pytest: full cleanup between tests

In a module, I have two tests:

@pytest.fixture
def myfixture(request):
    prepare_stuff()
    yield 1
    clean_stuff()
    # time.sleep(10) # in doubt, I tried that, did not help

def test_1(myfixture):
    a = somecode()
    assert a==1

def test_2(myfixture):
    b = somecode()
    assert b==1

case 1

When these two tests are executed individually, all is ok, i.e. both

pytest ./test_module.py:test_1

and immediately after:

pytest ./test_module.py:test_2

run until completion and pass with success.

case 2

But:

pytest ./test_module.py -k "test_1 or test_2"

reports:

collected 2 items
test_module.py .

and hangs forever (after investigation: test_1 completed successfully, but the second call to prepare_stuff hangs).

question

In my specific setup prepare_stuff, clean_stuff and somecode are quite evolved, i.e. they create and delete some shared memory segments, which when done wrong can results in some hanging. So some issue here is possible.

But my question is: are there things occurring between two calls of pytest (case 1) that do not occur between the call of test_1 and test_2 from the same "pytest process" (case 2), which could explain why "case 1" works ok while "case 2" hangs between test_1 and test_2 ? If so, is there a way to "force" the same "cleanup" to occur between test_1 and test_2 for "case 2" ?

Note: I already tried to specify the scope of "myfixture" to "function", and also double checked that "clean_stuff" is called after "test_1", even in "case 2".

from pytest: full cleanup between tests