Summary:

multiprocessing.Pool contains a bug when the script is invoked with pdb.



Steps to reproduce:

Consider the following script:

```
from multiprocessing import Pool

def f(x):
    return x*x

if __name__ == '__main__':
    with Pool(5) as p:
        print(p.map(f, [1, 2, 3]))
```

When called as `python3.10 test.py`, this works fine, and returns `[1, 4, 9]`

When called as `python3.10 -m pdb test.py`, it gives the following exception:

```
freek@minnie ~ » python3.9 -m pdb test.py
> /Users/freek/test.py(3)<module>()
-> from multiprocessing import Pool
(Pdb) c
Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/pdb.py", line 1704, in main
    pdb._runscript(mainpyfile)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/pdb.py", line 1573, in _runscript
    self.run(statement)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/bdb.py", line 580, in run
    exec(cmd, globals, locals)
  File "<string>", line 1, in <module>
  File "/Users/freek/test.py", line 3, in <module>
    from multiprocessing import Pool
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py", line 119, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/pool.py", line 212, in __init__
    self._repopulate_pool()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/pool.py", line 303, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/pool.py", line 326, in _repopulate_pool_static
    w.start()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 183, in get_preparation_data
    main_mod_name = getattr(main_module.__spec__, "name", None)
AttributeError: module '__main__' has no attribute '__spec__'
```

I can reproduce this in Python 3.8.7, 3.9.1, 3.10.0a3. It works fine in Python 3.7.9.

A workaround is to define `__spec__` in __main__:

```
from multiprocessing import Pool

def f(x):
    return x*x

if __name__ == '__main__':
    __spec__ = None
    with Pool(5) as p:
        print(p.map(f, [1, 2, 3]))
```

I'd say that multiprocessing/spawn.py", line 183, in get_preparation_data is flawed
    main_mod_name = getattr(main_module.__spec__, "name", None)