SmartContract: restrict the number of allowed notifications

src/Neo/SmartContract/ApplicationEngine.Runtime.cs

@@ -36,6 +36,11 @@ partial class ApplicationEngine
         /// </summary>
         public const int MaxNotificationSize = 1024;
 
+        /// <summary>
+        /// The maximum number of notifications per application execution.
+        /// </summary>
+        public const int MaxNotificationCount = 512;
+
         private uint random_times = 0;
 
         /// <summary>
@@ -394,9 +399,16 @@ protected internal void RuntimeNotifyV1(byte[] eventName, Array state)
         /// <param name="state">The arguments of the event.</param>
         protected internal void SendNotification(UInt160 hash, string eventName, Array state)
         {
+            notifications ??= new List<NotifyEventArgs>();
+            // Restrict the number of notifications for Application executions. Do not check
+            // persisting triggers to avoid native persist failure. Do not check verification
+            // trigger since verification context is loaded with ReadOnly flag.
+            if (IsHardforkEnabled(Hardfork.HF_Echidna) && Trigger == TriggerType.Application && notifications.Count >= MaxNotificationCount)
+            {
+                throw new InvalidOperationException($"Maximum number of notifications `{MaxNotificationCount}` is reached.");
+            }
             NotifyEventArgs notification = new(ScriptContainer, hash, eventName, (Array)state.DeepCopy(asImmutable: true));
             Notify?.Invoke(this, notification);
-            notifications ??= new List<NotifyEventArgs>();
             notifications.Add(notification);
             CurrentContext.GetState<ExecutionContextState>().NotificationCount++;
         }