TIdTCPServer hangs when setting Active = false

I was looking at this example for using TIdTCPServer/client components and I found that if there are any clients then the server component will hang when you change active to false. Specifically, it hangs on the call to the Windows "ExitThread" function call for the context thread.

To reproduce the behavior:

1. run the server,
2. click the "Start Server" button,
3. run a client,
4. click the connect button
5. click the "Stop Server" button

I want a simple TCP server to monitor a process over the LAN but I can't figure out how to prevent this lock up. I have found a lot of information that skirts around this but nothing has made sense to me yet. I'm using Delphi 10.2 on Win 8.1 with Indy 10.6.2.5366.

ExitThread() can't hang, unless a DLL is misbehaving in its DllMain/DllEntryPoint() handler, causing a deadlock in the DLL loader. But, the server's Active property setter can certainly hang, such as if any of the client threads are deadlocked.

The example you linked to is NOT a good example to follow. The threaded event handlers are doing things that are not thread-safe. They are accessing UI controls without syncing with the main UI thread, which can cause many problems including deadlocks and dead UI controls. And the server's broadcast method is implemented all wrong, making it prone to deadlocks, crashes, and data corruption.

Whoever wrote that example (not me) clearly didn't know what they were doing. It needs to be rewritten to take thread safety into account properly. Try something more like this instead:

unit UServer;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, IdContext, IdComponent, Vcl.StdCtrls,
  IdBaseComponent, IdCustomTCPServer, IdTCPServer, Vcl.ExtCtrls;

type
  TFServer = class(TForm)
    Title         : TLabel;

    btn_start     : TButton;
    btn_stop      : TButton;
    btn_clear     : TButton;

    clients_connected : TLabel;

    IdTCPServer   : TIdTCPServer;
    Label1        : TLabel;
    Panel1        : TPanel;
    messagesLog   : TMemo;

    procedure FormShow(Sender: TObject);

    procedure btn_startClick(Sender: TObject);
    procedure btn_stopClick(Sender: TObject);
    procedure btn_clearClick(Sender: TObject);

    procedure IdTCPServerConnect(AContext: TIdContext);
    procedure IdTCPServerDisconnect(AContext: TIdContext);
    procedure IdTCPServerExecute(AContext: TIdContext);
    procedure IdTCPServerStatus(ASender: TObject; const AStatus: TIdStatus;
                                const AStatusText: string);

  private
    { Private declarations }

    procedure broadcastMessage(p_message : string);
    procedure Log(p_who, p_message: string);
    procedure UpdateClientsConnected(ignoreOne: boolean);

  public
    { Public declarations }

  end;
  // ...

var
  FServer     : TFServer;

implementation

uses
  IdGlobal, IdYarn, IdThreadSafe;

{$R *.dfm}

// ... listening port
const
  GUEST_CLIENT_PORT = 20010;

// *****************************************************************************
//   CLASS : TMyContext
//           HELPER CLASS FOR QUEUING OUTBOUND MESSAGES TO A CLIENT
// *****************************************************************************
type
  TMyContext = class(TIdServerContext)
  private
    FQueue: TIdThreadSafeStringList;
    FAnyInQueue: Boolean;
  public
    constructor Create(AConnection: TIdTCPConnection; AYarn: TIdYarn; AList: TIdContextThreadList = nil); override;
    destructor Destroy; override;
    procedure AddToQueue(p_message: string);
    procedure CheckQueue;
  end;

constructor TMyContext.Create(AConnection: TIdTCPConnection; AYarn: TIdYarn; AList: TIdContextThreadList = nil);
begin
  inherited Create(AConnection, AYarn, AList);
  FQueue := TIdThreadSafeStringList.Create;
  FAnyQueued := false;
end;

destructor TMyContext.Destroy;
begin
  FQueue.Free;
  inherited;
end;

procedure TMyContext.AddToQueue(p_message: string);
begin
  with FQueue.Lock do
  try
    Add(p_message);
    FAnyInQueue := true;
  finally
    FQueue.Unlock;
  end;
end;

procedure TMyContext.CheckQueue;
var
  queue, tmpList  : TStringList;
  i               : integer;
begin
  if not FAnyInQueue then Exit;
  tmpList := TStringList.Create;
  try
    queue := FQueue.Lock;
    try
      tmpList.Assign(queue);
      queue.Clear;
      FAnyInQueue := false;
    finally
      FQueue.Unlock;
    end;
    for i := 0 to tmpList.Count - 1 do begin
      Connection.IOHandler.WriteLn(tmpList[i]);
    end;
  finally
    tmpList.Free;
  end;
end;
// .............................................................................


// *****************************************************************************
//   EVENT : onShow()
//           ON FORM SHOW
// *****************************************************************************
procedure TFServer.FormShow(Sender: TObject);
begin
  // ... INITIALIZE:

  // ... clear message log
  messagesLog.Lines.Clear;

  // ... zero to clients connected
  clients_connected.Caption := IntToStr(0);

  // ... set buttons
  btn_start.Visible := true;
  btn_start.Enabled := true;
  btn_stop.Visible  := false;

  // ... set context class
  IdTCPServer.ContextClass := TMyContext;
end;
// .............................................................................


// *****************************************************************************
//   EVENT : btn_startClick()
//           CLICK ON START BUTTON
// *****************************************************************************
procedure TFServer.btn_startClick(Sender: TObject);
begin
  btn_start.Enabled := false;

  // ... START SERVER:

  // ... clear the Bindings property ( ... Socket Handles )
  IdTCPServer.Bindings.Clear;
  // ... Bindings is a property of class: TIdSocketHandles;

  // ... add listening ports:

  // ... add a port for connections from guest clients.
  IdTCPServer.Bindings.Add.Port := GUEST_CLIENT_PORT;
  // ... etc..

  // ... ok, Active the Server!
  IdTCPServer.Active  := true;

  // ... hide start button
  btn_start.Visible   := false;

  // ... show stop button
  btn_stop.Visible    := true;
  btn_stop.Enabled    := true;

  // ... message log
  Log('SERVER', 'STARTED!');

end;
// .............................................................................


// *****************************************************************************
//   EVENT : btn_stopClick()
//           CLICK ON STOP BUTTON
// *****************************************************************************
procedure TFServer.btn_stopClick(Sender: TObject);
begin
  btn_stop.Enabled := false;

  // ... before stopping the server ... send 'good bye' to all clients connected
  broadcastMessage( 'Goodbye my Clients :)');

  // ... stop server!
  IdTCPServer.Active := false;

  // ... hide stop button
  btn_stop.Visible   := false;

  // ... show start button
  btn_start.Visible  := true;
  btn_start.Enabled  := true;

  // ... message log
  Log('SERVER', 'STOPPED!');

end;
// .............................................................................


// *****************************************************************************
//   EVENT : btn_clearClick()
//           CLICK ON CLEAR BUTTON
// *****************************************************************************
procedure TFServer.btn_clearClick(Sender: TObject);
begin
  //... clear messages log
  MessagesLog.Lines.Clear;
end;
// .............................................................................


// *****************************************************************************
//   EVENT : onConnect()
//           OCCURS ANY TIME A CLIENT IS CONNECTED
// *****************************************************************************
procedure TFServer.IdTCPServerConnect(AContext: TIdContext);
var
  PeerIP      : string;
  PeerPort    : TIdPort;
begin

  // ... OnConnect is a TIdServerThreadEvent property that represents the event
  //     handler signalled when a new client connection is connected to the server.

  // ... Use OnConnect to perform actions for the client after it is connected
  //     and prior to execution in the OnExecute event handler.

  // ... see indy doc:
  //     http://www.indyproject.org/sockets/docs/index.en.aspx

  // ... getting IP address and Port of Client that connected
  PeerIP    := AContext.Binding.PeerIP;
  PeerPort  := AContext.Binding.PeerPort;

  // ... message log ...........................................................
  Log('SERVER', 'Client Connected! Peer=' + PeerIP + ':' + IntToStr(PeerPort));
  // ...

  // ... update number of clients connected
  UpdateClientsConnected(false);
  // ...

  // ... send the Welcome message to Client connected
  AContext.Connection.IOHandler.WriteLn('Welcome GUEST Client :)');

end;
// .............................................................................


// *****************************************************************************
//   EVENT : onDisconnect()
//           OCCURS ANY TIME A CLIENT IS DISCONNECTED
// *****************************************************************************
procedure TFServer.IdTCPServerDisconnect(AContext: TIdContext);
var
  PeerIP      : string;
  PeerPort    : TIdPort;
begin

  // ... getting IP address and Port of Client that connected
  PeerIP    := AContext.Binding.PeerIP;
  PeerPort  := AContext.Binding.PeerPort;

  // ... message log ...........................................................
  Log('SERVER', 'Client Disconnected! Peer=' + PeerIP + ':' + IntToStr(PeerPort));
  // ...

  // ... update number of clients connected
  UpdateClientsConnected(true);
  // ...

end;
// .............................................................................


// *****************************************************************************
//   EVENT : onExecute()
//           ON EXECUTE THREAD CLIENT
// *****************************************************************************
procedure TFServer.IdTCPServerExecute(AContext: TIdContext);
var
  PeerIP        : string;
  PeerPort      : TIdPort;
  msgFromClient : string;
begin

  // ... OnExecute is a TIdServerThreadEvents event handler used to execute
  //     the task for a client connection to the server.

  // ... check for pending broadcast messages to the client
  TMyContext(AContext).CheckQueue;
  // ...

  // check for inbound messages from client
  if AContext.Connection.IOHandler.InputBufferIsEmpty then
  begin
    AContext.Connection.IOHandler.CheckForDataOnSource(100);
    AContext.Connection.IOHandler.CheckForDisconnect;
    if AContext.Connection.IOHandler.InputBufferIsEmpty then Exit;
  end;

  // ... received a message from the client

  // ... get message from client
  msgFromClient := AContext.Connection.IOHandler.ReadLn;

  // ... getting IP address, Port and PeerPort from Client that connected
  PeerIP    := AContext.Binding.PeerIP;
  PeerPort  := AContext.Binding.PeerPort;

  // ... message log ...........................................................
  Log('CLIENT', '(Peer=' + PeerIP + ':' + IntToStr(PeerPort) + ') ' + msgFromClient);
  // ...

  // ... process message (request) from Client

  // ...

  // ... send response to Client

  AContext.Connection.IOHandler.WriteLn('... response from server :)');

end;
// .............................................................................


// *****************************************************************************
//   EVENT : onStatus()
//           ON STATUS CONNECTION
// *****************************************************************************
procedure TFServer.IdTCPServerStatus(ASender: TObject; const AStatus: TIdStatus;
                                     const AStatusText: string);
begin

  // ... OnStatus is a TIdStatusEvent property that represents the event handler
  //     triggered when the current connection state is changed...

  // ... message log
  Log('SERVER', AStatusText);
end;
// .............................................................................


// *****************************************************************************
//   PROCEDURE : broadcastMessage()
//               BROADCAST A MESSAGE TO ALL CLIENTS CONNECTED
// *****************************************************************************
procedure TFServer.broadcastMessage( p_message : string );
var
  tmpList      : TIdContextList;
  contexClient : TIdContext;
  i            : integer;
begin

  // ... send a message to all clients connected

  // ... get context Locklist
  tmpList := IdTCPServer.Contexts.LockList;
  try
    for i := 0 to tmpList.Count-1 do begin
      // ... get context ( thread of i-client )
      contexClient := tmpList[i];

      // ... queue message to client
      TMyContext(contexClient).AddToQueue(p_message);
    end;
  finally
    // ... unlock list of clients!
    IdTCPServer.Contexts.UnlockList;
  end;

end;
// .............................................................................


// *****************************************************************************
//   PROCEDURE : Log()
//               LOG A MESSAGE TO THE UI
// *****************************************************************************
procedure TFServer.Log(p_who, p_message : string);
begin
  TThread.Queue(nil,
    procedure
    begin
      MessagesLog.Lines.Add('[' + p_who + '] - ' + FormatDateTime('yyyy-mm-dd hh:nn:ss', Now) + ': ' + p_message);
    end
  );
end;
// .............................................................................


// *****************************************************************************
//   PROCEDURE : UpdateClientsConnected()
//               DISPLAY THE NUMBER OF CLIENTS CONNECTED
// *****************************************************************************
procedure TFServer.UpdateClientsConnected(ignoreOne: Boolean);
var
  NumClients: integer;
begin
  with IdTCPServer.Contexts.LockList do
  try
    NumClients := Count;
  finally
    IdTCPServer.Contexts.UnlockList;
  end;

  if ignoreOne then Dec(NumClients);

  TThread.Queue(nil,
    procedure
    begin
      clients_connected.Caption := IntToStr(NumClients);
    end
  );
end;
// .............................................................................

end.

unit UClient;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, IdBaseComponent,
  IdComponent, IdTCPConnection, IdTCPClient, IdThreadComponent;

type
  TFClient = class(TForm)

    Label1        : TLabel;
    Label2        : TLabel;

    messageToSend : TMemo;
    messagesLog   : TMemo;

    btn_connect   : TButton;
    btn_disconnect: TButton;
    btn_send      : TButton;

    // ... TIdTCPClient
    IdTCPClient       : TIdTCPClient;

    // ... TIdThreadComponent
    IdThreadComponent : TIdThreadComponent;

    procedure FormShow(Sender: TObject);

    procedure btn_connectClick(Sender: TObject);
    procedure btn_disconnectClick(Sender: TObject);
    procedure btn_sendClick(Sender: TObject);

    procedure IdTCPClientConnected(Sender: TObject);
    procedure IdTCPClientDisconnected(Sender: TObject);

    procedure IdThreadComponentRun(Sender: TIdThreadComponent);


  private
    { Private declarations }

    procedure Log(p_who, p_message: string);

  public
    { Public declarations }

  end;

var
  FClient     : TFClient;

implementation

{$R *.dfm}

// ... listening port: GUEST CLIENT
const
  GUEST_PORT = 20010;

// *****************************************************************************
//   EVENT : onShow()
//           ON SHOW FORM
// *****************************************************************************
procedure TFClient.FormShow(Sender: TObject);
begin

  // ... INITAILIZE

  // ... message to send
  messageToSend.Clear;
  messageToSend.Enabled     := false;

  // ... log
  messagesLog.Clear;

  // ... buttons
  btn_connect.Enabled       := true;
  btn_disconnect.Enabled    := false;
  btn_send.Enabled          := false;

end;
// .............................................................................


// *****************************************************************************
//   EVENT : btn_connectClick()
//           CLICK ON CONNECT BUTTON
// *****************************************************************************
procedure TFClient.btn_connectClick(Sender: TObject);
begin
  btn_connect.Enabled := false;

  // ... try to connect to Server
  try
    IdTCPClient.Connect;
  except
    on E: Exception do begin
      Log('CLIENT', 'CONNECTION ERROR! ' + E.Message);
      btn_connect.Enabled := true;
    end;
  end;

end;
// .............................................................................


// *****************************************************************************
//   EVENT : btn_disconnectClick()
//           CLICK ON DISCONNECT BUTTON
// *****************************************************************************
procedure TFClient.btn_disconnectClick(Sender: TObject);
begin
  btn_disconnect.Enabled := false;

  // ... disconnect from Server
  IdTCPClient.Disconnect;

  // ... set buttons
  btn_connect.Enabled       := true;
  btn_send.Enabled          := false;

  // ... message to send
  messageToSend.Enabled     := false;
end;
// .............................................................................


// *****************************************************************************
//   EVENT : onConnected()
//           OCCURS WHEN CLIENT IS CONNECTED
// *****************************************************************************
procedure TFClient.IdTCPClientConnected(Sender: TObject);
begin
  // ... messages log
  Log('CLIENT', 'CONNECTED!');

  // ... after connection is ok, run the Thread ... waiting messages 
  //     from server
  IdThreadComponent.Active := true;

  // ... set buttons
  btn_disconnect.Enabled    := true;
  btn_send.Enabled          := true;

  // ... enable message to send
  messageToSend.Enabled     := true;
end;
// .............................................................................


// *****************************************************************************
//   EVENT : onDisconnected()
//           OCCURS WHEN CLIENT IS DISCONNECTED
// *****************************************************************************
procedure TFClient.IdTCPClientDisconnected(Sender: TObject);
begin
  // ... message log
  Log('CLIENT', 'DISCONNECTED!');
end;
// .............................................................................


// *****************************************************************************
//   EVENT : btn_sendClick()
//           CLICK ON SEND BUTTON
// *****************************************************************************
procedure TFClient.btn_sendClick(Sender: TObject);
begin
  // ... send message to Server
  IdTCPClient.IOHandler.WriteLn(messageToSend.Text);
end;
// .............................................................................


// *****************************************************************************
//   EVENT : onRun()
//           OCCURS WHEN THE SERVER SEND A MESSAGE TO CLIENT
// *****************************************************************************
procedure TFClient.IdThreadComponentRun(Sender: TIdThreadComponent);
var
  msgFromServer : string;
begin
  // ... read message from server
  msgFromServer := IdTCPClient.IOHandler.ReadLn();

  // ... messages log
  Log('SERVER', msgFromServer);
end;
// .............................................................................


// *****************************************************************************
//   FUNCTION : Log()
//              LOGS A MESSAGE TO THE UI
// *****************************************************************************
procedure TFClient.Log(p_who, p_message: string);
begin
  TThread.Queue(nil,
    procedure
    begin
      MessagesLog.Lines.Add('[' + p_who + '] - ' + FormatDateTime('yyyy-mm-dd hh:nn:ss', Now) + ': ' + p_message);
    end
  );
end;
// .............................................................................

end.