rokojori_action_library/Tools/mcp/McpServer.cs

#if TOOLS

using Godot;
using System;
using System.Collections.Concurrent;
using System.Net;
using System.Text;

namespace Rokojori.Tools
{
  using Rokojori;

  // Owned by RokojoriPlugin: Start()/Stop() from _EnterTree/_ExitTree,
  // ProcessPendingRequests() from the existing _Process tick.
  //
  // Threading: HttpListener accepts via BeginGetContext/EndGetContext (thread-pool callbacks),
  // not a dedicated foreground/background Thread blocked in GetContext() - no thread is ever
  // blocked waiting on anything in this class, which matters because a blocked thread with a
  // stack frame into this assembly keeps Godot's C# hot reload from unloading it until that
  // thread returns, and relying on Stop()/Close() to reliably abort such a thread has not held
  // up in practice.
  //
  // More fundamentally: the HTTP callback thread does raw byte I/O only - read the request body
  // in, hand it off untouched, write the response bytes out when told to. It never parses JSON,
  // dispatches protocol methods, or executes a tool. That's not just about tools/call touching
  // Godot APIs directly (ResourceLoader, PackedScene, the scene tree) - the shared JSON library
  // this server depends on (JSONParser) calls RJLog on malformed input, and RJLog ultimately
  // calls GD.Print/GD.PrintErr, which are themselves Godot engine API calls. Every one of those
  // is only safe from the main thread; calling into the engine from a background thread can
  // corrupt native-side state in ways that surface as hot-reload failures. So ALL of it - JSON.
  // Parse included - is deferred to ProcessPendingRequests() on the main thread via
  // pendingRequests, and the response is written from there too.
  public class McpServer
  {
    public const string FlagFilePath = "res://.godot-mcp";
    public const int PortRangeStart = 9400;
    public const int PortRangeCount = 20;

    HttpListener listener;
    volatile bool running = false;
    int boundPort = -1;
    string projectPath = "";

    readonly McpProtocolHandler protocol = new McpProtocolHandler();
    readonly ConcurrentQueue<PendingRequest> pendingRequests = new ConcurrentQueue<PendingRequest>();

    public bool IsRunning { get { return running; } }
    public int BoundPort { get { return boundPort; } }

    public void Start()
    {
      if ( running )
      {
        return;
      }

      var flagGlobalPath = ProjectSettings.GlobalizePath( FlagFilePath );

      if ( ! FilesSync.FileExists( flagGlobalPath ) )
      {
        RJLog.Log( "MCP", "Flag file not found (" + FlagFilePath + ") - server not starting." );
        return;
      }

      projectPath = ProjectSettings.GlobalizePath( "res://" );

      for ( var port = PortRangeStart; port < PortRangeStart + PortRangeCount; port++ )
      {
        if ( TryBind( port ) )
        {
          boundPort = port;
          break;
        }
      }

      if ( listener == null )
      {
        RJLog.Error( "MCP", "Could not bind any port in range", PortRangeStart, "-", ( PortRangeStart + PortRangeCount - 1 ) );
        return;
      }

      running = true;
      BeginAccept();

      RJLog.Log( "MCP", "Server started on port", boundPort, "for project", projectPath );
    }

    public void Stop()
    {
      if ( ! running )
      {
        return;
      }

      running = false;

      // Any request still waiting for a future _Process() tick to dequeue it would otherwise sit
      // unanswered forever if that tick never comes (e.g. the editor pausing the main loop to
      // hot-reload this very assembly). Stop() always runs on the main thread, never concurrently
      // with _Process(), so it can safely stand in for the tick that won't arrive and answer any
      // still-queued request right now instead of leaving its HTTP response hanging.
      while ( pendingRequests.TryDequeue( out var pending ) )
      {
        AbortPendingRequest( pending );
      }

      // Closing the listener faults any pending BeginGetContext registration, which invokes
      // OnGetContext on a thread-pool callback thread with an exception from EndGetContext() -
      // handled there by simply not chaining another BeginAccept(). No dedicated thread to join.
      try
      {
        listener?.Stop();
        listener?.Close();
      }
      catch ( Exception )
      {
      }

      listener = null;
      boundPort = -1;

      RJLog.Log( "MCP", "Server stopped." );
    }

    // Stop() runs on the main thread, so parsing here (to recover the request's id for a clean
    // JSON-RPC error) is safe even though JSON.Parse can call RJLog on malformed input.
    void AbortPendingRequest( PendingRequest pending )
    {
      JSONData id = new JSONValue();

      try
      {
        var parsed = JSON.Parse( pending.body );
        var envelope = parsed != null ? parsed.AsObject() : null;
        var parsedId = envelope != null ? envelope.Get( "id" ) : null;

        if ( parsedId != null )
        {
          id = parsedId;
        }
      }
      catch ( Exception )
      {
      }

      TryWriteJson( pending.response, protocol.ToolCallResult( id, ToolCallOutcome.Error( "MCP server is stopping" ) ) );
    }

    void BeginAccept()
    {
      if ( ! running || listener == null )
      {
        return;
      }

      try
      {
        listener.BeginGetContext( OnGetContext, null );
      }
      catch ( Exception )
      {
        // Listener already stopped/closed - nothing to accept.
      }
    }

    void OnGetContext( IAsyncResult ar )
    {
      HttpListenerContext context;

      try
      {
        context = listener.EndGetContext( ar );
      }
      catch ( Exception )
      {
        // Stop() closed the listener while this accept was pending - end the chain here.
        return;
      }

      try
      {
        HandleContext( context );
      }
      catch ( Exception e )
      {
        // This runs on a thread-pool callback thread, not the main thread. Console.Error is
        // plain .NET, not a Godot API - unlike RJLog, it carries no risk of touching the engine
        // from the wrong thread (see the class comment above).
        Console.Error.WriteLine( "[MCP] Unhandled error handling request: " + e );

        try
        {
          context.Response.StatusCode = 500;
          context.Response.OutputStream.Close();
        }
        catch ( Exception )
        {
        }
      }
      finally
      {
        BeginAccept();
      }
    }

    // Drains requests queued by the HTTP callback thread and handles them fully - JSON parsing,
    // protocol dispatch, tool execution, and writing the HTTP response - on the main thread.
    // Nothing on the HTTP side ever blocks waiting for this to happen.
    public void ProcessPendingRequests()
    {
      while ( pendingRequests.TryDequeue( out var pending ) )
      {
        HandleJsonRpc( pending.body, pending.response );
      }
    }

    bool TryBind( int port )
    {
      try
      {
        var candidate = new HttpListener();
        // http.sys matches the Host header against registered prefixes exactly - "localhost"
        // and "127.0.0.1" don't match each other, so both are registered since the MCP client
        // config could plausibly use either.
        candidate.Prefixes.Add( "http://localhost:" + port + "/" );
        candidate.Prefixes.Add( "http://127.0.0.1:" + port + "/" );
        candidate.Start();
        listener = candidate;
        return true;
      }
      catch ( HttpListenerException )
      {
        return false;
      }
      catch ( System.Net.Sockets.SocketException )
      {
        return false;
      }
    }

    // Runs on the HTTP callback thread. The /project-path and 404 branches only touch plain .NET
    // request/response objects and cached strings/ints, so they're answered directly here. A
    // /mcp POST body is handed off untouched - see the class comment for why.
    void HandleContext( HttpListenerContext context )
    {
      var request = context.Request;
      var response = context.Response;

      if ( request.HttpMethod == "GET" && request.Url.AbsolutePath == "/project-path" )
      {
        var discovery = new JSONObject();
        discovery.Set( "projectPath", projectPath );
        discovery.Set( "port", (double) boundPort );
        WriteJson( response, 200, discovery );
        return;
      }

      if ( request.HttpMethod != "POST" || request.Url.AbsolutePath != "/mcp" )
      {
        response.StatusCode = 404;
        response.OutputStream.Close();
        return;
      }

      string body;

      using ( var reader = new System.IO.StreamReader( request.InputStream, request.ContentEncoding ?? Encoding.UTF8 ) )
      {
        body = reader.ReadToEnd();
      }

      pendingRequests.Enqueue( new PendingRequest( body, response ) );
    }

    // Runs on the main thread only, dequeued from pendingRequests by ProcessPendingRequests().
    void HandleJsonRpc( string body, HttpListenerResponse response )
    {
      JSONObject envelope;

      try
      {
        var parsed = JSON.Parse( body );
        envelope = parsed != null ? parsed.AsObject() : null;

        if ( envelope == null )
        {
          throw new Exception( "Request body is not a JSON object" );
        }
      }
      catch ( Exception )
      {
        TryWriteJson( response, JsonRpc.BuildError( new JSONValue(), -32700, "Parse error" ) );
        return;
      }

      var id = envelope.Get( "id" );
      var isNotification = JsonRpc.IsNotification( envelope );
      var method = JsonRpc.GetMethod( envelope );
      var paramsObj = envelope.GetObject( "params" );

      if ( isNotification )
      {
        TryWriteEmpty( response, 202 );
        return;
      }

      switch ( method )
      {
        case "initialize":
          TryWriteJson( response, protocol.Initialize( id, paramsObj ) );
          break;

        case "tools/list":
          TryWriteJson( response, protocol.ToolsList( id ) );
          break;

        case "tools/call":
          HandleToolCall( id, paramsObj, response );
          break;

        default:
          TryWriteJson( response, JsonRpc.BuildError( id, -32601, "Method not found: " + method ) );
          break;
      }
    }

    void HandleToolCall( JSONData id, JSONObject paramsObj, HttpListenerResponse response )
    {
      var toolName = JsonUtil.GetStringOrNull( paramsObj, "name" );
      var arguments = paramsObj != null ? paramsObj.GetObject( "arguments" ) : null;

      if ( arguments == null )
      {
        arguments = new JSONObject();
      }

      ToolCallOutcome outcome;

      try
      {
        outcome = protocol.ExecuteTool( toolName, arguments );
      }
      catch ( Exception e )
      {
        RJLog.Error( "MCP", "Tool execution failed", toolName, e );
        outcome = ToolCallOutcome.Error( e.Message );
      }

      TryWriteJson( response, protocol.ToolCallResult( id, outcome ) );
    }

    void TryWriteJson( HttpListenerResponse response, JSONData data )
    {
      try
      {
        WriteJson( response, 200, data );
      }
      catch ( Exception e )
      {
        // Client likely disconnected before the response could be written - nothing to recover.
        RJLog.Error( "MCP", "Failed to write response", e );
      }
    }

    void TryWriteEmpty( HttpListenerResponse response, int statusCode )
    {
      try
      {
        WriteEmpty( response, statusCode );
      }
      catch ( Exception e )
      {
        RJLog.Error( "MCP", "Failed to write response", e );
      }
    }

    static void WriteJson( HttpListenerResponse response, int statusCode, JSONData data )
    {
      var text = data == null ? "" : JSON.Stringify( data );
      var bytes = Encoding.UTF8.GetBytes( text );

      response.StatusCode = statusCode;
      response.ContentType = "application/json";
      response.ContentLength64 = bytes.Length;
      response.OutputStream.Write( bytes, 0, bytes.Length );
      response.OutputStream.Close();
    }

    static void WriteEmpty( HttpListenerResponse response, int statusCode )
    {
      response.StatusCode = statusCode;
      response.ContentLength64 = 0;
      response.OutputStream.Close();
    }
  }
}

#endif