Hi,

Currrently, is it allowed for process worker to submit new task to its parent executor?

Considering the following Python script:

```python3
import concurrent.futures

with concurrent.futures.ProcessPoolExecutor(max_workers=12) as ppe:
    def hello(n: int) -> int:
        if n == 0:
            return 0
        return n+ ppe.submit(hello, n-1).result()

    future = ppe.submit(hello, 1)
    print(future.result())

```

It will never stuck and never ended.

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