Issue: Multiprocessing windows error

As described here https://huggingface.co/blog/fine-tune-xlsr-wav2vec2

When using the num_proc argument on windows the whole Python environment crashes and hanging in loop.
For example at the map_to_array part.
An error occures because the cache file already exists and windows throws and error. After this the log crashes into an loop 

Comments:
Hi ! Thanks for reporting.
This looks like a bug, could you try to provide a minimal code example that reproduces the issue ? This would be very helpful !

Otherwise I can try to run the wav2vec2 code above on my side but probably not this week..

```
from datasets import load_dataset

dataset = load_dataset('glue', 'mrpc', split='train')


updated_dataset = dataset.map(lambda example: {'sentence1': 'My sentence: ' + example['sentence1']}, num_proc=4)

```






I was able to copy some of the shell 
This is repeating every half second
Win 10, Anaconda with python 3.8, datasets installed from main branche
```

  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 287, in _fixup_main_from_path
    _check_not_importing_main()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 116, in spawn_main
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 134, in _check_not_importing_main
    main_content = runpy.run_path(main_path,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 265, in run_path
    exitcode = _main(fd, parent_sentinel)
    raise RuntimeError('''
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 125, in _main
RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.    return _run_module_code(code, init_globals, run_name,
    prepare(preparation_data)

  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 97, in _run_module_code
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 236, in prepare
    _run_code(code, mod_globals, init_globals,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 87, in _run_code
    _fixup_main_from_path(data['init_main_from_path'])
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 287, in _fixup_main_from_path
    exec(code, run_globals)
  File "F:\Codes\Python Apps\asr\test.py", line 6, in <module>
    updated_dataset = dataset.map(lambda example: {'sentence1': 'My sentence: ' + example['sentence1']}, num_proc=4)
    main_content = runpy.run_path(main_path,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\datasets\arrow_dataset.py", line 1370, in map
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 265, in run_path
    with Pool(num_proc, initargs=(RLock(),), initializer=tqdm.set_lock) as pool:
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\context.py", line 119, in Pool
    return _run_module_code(code, init_globals, run_name,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
    return Pool(processes, initializer, initargs, maxtasksperchild,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 87, in _run_code
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 212, in __init__
    exec(code, run_globals)
  File "F:\Codes\Python Apps\asr\test.py", line 6, in <module>
    self._repopulate_pool()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 303, in _repopulate_pool
    updated_dataset = dataset.map(lambda example: {'sentence1': 'My sentence: ' + example['sentence1']}, num_proc=4)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\datasets\arrow_dataset.py", line 1370, in map
    return self._repopulate_pool_static(self._ctx, self.Process,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 326, in _repopulate_pool_static
    with Pool(num_proc, initargs=(RLock(),), initializer=tqdm.set_lock) as pool:
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\context.py", line 119, in Pool
    w.start()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\process.py", line 121, in start
    return Pool(processes, initializer, initargs, maxtasksperchild,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 212, in __init__
    self._popen = self._Popen(self)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\context.py", line 327, in _Popen
    self._repopulate_pool()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 303, in _repopulate_pool
    return Popen(process_obj)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\popen_spawn_win32.py", line 45, in __init__
    return self._repopulate_pool_static(self._ctx, self.Process,
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 326, in _repopulate_pool_static
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 154, in get_preparation_data
    _check_not_importing_main()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 134, in _check_not_importing_main
    w.start()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\process.py", line 121, in start
    raise RuntimeError('''
RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...
```

Thanks this is really helpful !
I'll try to reproduce on my side and come back to you

if __name__ == '__main__':


This line before calling the map function stops the error but the script still repeats endless

Indeed you needed `if __name__ == '__main__'` since accoding to [this stackoverflow post](https://stackoverflow.com/a/18205006):

> On Windows the subprocesses will import (i.e. execute) the main module at start. You need to insert an if __name__ == '__main__': guard in the main module to avoid creating subprocesses recursively.

Regarding the hanging issue, can you try to update `dill` and `multiprocess` ?

It's already on the newest version

```
Traceback (most recent call last):
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\shutil.py", line 791, in move
    os.rename(src, real_dst)
FileExistsError: [WinError 183] Eine Datei kann nicht erstellt werden, wenn sie bereits vorhanden ist: 'D:\\huggingfacecache\\common_voice\\de\\6.1.0\\0041e06ab061b91d0a23234a2221e87970a19cf3a81b20901474cffffeb7869f\\tmpx9fl_jg8' -> 'D:\\huggingfacecache\\common_voice\\de\\6.1.0\\0041e06ab061b91d0a23234a2221e87970a19cf3a81b20901474cffffeb7869f\\cache-9b4f203a63742dfc.arrow'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 125, in _main
    prepare(preparation_data)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 236, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 287, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 265, in run_path
    return _run_module_code(code, init_globals, run_name,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "F:\Codes\Python Apps\asr\cvtrain.py", line 243, in <module>
    common_voice_train = common_voice_train.map(remove_special_characters, remove_columns=["sentence"])
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\datasets\arrow_dataset.py", line 1339, in map
    return self._map_single(
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\datasets\arrow_dataset.py", line 203, in wrapper
    out: Union["Dataset", "DatasetDict"] = func(self, *args, **kwargs)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\datasets\fingerprint.py", line 337, in wrapper
    out = func(self, *args, **kwargs)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\datasets\arrow_dataset.py", line 1646, in _map_single
    shutil.move(tmp_file.name, cache_file_name)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\shutil.py", line 805, in move
    copy_function(src, real_dst)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\shutil.py", line 435, in copy2
    copyfile(src, dst, follow_symlinks=follow_symlinks)
  0%|                                                                                                                                                                                                 | 0/27771 [00:00<?, ?ex/s] 
 File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\shutil.py", line 264, in copyfile
    with open(src, 'rb') as fsrc, open(dst, 'wb') as fdst:
OSError: [Errno 22] Invalid argument: 'D:\\huggingfacecache\\common_voice\\de\\6.1.0\\0041e06ab061b91d0a23234a2221e87970a19cf3a81b20901474cffffeb7869f\\cache-9b4f203a63742dfc.arrow'
```

I was adding freeze support before calling the mapping function like this
if __name__ == '__main__':
    freeze_support()
   dataset.map(....)

Usually OSError of an arrow file on windows means that the file is currently opened as a dataset object, so you can't overwrite it until the dataset object falls out of scope.
Can you make sure that there's no dataset object that loaded the `cache-9b4f203a63742dfc.arrow` file ?

Now I understand
The error occures because the script got restarted in another thread, so the object is already loaded.
Still don't have an idea why a new thread starts the whole script again