SmartContract: restrict the number of allowed notifications

Fix the problem described in
nspcc-dev/neo-go#3490. Port the solution from
nspcc-dev/neo-go#3640.

MaxNotificationsCount constraint is chosen based on the Mainnet
statistics of the number of notifications per every transaction, ref.
nspcc-dev/neo-go#3490 (comment).

Signed-off-by: Anna Shaleva <[email protected]>

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>
@@ -395,8 +400,15 @@ protected internal void RuntimeNotifyV1(byte[] eventName, Array state)
         protected internal void SendNotification(UInt160 hash, string eventName, Array state)
         {
             NotifyEventArgs notification = new(ScriptContainer, hash, eventName, (Array)state.DeepCopy(asImmutable: true));
-            Notify?.Invoke(this, notification);
             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.");
+            }
+            Notify?.Invoke(this, notification);
             notifications.Add(notification);
             CurrentContext.GetState<ExecutionContextState>().NotificationCount++;
         }