Program: 20998
Project: 20998
Buggy source:
private void startDownloadThread(DownloadLink dlink){
  Interaction.handleInteraction(Interaction.INTERACTION_BEFORE_DOWNLOAD,dlink);
  String outputFilePath=dlink.getFileOutput();
  int index=outputFilePath.lastIndexOf(PATH_SEPARATOR);
  String outputFolder=outputFilePath.substring(0,index);
  File destinationFolder=new File(outputFolder);
  if (!destinationFolder.exists()) {
    if (!destinationFolder.mkdir()) {
      logger.warning("String_Node_Str" + destinationFolder + "String_Node_Str");
      LinkStatus status=dlink.getLinkStatus();
      status.setErrorMessage("String_Node_Str" + outputFolder);
      status.addStatus(LinkStatus.ERROR_LOCAL_IO);
      dlink.setEnabled(false);
      dlink.requestGuiUpdate();
    }
  }
  long freeDiskSpace=destinationFolder.getUsableSpace();
  long fileSize=dlink.getDownloadSize();
  if (freeDiskSpace > fileSize) {
    SingleDownloadController download=new SingleDownloadController(dlink);
    logger.info("String_Node_Str" + dlink.getHost());
    dlink.getLinkStatus().setActive(true);
    this.activateDownload(dlink,download);
    if (!stopMarkTracker.contains(dlink))     stopMarkTracker.add(dlink);
    download.start();
  }
 else {
    String fileName=dlink.getName();
    logger.warning("String_Node_Str" + fileName + "String_Node_Str"+ outputFolder+ "String_Node_Str"+ fileSize+ "String_Node_Str"+ freeDiskSpace+ "String_Node_Str");
    String title=JDL.L("String_Node_Str","String_Node_Str" + outputFolder + "String_Node_Str");
    String message=JDL.L("String_Node_Str","String_Node_Str" + fileName + "String_Node_Str");
    UserIO.getInstance().requestConfirmDialog(UserIO.NO_CANCEL_OPTION,title,message);
    LinkStatus status=dlink.getLinkStatus();
    status.setErrorMessage(message);
    status.addStatus(LinkStatus.ERROR_LOCAL_IO);
    dlink.setEnabled(false);
    dlink.requestGuiUpdate();
  }
}

Fixed version (ground truth):
private void startDownloadThread(DownloadLink dlink){
  Interaction.handleInteraction(Interaction.INTERACTION_BEFORE_DOWNLOAD,dlink);
  String outputFilePath=dlink.getFileOutput();
  int index=outputFilePath.lastIndexOf(PATH_SEPARATOR);
  String outputFolder=outputFilePath.substring(0,index);
  File destinationFolder=new File(outputFolder);
  if (!destinationFolder.exists()) {
    boolean createFolder=destinationFolder.mkdirs();
    if (!createFolder) {
      logger.warning("String_Node_Str" + destinationFolder + "String_Node_Str");
      LinkStatus status=dlink.getLinkStatus();
      status.setErrorMessage("String_Node_Str" + outputFolder);
      status.addStatus(LinkStatus.ERROR_LOCAL_IO);
      dlink.setEnabled(false);
      dlink.requestGuiUpdate();
    }
 else {
      logger.info("String_Node_Str" + destinationFolder + "String_Node_Str");
    }
  }
  long freeDiskSpace=destinationFolder.getUsableSpace();
  long fileSize=dlink.getDownloadSize();
  if (freeDiskSpace > fileSize) {
    SingleDownloadController download=new SingleDownloadController(dlink);
    logger.info("String_Node_Str" + dlink.getHost());
    dlink.getLinkStatus().setActive(true);
    this.activateDownload(dlink,download);
    if (!stopMarkTracker.contains(dlink))     stopMarkTracker.add(dlink);
    download.start();
  }
 else {
    String fileName=dlink.getName();
    logger.warning("String_Node_Str" + fileName + "String_Node_Str"+ outputFolder+ "String_Node_Str"+ fileSize+ "String_Node_Str"+ freeDiskSpace+ "String_Node_Str");
    String title=JDL.L("String_Node_Str","String_Node_Str" + outputFolder + "String_Node_Str");
    String message=JDL.L("String_Node_Str","String_Node_Str" + fileName + "String_Node_Str");
    UserIO.getInstance().requestConfirmDialog(UserIO.NO_CANCEL_OPTION,title,message);
    LinkStatus status=dlink.getLinkStatus();
    status.setErrorMessage(message);
    status.addStatus(LinkStatus.ERROR_LOCAL_IO);
    dlink.setEnabled(false);
    dlink.requestGuiUpdate();
  }
}