Python crash in ./run-cbmc-proofs.py -jN when N is large

[rod-chapman]

In the proof of the mldsa-native project, running the run-cbmc-proofs.py script with -jN
for N >= 2, we always see a Python crash (exactly once per run). For example, on an EC2 r7g.16xlarge instance
with 64 cores, and running CBMC 6.8.0:

$ time ./run-cbmc-proofs.py --summarize --no-coverage -j64

For your convenience, the output of this run will be symbolically linked to  /home/ubuntu/pqcp/mldref/proofs/cbmc/output/latest/html/index.html 

Configuring CBMC proofs: 138 / 138
[1145/2071] crypto_sign_keypair: checking function contracts                                                                                  Contract 'randombytes' not found, deriving empty pure contract 'contract::randombytes' from function 'randombytes'

[1824/2071] polyveck_unpack_t0: checking safety properties                                                                                    Exception ignored in: <function BaseSubprocessTransport.__del__ at 0xfffff61d6b60>

Traceback (most recent call last):

  File "/nix/store/40qg7a2iaxnxkra9wk327xi50687pap9-python3-3.12.11/lib/python3.12/asyncio/base_subprocess.py", line 126, in __del__

    self.close()

  File "/nix/store/40qg7a2iaxnxkra9wk327xi50687pap9-python3-3.12.11/lib/python3.12/asyncio/base_subprocess.py", line 104, in close

    proto.pipe.close()

  File "/nix/store/40qg7a2iaxnxkra9wk327xi50687pap9-python3-3.12.11/lib/python3.12/asyncio/unix_events.py", line 568, in close

    self._close(None)

  File "/nix/store/40qg7a2iaxnxkra9wk327xi50687pap9-python3-3.12.11/lib/python3.12/asyncio/unix_events.py", line 592, in _close

    self._loop.call_soon(self._call_connection_lost, exc)

  File "/nix/store/40qg7a2iaxnxkra9wk327xi50687pap9-python3-3.12.11/lib/python3.12/asyncio/base_events.py", line 799, in call_soon

    self._check_closed()

  File "/nix/store/40qg7a2iaxnxkra9wk327xi50687pap9-python3-3.12.11/lib/python3.12/asyncio/base_events.py", line 545, in _check_closed

    raise RuntimeError('Event loop is closed')

RuntimeError: Event loop is closed

This "event loop is closed" appears to be a common problem with concurrent programming in Python.

Can it be fixed please?

[tautschnig]

I have tried 200 times on a r7g.16xlarge, but so far none of those runs resulted in the error.