Delphi simple TCP server hangs. The form freezes but the server continues managing the clients.

Question

I'm using a form with an IdTCPServer on it managing strings from the client with a AThread.connection.readln/writeln system. The string handling works and that isn't the problem.

The thing is, the form with the server on it hangs and will not load, but it still managed all the clients connected to it so it IS running but it just doesn't work as a form. I'll make a guess that its sitting on a readline or something... but I have NO idea how i can fix this at this moment in time.

Please help.

procedure TMonitorFrm.ServerExecute(AThread: TIdPeerThread);

    procedure post(PostMessage:string);
    begin
            try
                    AThread.Connection.WriteLn(PostMessage);
            except
                    showmessage('Cannot post');
            end;
    end;

var
        ActClient       : PClient;
        sTemp,
        CommBlock,
        NewCommBlock,
        ReceiverName,
        sContent,
        sSQL,
        sCommand        : String;
        iCount2,
        iCount          : Integer;

        sldb    : TSQLiteDatabase;
        sltb    : TSQLiteTable;

begin
        if not AThread.Terminated and AThread.Connection.Connected then
        begin
                CommBlock := AThread.Connection.ReadLn();
                ActClient := PClient(AThread.Data);
                ActClient.LastAction := Now;
                sCommand := copy(CommBlock,0,pos(',',CommBlock)-1); {seperate command}
                sContent := copy(CommBlock,pos(',',CommBlock)+1,length(CommBlock)-(pos(',',CommBlock)+1)); {seperate data block}
                iCount:= 0 ;

            if sCommand = 'Announce' then //SPECIAL
            begin
                    { Do stuff for this command...}
            end

            else if sCommand = 'CheckSect' then
                    {Etcetera...}

procedure TMonitorFrm.FormCreate(Sender: TObject);
var
        sCompetitionID  : string;
        sldb    : TSQLiteDatabase;
        sltb    : TSQLiteTable;
begin
        Clients := TThreadList.Create;
        Server.Active := True;
        AreaPnlList := TComponentList.Create;
        SectionPnlList := TComponentList.Create;
        Repeat until InputQuery('Competition Select', 'Please type the ID of the competition', sCompetitionID);
        iCompetitionID:=StrToInt(sCompetitionID);
        OpenDatabase(slDb);
        sltb:=slDb.GetTable('SELECT * FROM SectionTable WHERE CompetitionID='+sCompetitionID);
        Frame31.CreateSections(sltb,Frame31);
        sltb.Free;
        CloseDatabase(slDb);
{
This section needs to check the SQLite databases for sections and list them in the display window and makes a drag n drop profile...
}
end;

Answer 1

If you think you cannot reliably trace the execution with the debugger, you can add some explicit trace information in your code.
Use OutputDebugString to log information either in the IDE EventLog when running from the IDE or to a DbgView window (from SysInternals) when running outside Delphi.

Answer 2

Indy uses blocking sockets. It's supposed to hang the current thread. To use Indy components in the VCL thread while keeping the VCL thread responsive, put a TIdAntifreeze component on the form as well. The Indy components are aware of that component and will yield control to it periodically so that your VCL thread can continue processing messages.

Answer 3

Remember that VCL components are not thread safe, so you cannot access them directly from thread, otherwise something will go wrong, eg main thread can get stuck.

So check that TMonitorFrm.ServerExecute doesn't do anything with forms and components. If you need to update something, use Synchronize.

Answer 4

This is how to stop the Readln causing it to freeze. You need to check if there is something there to read first. There are a couple ways to do this, you can make the readln timeout

Repeat
    CommBlock := '';
    CommBlock := AThread.Connection.ReadLn('',10);
    Application.ProcessMessages;
until Commblock <> '';

Or you can check if the connection is readable before tryign to read.

Repeat
    CommBlock:='';
    if AThread.Connection.IOHandler.Readable()then
    begin
            CommBlock := AThread.Connection.ReadLn();
    end
    else
    begin
            application.ProcessMessages;
    end;
until commblock <> '';

Either way, the key line is Application.ProcessMessages as this allows the main thread to check its messages and keep running.