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I played with ./configure --with-experimental-isolated-subinterpreters. I tried to run "pip list" in parallel in multiple interpreters.
I hit multiple issues:
* non-atomic reference count of Python objects shared by multiple interpreters, objects shared via static types for example.
* resolve_slotdups() uses a static variable
* pip requires _xxsubinterpreters.create(isolated=False): the vendored distro package runs the lsb_release command with subprocess.
* Race conditions in PyType_Ready() on static types:
 * Objects/typeobject.c:5494: PyType_Ready: Assertion "(type->tp_flags & (1UL << 13)) == 0" failed
 * Race condition in add_subclass()
* parser_init() doesn't support subinterpreters
* unicode_dealloc() fails to delete an interned string in the Unicode interned dictionary => https://bugs.python.org/issue40521#msg383829
To run "pip list", I used:
CODE = """
import runpy
import sys
import traceback
sys.argv = ["pip", "list"]
try:
 runpy.run_module("pip", run_name="__main__", alter_sys=True)
except SystemExit:
 pass
except Exception as exc:
 traceback.print_exc()
 print("BUG", exc)
 raise
"""