I wrote 3 scripts to reproduce the bug in a more reliable way. So I just have to type "bisect" and it runs the test 12 times.

(1) bisect.bat:
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@"C:\vstinner\python\3.9\PCbuild\amd64\python_d.exe" bisect.py
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(2) bisect.py:
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import subprocess
import os
import sys

BISECT = False

def run(*args):
    print("+ ", ' '.join(args))
    env = dict(os.environ)
    env['PYTHONFAULTHANDLER'] = '1'
    proc = subprocess.run(args, env=env)
    exitcode = proc.returncode
    if exitcode:
        print()
        print(f"COMMAND FAILED: {exitcode}")
        if BISECT:
            print()
            print("type: git bisect bad")
        sys.exit(exitcode)

python = sys.executable
#script = "win_py399_crash_reproducer.py"
script = "bug.py"
nrun = 12
for i in range(1, nrun+1):
    print(f"Run #{i}/{nrun}")
    if i % 2:
        run(python, script)
    else:
        run(python, "-X", "dev", script)

if BISECT:
    print()
    print("Not reproduced")
    print()
    run("git", "checkout", ".")
    run("git", "bisect", "good")
---


(3) win_py399_crash_reproducer.py (import "_sre"):
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# When this program is run on windows using python 3.9.9 it crashes about 50%
# of the time.

import _testcapi
import threading

code = """
import _sre
print("exit subinterpreter")
"""

def doIt():
    _testcapi.run_in_subinterp(code)

tt=[]

for i in range(16):
    t = threading.Thread(target=doIt)
    t.start()
    tt.append(t)

for t in tt:
    t.join()
print("exit main")
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Example:
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vstinner@DESKTOP-DK7VBIL C:\vstinner\python\3.9>bisect
Run #1/12
+  C:\vstinner\python\3.9\PCbuild\amd64\python_d.exe bug.py
Run #2/12
+  C:\vstinner\python\3.9\PCbuild\amd64\python_d.exe -X dev bug.py
Run #3/12
+  C:\vstinner\python\3.9\PCbuild\amd64\python_d.exe bug.py
Windows fatal exception: access violation
(...)
Current thread 0x00000420 (most recent call first):
  File "C:\vstinner\python\3.9\bug.py", line 13 in doIt
  File "C:\vstinner\python\3.9\lib\threading.py", line 910 in run
  File "C:\vstinner\python\3.9\lib\threading.py", line 973 in _bootstrap_inner
  File "C:\vstinner\python\3.9\lib\threading.py", line 930 in _bootstrap
(...)
COMMAND FAILED: 3221225477
---