Socket.ReadTimeout Exception handling

I am using Delphi 12.1 and Indy 10 to implement a client/server application, in which the client asks for data and waits until the server sends it:

procedure TSocketThread.Execute;
var
  Buffer: TIdBytes;
begin
  try
    FClient.Socket.ReadTimeout := 1000;
    // Send the message to the server
    FClient.Socket.WriteLn(FMessage);
    // Allocate a buffer to read the response
    SetLength(Buffer, (4096+1024)*4 + 4); // +4 for CRC
    // Read bytes from the socket
    FClient.IOHandler.ReadBytes(Buffer, Length(Buffer), False); // Read bytes from the server
    // Synchronize the response to the main thread for UI updates
    Synchronize(procedure
    begin
      // Update UI or handle the received response here
     Form1.MemoResponse.Lines.Add('Received Data OK');
    end);
  except
    on E: Exception do
    begin
      // Handle exceptions and notify the user
      Synchronize(procedure
      begin
         ShowMessage('Error: ' + E.Message);
      end);
    end;
  end;
end;

Now, I want to include a safety function using Socket.ReadTimeout for the case of not enough bytes being received. If this is the case, the exception is being raised as expected.

I would like to know, how I can capture this specific expection of Socket.ReadTimeout such as:

    on E: EIdSocketError do
    begin
      // Handle Socket.ReadTimeout
    // Send the message to the server again
    FClient.Socket.WriteLn(FMessage);    
    end;

However, this might not be possible, since EIdSocketError can not be found.

Also, do you think that asking for a new message by exceptions is a good way of doing it (I would also limit this to a limited number of new requests)?

Solution

EIdSocketError is declared in the IdStack unit, but the read timeout error is EIdReadTimeout which is declared in the EIdExceptionCore unit. Add those units to your uses clause as needed.

Requesting a new message in reply to a read timeout exception is not a good idea. At the point when that error occurs, you don't know if the socket is still in a good state anymore. There may have been some bytes already received, leaving a partial message in the buffer. So, if you request a new message without clearing out a partial message left behind from before the timeout, you will corrupt your reading.

1 second is a pretty small timeout, I would recommend at least 5-10+ seconds instead, and if a read timeout does occur then the only sane thing to do is reconnect the connection and start over.

On the other hand, you can use the CheckForDataOnSource() method to wait for data to arrive without actually reading it, so that you don't try to read partial messages, eg:

procedure TSocketThread.Execute;
var
  Buffer: TIdBytes;
  I: Integer;
begin
  try
    FClient.Socket.ReadTimeout := 5000;

    for I := 1 to 5 do
    begin
      // Send the message to the server
      FClient.Socket.WriteLn(FMessage);

      if FClient.Socket.CheckForDataOnSource(1000) then
      begin
        // Allocate a buffer to read the response
        SetLength(Buffer, (4096+1024)*4 + 4); // +4 for CRC
        // Read bytes from the socket
        FClient.IOHandler.ReadBytes(Buffer, Length(Buffer), False); // Read bytes from the server

        // Synchronize the response to the main thread for UI updates
        Synchronize(procedure
        begin
          // Update UI or handle the received response here
          Form1.MemoResponse.Lines.Add('Received Data OK');
        end);

        Exit;
      end;
    end;

    Synchronize(procedure
    begin
      // Handle timeout and notify the user
      Form1.MemoResponse.Lines.Add('Error: No response received');
    end);

  except
    on E: Exception do
    begin
      // Handle exceptions and notify the user
      Synchronize(procedure
      begin
        Form1.MemoResponse.Lines.Add('Error: ' + E.Message);
      end);
    end;
  end;
end;