Hi all,

Preface this by saying I know next to nothing about multiprocessing and my debugging skills are failing me.

I'm trying to run a simple rio mbtiles command on Windows (Python 3.8, rasterio 1.2.9, rio_mbtiles 1.6.0) with a small test GeoTiff in Web Mercator using this syntax:

  > rio mbtiles
      --resampling average
      --zoom-levels 5..8
      --format PNG
      -j 4
      --progress-bar
      test.tif
      test.mbtiles

I often get this error stack trace:

  """
  File "c:\users\gregorma\envs\default-38\lib\site-packages\mbtiles\cf.py",
  line 62, in process_tiles
    tile, contents = future.result()
  File "c:\users\gregorma\appdata\local\programs\python\python38\
  lib\concurrent\futures\_base.py", line 432, in result
    return self.__get_result()
  File "c:\users\gregorma\appdata\local\programs\python\python38\
  lib\concurrent\futures\_base.py", line 388, in __get_result
    raise self._exception
  concurrent.futures.process.BrokenProcessPool: A process in the process pool
  was terminated abruptly while the future was running or pending.
  """

I can set up the same environment using WSL (Debian) and run the same command and it works flawlessly.  I don't think it's relevant but I'm using virtualenvs on both Windows and WSL.

I've read numerous posts about including either the if __name__ == "__main__" guard or (with multiprocessing) using freeze_support for helping with these issues on Windows, but have no idea where these would be inserted for this command (either at rio itself or within any of the plugins that use concurrent.futures). 

Has anyone had luck running rio mbtiles on a Windows box?  I obviously have the WSL workaround if needed.  Please let me know if more information is needed.

Thanks for any advice,
matt