import io
from threading import Thread, Barrier

shared = io.BytesIO(b'x' * 64)
N_W, N_R = 2, 8
barrier = Barrier(N_W + N_R)

def writer():
    barrier.wait()
    for i in range(20000):
        try:
            shared.seek(0)
            shared.write(b'a' * (64 + (i & 63)))
        except Exception:
            pass

def reader():
    barrier.wait()
    for _ in range(20000):
        try:
            shared.seek(0)
            shared.read()
        except Exception:
            pass

if __name__ == "__main__":
    threads = [Thread(target=writer) for _ in range(N_W)]
    threads += [Thread(target=reader) for _ in range(N_R)]
    for t in threads: t.start()
    for t in threads: t.join()

==================
WARNING: ThreadSanitizer: data race (pid=2292869)
  Read of size 8 at 0x7fffbc080190 by thread T2:
    #0 _PyObject_MiRealloc /cpython/Objects/obmalloc.c:360:13 
    #1 PyObject_Realloc /cpython/Objects/obmalloc.c:1731:12 
    #2 _PyBytes_Resize /cpython/Objects/bytesobject.c:3389:9 
    #3 resize_buffer_lock_held /cpython/./Modules/_io/bytesio.c:176:13 
    #4 write_bytes_lock_held /cpython/./Modules/_io/bytesio.c:216:13 
    #5 _io_BytesIO_write_impl /cpython/./Modules/_io/bytesio.c:782:20 
    #6 bytesio_setstate_lock_held /cpython/./Modules/_io/bytesio.c:927:14 
    #7 bytesio_setstate /cpython/./Modules/_io/bytesio.c:980:11 
    #8 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:4261:35 
    ...
  Previous atomic write of size 8 at 0x7fffbc080190 by thread T8:
    #0 _Py_atomic_compare_exchange_ssize /cpython/./Include/cpython/pyatomic_gcc.h:130:10 
    #1 _Py_DecRefSharedIsDead /cpython/Objects/object.c:406:15 
    #2 _Py_DecRefSharedDebug /cpython/Objects/object.c:425:9 
    #3 _Py_DecRefShared /cpython/Objects/object.c:433:5 
    #4 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h 
    ...
SUMMARY: ThreadSanitizer: data race /cpython/Objects/obmalloc.c:360:13 in _PyObject_MiRealloc