Free disk space

It is not obvious how to get the free space, in a directory. Here is working code from the CudaText project.

uses SysUtils;

function AppDiskGetFreeSpace(const fn: string): Int64;
begin
  {$ifdef linux}
  //this crashes on FreeBSD 12 x64
  exit(SysUtils.DiskFree(SysUtils.AddDisk(ExtractFileDir(fn))));
  {$endif}

  {$ifdef windows}
  exit(SysUtils.DiskFree(SysUtils.GetDriveIDFromLetter(ExtractFileDrive(fn))));
  {$endif}

  //cannot detect
  exit(-1);
end;

The "fn" parameter should be any filename in the needed directory. E.g. if we need to get free space in the directory '/home/user/media', pass parameter fn as '/home/user/media/a'.

Note: code does not support Unixes other than Linux, because real tests show, that code for "$ifdef linux" part is crashing on FreeBSD 12 x64. Under FPC 3.2-fixes. Gitlab issue.