Delphi XE2 / Indy TIdTCPServer / "Connection reset by peer"

Question

Your Clients list is not protected from multithreaded access. TIdTCPServer is a multi-threaded component, each client runs in its own worker thread. You need to take that into account. I suggest you get rid of your Clients list altogether and use the TIdTCPServer.Contexts property instead. Otherwise, you need to protect your Clients list, such as by changing it to a TThreadList, or at least wrapping it with a TCriticalSection (which is what TThreadList does internally).

Another problem I see is that you are setting your Client.DNS field to the wrong value, which may affect your communications depending on what you are using Client.DNS for exactly.

Try this instead:

procedure TMainHost.idTCPServerNewConnect(AContext: TIdContext);
var
  Client: TSimpleClient;
begin
  Client := TSimpleClient.Create();

  Client.IP := AContext.Binding.PeerIP;
  Client.DNS := GStack.HostByAddress(Client.IP, AContext.Binding.IPVersion);
  Client.Conectado := True;
  Client.Port := AContext.Binding.Port;
  Client.Name := 'Central';
  Client.Thread := AContext;

  AContext.Data := Client;

  // this may or may not need to be Synchronized, depending on what it actually does...
  if (MainEstrutura.current_central.IP = Client.IP) then
  begin
    MainEstrutura.current_central.Conectado := true;
    MainEstrutura.envia_configuracao;
  end;
end;

procedure TMainHost.idTCPServerNewDisconnect(AContext: TIdContext);
var
  Client: TSimpleClient;
begin
  { Retrieve Client Record from Data pointer }
  Client := TSimpleClient(AContext.Data);
  { Free the Client object }
  FreeAndNil(Client);
  AContext.Data := nil;    
end;

procedure TMainHost.DirectTCPMessage(IP: String; TheMessage: String);
var
  List: TIdContextList; // or TList in an earlier version that did not have TIdContextList yet
  Context: TIdContext;
  i: Integer;
  Msg: String;
begin
  Msg := Trim(TheMessage);

  List := idTCPServerNew.Contexts.LockList;
  try
    for i := 0 to List.Count - 1 do
    begin
      Context := Context(List[i]);
      if TSimpleClient(Context.Data).IP = IP then
      begin
        try
          Context.Connection.IOHandler.WriteLn(Msg);
        except
        end;
        Break;
      end;
    end;
  finally
    idTCPServerNew.Contexts.UnlockList;
  end;
end;

With that said, if your server sends any data from inside of the OnExecute event or CommandsHandlers collection then this approach of sending a message to a client from outside of its thread is not safe, as you risk overlapping data that corrupts the communication with that client. A safer approach is to queue the outgoing data and have the OnExecute event send the data when it is safe to do so, eg:

procedure TMainHost.idTCPServerNewConnect(AContext: TIdContext);
var
  Client: TSimpleClient;
begin
  Client := TSimpleClient.Create();
  ...
  Client.Queue := TIdThreadSafeStringList.Create; // <-- add this
  ...
end;

procedure TMainHost.idTCPServerNewExecute(AContext: TIdContext);
var
  List: TStringList;
  I: Integer;
begin
  Client := TSimpleClient(AContext.Data);
  ...
  List := Client.Queue.Lock;
  try
    while List.Count > 0 do
    begin
      AContext.Connection.IOHandler.WriteLn(List[0]);
      List.Delete(0);
    end;
  finally
    Client.Queue.Unlock;
  end;
  ...
end;

procedure TMainHost.DirectTCPMessage(IP: String; TheMessage: String);
var
  List: TIdContextList; // or TList in an earlier version that did not have TIdContextList yet
  Context: TIdContext;
  i: Integer;
  Msg: String;
begin
  Msg := Trim(TheMessage);

  List := idTCPServerNew.Contexts.LockList;
  try
    for i := 0 to List.Count - 1 do
    begin
      Context := Context(List[i]);
      if TSimpleClient(Context.Data).IP = IP then
      begin
        TSimpleClient(Context.Data).Queue.Add(Msg);
        Break;
      end;
    end;
  finally
    idTCPServerNew.Contexts.UnlockList;
  end;
end;

Update: that being said, I would suggest deriving TSimpleClient from TIdServerContext and assign that to the server's ContextsClass property, then you don't need to use the TIdContext.Data property anymore:

type
  TSimpleClient = class(TIdServerContext)
  public
    Queue: TIdThreadSafeStringList;
    ...
    // or TThreadList in an earlier version that did not have TIdContextThreadList yet
    constructor Create(AConnection: TIdTCPConnection; AYarn: TIdYarn; AList: TIdContextThreadList = nil); override;
    destructor Destroy; override;
  end;

constructor TSimpleClient.Create(AConnection: TIdTCPConnection; AYarn: TIdYarn; AList: TIdContextThreadList = nil);
begin
  inherited;
  Queue := TIdThreadSafeStringList.Create;
  ...
end;

destructor TSimpleClient.Destroy;
begin
  ...
  Queue.Free;
  inherited;
end;

procedure TMainHost.FormCreate(Sener: TObject);
begin
  // this must be assigned before the server is activated
  idTCPServerNew.ContextClass := TSimpleClient;
end;

procedure TMainHost.idTCPServerNewConnect(AContext: TIdContext);
var
  Client: TSimpleClient;
  ...
 begin
  Client := AContext as TSimpleClient;
  // use Client as needed...
end;

procedure TMainHost.idTCPServerNewExecute(AContext: TIdContext);
var
  Client: TSimpleClient;
  ...
begin
  Client := AContext as TSimpleClient;
  // use Client as needed...
end;

procedure TMainHost.DirectTCPMessage(IP: String; TheMessage: String);
var
  List: TIdContextList; // or TList in an earlier version that did not have TIdContextList yet
  Client: TSimpleClient;
  i: Integer;
  Msg: String;
begin
  Msg := Trim(TheMessage);

  List := idTCPServerNew.Contexts.LockList;
  try
    for i := 0 to List.Count - 1 do
    begin
      Client := TIdContext(Context(List[i])) as TSimpleClient;
      if Client.IP = IP then
      begin
        Client.Queue.Add(Msg);
        Break;
      end;
    end;
  finally
    idTCPServerNew.Contexts.UnlockList;
  end;
end;