Repeated creation of ProcessPoolExecutor leads to "Too many open files"

[JelleZijlstra]

Bug report

Bug description:

Running this code:

import concurrent.futures.process
for _ in range(100):
    x = concurrent.futures.process.ProcessPoolExecutor()
    x.submit(lambda: 42)

Results (most of the time) in an error indicating the system has run out of file descriptors, for example:

Traceback (most recent call last):
  File "<python-input-3>", line 4, in <module>
    x.submit(lambda: 42)
    ~~~~~~~~^^^^^^^^^^^^
  File "/Users/jelle/.pyenv/versions/3.13.0rc2/lib/python3.13/concurrent/futures/process.py", line 811, in submit
    self._adjust_process_count()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Users/jelle/.pyenv/versions/3.13.0rc2/lib/python3.13/concurrent/futures/process.py", line 770, in _adjust_process_count
    self._spawn_process()
    ~~~~~~~~~~~~~~~~~~~^^
  File "/Users/jelle/.pyenv/versions/3.13.0rc2/lib/python3.13/concurrent/futures/process.py", line 788, in _spawn_process
    p.start()
    ~~~~~~~^^
  File "/Users/jelle/.pyenv/versions/3.13.0rc2/lib/python3.13/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
                  ~~~~~~~~~~~^^^^^^
  File "/Users/jelle/.pyenv/versions/3.13.0rc2/lib/python3.13/multiprocessing/context.py", line 289, in _Popen
    return Popen(process_obj)
  File "/Users/jelle/.pyenv/versions/3.13.0rc2/lib/python3.13/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/Users/jelle/.pyenv/versions/3.13.0rc2/lib/python3.13/multiprocessing/popen_fork.py", line 20, in __init__
    self._launch(process_obj)
    ~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/Users/jelle/.pyenv/versions/3.13.0rc2/lib/python3.13/multiprocessing/popen_spawn_posix.py", line 53, in _launch
    parent_r, child_w = os.pipe()
                        ~~~~~~~^^
OSError: [Errno 24] Too many open files

The exact error varies. For example, one time the error happened on an os.getcwd() call. One time one of the futures printed as <Future at 0x1035b9950 state=finished raised PicklingError>.

I originally encountered this in the CI for Black, where it only happened on MacOS in 3.13, but the code sample above raises errors on 3.11 and 3.12 too when I run it locally. Still, it's possible that the GC changes in 3.13 make the problem more frequent.

Adding gc.collect() to the loop body fixes the problem. This suggests there is some cycle involving the executor that isn't getting collected until the GC kicks in.

CPython versions tested on:

3.11, 3.12, 3.13

Operating systems tested on:

macOS