[issue33725] Python crashes on macOS after fork with no exec

[Mouse]

Mouse <uri at ll.mit.edu> added the comment:

Also, adding `p.join()` immediately after `p.start()` in my sample code showed this timing:
```
$ time python3.8 multi1.py 
worker 0 done, got 100 numbers
worker 1 done, got 0 numbers
worker 2 done, got 0 numbers
worker 3 done, got 0 numbers

real	0m2.342s
user	0m0.227s
sys	0m0.111s
$ 
```

Setting instead start to `fork` showed this timing:
```
$ time python3.8 multi1.py 
worker 2 done, got 25 numbers
worker 0 done, got 25 numbers
worker 1 done, got 25 numbers
worker 3 done, got 25 numbers

real	0m0.537s
user	0m0.064s
sys	0m0.040s
$ 
```

The proposed fix is roughly four times slower, compared to reverting start to `fork`.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33725>
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