scadalink-design/deprecated/lmxproxy/src-reference/ZB.MOM.WW.LmxProxy.Host/Services/StatusWebServer.cs

using System;
using System.IO;
using System.Net;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using Serilog;
using ZB.MOM.WW.LmxProxy.Host.Configuration;

namespace ZB.MOM.WW.LmxProxy.Host.Services
{
    /// <summary>
    ///     HTTP web server that serves status information for the LmxProxy service
    /// </summary>
    public class StatusWebServer : IDisposable
    {
        private static readonly ILogger Logger = Log.ForContext<StatusWebServer>();

        private readonly WebServerConfiguration _configuration;
        private readonly StatusReportService _statusReportService;
        private CancellationTokenSource? _cancellationTokenSource;
        private bool _disposed;
        private HttpListener? _httpListener;
        private Task? _listenerTask;

        /// <summary>
        ///     Initializes a new instance of the StatusWebServer class
        /// </summary>
        /// <param name="configuration">Web server configuration</param>
        /// <param name="statusReportService">Service for collecting status information</param>
        public StatusWebServer(WebServerConfiguration configuration, StatusReportService statusReportService)
        {
            _configuration = configuration ?? throw new ArgumentNullException(nameof(configuration));
            _statusReportService = statusReportService ?? throw new ArgumentNullException(nameof(statusReportService));
        }

        /// <summary>
        ///     Disposes the web server and releases resources
        /// </summary>
        public void Dispose()
        {
            if (_disposed)
            {
                return;
            }

            _disposed = true;

            Stop();

            _cancellationTokenSource?.Dispose();
            _httpListener?.Close();
        }

        /// <summary>
        ///     Starts the HTTP web server
        /// </summary>
        /// <returns>True if started successfully, false otherwise</returns>
        public bool Start()
        {
            try
            {
                if (!_configuration.Enabled)
                {
                    Logger.Information("Status web server is disabled");
                    return true;
                }

                Logger.Information("Starting status web server on port {Port}", _configuration.Port);

                _httpListener = new HttpListener();

                // Configure the URL prefix
                string prefix = _configuration.Prefix ?? $"http://+:{_configuration.Port}/";
                if (!prefix.EndsWith("/"))
                {
                    prefix += "/";
                }

                _httpListener.Prefixes.Add(prefix);
                _httpListener.Start();

                _cancellationTokenSource = new CancellationTokenSource();
                _listenerTask = Task.Run(() => HandleRequestsAsync(_cancellationTokenSource.Token));

                Logger.Information("Status web server started successfully on {Prefix}", prefix);
                return true;
            }
            catch (Exception ex)
            {
                Logger.Error(ex, "Failed to start status web server");
                return false;
            }
        }

        /// <summary>
        ///     Stops the HTTP web server
        /// </summary>
        /// <returns>True if stopped successfully, false otherwise</returns>
        public bool Stop()
        {
            try
            {
                if (!_configuration.Enabled || _httpListener == null)
                {
                    return true;
                }

                Logger.Information("Stopping status web server");

                _cancellationTokenSource?.Cancel();

                if (_listenerTask != null)
                {
                    try
                    {
                        _listenerTask.Wait(TimeSpan.FromSeconds(5));
                    }
                    catch (Exception ex)
                    {
                        Logger.Warning(ex, "Error waiting for listener task to complete");
                    }
                }

                _httpListener?.Stop();
                _httpListener?.Close();

                Logger.Information("Status web server stopped successfully");
                return true;
            }
            catch (Exception ex)
            {
                Logger.Error(ex, "Error stopping status web server");
                return false;
            }
        }

        /// <summary>
        ///     Main request handling loop
        /// </summary>
        private async Task HandleRequestsAsync(CancellationToken cancellationToken)
        {
            Logger.Information("Status web server listener started");

            while (!cancellationToken.IsCancellationRequested && _httpListener != null && _httpListener.IsListening)
            {
                try
                {
                    HttpListenerContext? context = await _httpListener.GetContextAsync();

                    // Handle request asynchronously without waiting
                    _ = Task.Run(async () =>
                    {
                        try
                        {
                            await HandleRequestAsync(context);
                        }
                        catch (Exception ex)
                        {
                            Logger.Error(ex, "Error handling HTTP request from {RemoteEndPoint}",
                                context.Request.RemoteEndPoint);
                        }
                    }, cancellationToken);
                }
                catch (ObjectDisposedException)
                {
                    // Expected when stopping the listener
                    break;
                }
                catch (HttpListenerException ex) when (ex.ErrorCode == 995) // ERROR_OPERATION_ABORTED
                {
                    // Expected when stopping the listener
                    break;
                }
                catch (Exception ex)
                {
                    Logger.Error(ex, "Error in request listener loop");

                    // Brief delay before continuing to avoid tight error loops
                    try
                    {
                        await Task.Delay(1000, cancellationToken);
                    }
                    catch (OperationCanceledException)
                    {
                        break;
                    }
                }
            }

            Logger.Information("Status web server listener stopped");
        }

        /// <summary>
        ///     Handles a single HTTP request
        /// </summary>
        private async Task HandleRequestAsync(HttpListenerContext context)
        {
            HttpListenerRequest? request = context.Request;
            HttpListenerResponse response = context.Response;

            try
            {
                Logger.Debug("Handling {Method} request to {Url} from {RemoteEndPoint}",
                    request.HttpMethod, request.Url?.AbsolutePath, request.RemoteEndPoint);

                // Only allow GET requests
                if (request.HttpMethod != "GET")
                {
                    response.StatusCode = 405; // Method Not Allowed
                    response.StatusDescription = "Method Not Allowed";
                    await WriteResponseAsync(response, "Only GET requests are supported", "text/plain");
                    return;
                }

                string path = request.Url?.AbsolutePath?.ToLowerInvariant() ?? "/";

                switch (path)
                {
                    case "/":
                        await HandleStatusPageAsync(response);
                        break;

                    case "/api/status":
                        await HandleStatusApiAsync(response);
                        break;

                    case "/api/health":
                        await HandleHealthApiAsync(response);
                        break;

                    default:
                        response.StatusCode = 404; // Not Found
                        response.StatusDescription = "Not Found";
                        await WriteResponseAsync(response, "Resource not found", "text/plain");
                        break;
                }
            }
            catch (Exception ex)
            {
                Logger.Error(ex, "Error handling HTTP request");

                try
                {
                    response.StatusCode = 500; // Internal Server Error
                    response.StatusDescription = "Internal Server Error";
                    await WriteResponseAsync(response, "Internal server error", "text/plain");
                }
                catch (Exception responseEx)
                {
                    Logger.Error(responseEx, "Error writing error response");
                }
            }
            finally
            {
                try
                {
                    response.Close();
                }
                catch (Exception ex)
                {
                    Logger.Warning(ex, "Error closing HTTP response");
                }
            }
        }

        /// <summary>
        ///     Handles the main status page (HTML)
        /// </summary>
        private async Task HandleStatusPageAsync(HttpListenerResponse response)
        {
            string statusHtml = await _statusReportService.GenerateHtmlReportAsync();
            await WriteResponseAsync(response, statusHtml, "text/html; charset=utf-8");
        }

        /// <summary>
        ///     Handles the status API endpoint (JSON)
        /// </summary>
        private async Task HandleStatusApiAsync(HttpListenerResponse response)
        {
            string statusJson = await _statusReportService.GenerateJsonReportAsync();
            await WriteResponseAsync(response, statusJson, "application/json; charset=utf-8");
        }

        /// <summary>
        ///     Handles the health API endpoint (simple text)
        /// </summary>
        private async Task HandleHealthApiAsync(HttpListenerResponse response)
        {
            bool isHealthy = await _statusReportService.IsHealthyAsync();
            string healthText = isHealthy ? "OK" : "UNHEALTHY";
            response.StatusCode = isHealthy ? 200 : 503; // Service Unavailable if unhealthy
            await WriteResponseAsync(response, healthText, "text/plain");
        }

        /// <summary>
        ///     Writes a response to the HTTP context
        /// </summary>
        private static async Task WriteResponseAsync(HttpListenerResponse response, string content, string contentType)
        {
            response.ContentType = contentType;
            response.Headers.Add("Cache-Control", "no-cache, no-store, must-revalidate");
            response.Headers.Add("Pragma", "no-cache");
            response.Headers.Add("Expires", "0");

            byte[] buffer = Encoding.UTF8.GetBytes(content);
            response.ContentLength64 = buffer.Length;

            using (Stream? output = response.OutputStream)
            {
                await output.WriteAsync(buffer, 0, buffer.Length);
            }
        }
    }
}