Improve error when if __name__ == "__main__" is omitted

What happened:

When running a small future submitting example the cluster fails to start and dumps reams of tracebacks.

What you expected to happen:

Prints the result.

Minimal Complete Verifiable Example:

$ conda create -n dask-what python dask distributed -y
$ conda activate dask-what 

# what.py
from dask.distributed import Client

f = lambda: 10 + 1

with Client() as client:
    print(client.submit(f).result())

$ python what.py
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.
2022-08-01 10:44:48,185 - distributed.nanny - ERROR - Failed to start process

Full traceback

/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/node.py:179: UserWarning: Port 8787 is already in use.
Perhaps you already have a cluster running?
Hosting the HTTP server on port 41555 instead
  warnings.warn(
/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/node.py:179: UserWarning: Port 8787 is already in use.
Perhaps you already have a cluster running?
Hosting the HTTP server on port 40205 instead
  warnings.warn(
/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/node.py:179: UserWarning: Port 8787 is already in use.
Perhaps you already have a cluster running?
Hosting the HTTP server on port 37055 instead
  warnings.warn(
/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/node.py:179: UserWarning: Port 8787 is already in use.
Perhaps you already have a cluster running?
Hosting the HTTP server on port 40399 instead
  warnings.warn(
2022-08-01 10:44:48,184 - distributed.nanny - ERROR - Failed to start process
Traceback (most recent call last):
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/nanny.py", line 436, in instantiate
    result = await self.process.start()
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/nanny.py", line 679, in start
    await self.process.start()
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/process.py", line 38, in _call_and_set_future
    res = func(*args, **kwargs)
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/process.py", line 199, in _start
    process.start()
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/multiprocessing/popen_spawn_posix.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/multiprocessing/spawn.py", line 154, in get_preparation_data
    _check_not_importing_main()
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/multiprocessing/spawn.py", line 134, in _check_not_importing_main
    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()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.
2022-08-01 10:44:48,185 - distributed.nanny - ERROR - Failed to start process

... repeated many times until keyboard interrupt

task: <Task pending name='Task-77' coro=<Nanny._on_exit() running at /home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/utils.py:778> wait_for=<Future pending cb=[Task.task_wakeup()]> cb=[set.remove()]>
Exception ignored in: <coroutine object Nanny._on_exit at 0x7f5117e3c120>
Traceback (most recent call last):
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/utils.py", line 777, in wrapper
  File "/home/jtomlinson/miniconda3/envs/dask-what/lib/python3.10/site-packages/distributed/utils.py", line 792, in __exit__
ImportError: sys.meta_path is None, Python is likely shutting down

... repeated many times

[jacobtomlinson]

Fixed it! I needed if __name__ == "__main__":.

from dask.distributed import Client


if __name__ == "__main__":

    f = lambda: 10 + 1

    with Client() as client:
        print(client.submit(f).result())

$ python what.py
11

[jacobtomlinson]

Thanks @jrbourbeau I hadn’t managed to discover #2708 despite searching for duplicates. That’s definitely the right place to track this.