }
+
+        public IEnumerator<T> GetEnumerator()
+        {
+            var entries = dictionary.Values.Cast<T>().ToArray();
+            foreach (var entry in entries) yield return entry;
+        }
+
+        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
     }
 }

neo/Network/P2P/TaskManager.cs

@@ -1,6 +1,7 @@
 using Akka.Actor;
 using Akka.Configuration;
 using Neo.IO.Actors;
+using Neo.IO.Caching;
 using Neo.Ledger;
 using Neo.Network.P2P.Payloads;
 using System;
@@ -24,7 +25,13 @@ private class Timer { }
 
         private readonly NeoSystem system;
         private const int MaxConncurrentTasks = 3;
-        private readonly HashSet<UInt256> knownHashes = new HashSet<UInt256>();
+
+        /// <summary>
+        /// The limit `Blockchain.Singleton.MemPool.Capacity * 2` was the same value used in ProtocolHandler
+        /// </summary>
+        private static readonly int MaxCachedHashes = Blockchain.Singleton.MemPool.Capacity * 2;
+        private readonly FIFOSet<UInt256> knownHashes = new FIFOSet<UInt256>(MaxCachedHashes);
+
         private readonly Dictionary<UInt256, int> globalTasks = new Dictionary<UInt256, int>();
         private readonly Dictionary<IActorRef, TaskSession> sessions = new Dictionary<IActorRef, TaskSession>();
         private readonly ICancelable timer = Context.System.Scheduler.ScheduleTellRepeatedlyCancelable(TimerInterval, TimerInterval, Context.Self, new Timer(), ActorRefs.NoSender);