Add missing file

BTCPayServer/WebsocketExtensions.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Net.WebSockets;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace BTCPayServer
+{
+    public static class WebsocketExtensions
+    {
+        /// <summary>
+        /// NGINX closes websocket connections after 1 min if there is no activity, so here we do some busy work every 30s
+        /// </summary>
+        /// <returns></returns>
+        public static async Task<WebSocketReceiveResult> ReceiveAndPingAsync(this WebSocket webSocket, ArraySegment<byte> buffer, CancellationToken cancellationToken = default)
+        {
+            var waiting = Task.Delay(TimeSpan.FromSeconds(30), cancellationToken);
+            var receiving = webSocket.ReceiveAsync(buffer, cancellationToken);
+wait:
+            var completed = await Task.WhenAny(waiting, receiving);
+            if (completed == waiting)
+            {
+                await webSocket.SendAsync(Encoding.UTF8.GetBytes("ping"), WebSocketMessageType.Text, true, cancellationToken);
+                waiting = Task.Delay(TimeSpan.FromSeconds(30), cancellationToken);
+                goto wait;
+            }
+            return receiving.Result;
+        }
+    }
+}