Fix PseudoFixtureDef reference cycle.

[a-feld]

Hi there! Apologies in advance for the long-winded description!

I was working on a talk for a local Python meetup on reference cycles in Python and noticed that a reference cycle was generated from within pytest which was causing test failures in my example code.

Here's the fixture I was running:

import pytest
import gc


@pytest.fixture(autouse=True)
def garbage_detect(request):
    # Clean up all objects
    gc.set_debug(0)
    gc.collect()

    # Save garbage and run test
    gc.set_debug(gc.DEBUG_SAVEALL)
    yield

    # Invoke the garbage collector
    gc.collect()

    # Check that there is no garbage
    garbage_expected = request.node.get_marker('garbage')

    if not garbage_expected:
        assert len(gc.garbage) == 0

The fixture is checking that garbage is not generated by the code under test. I was hitting test failures due to a reference cycle generated by the insertion of the request fixture into garbage_detect.

Here's an overview of the cycle

The PseudoFixtureDef class becomes garbage when the reference to the generated class is discarded in pytest_fixture_setup (details in the commit message).

This PR updates the implementation to avoid the situation where PseudoFixtureDef becomes garbage by using a class instance. I've also included a test which exposes the leaked PseudoFixtureDef types.

This isn't really a problem/bug per-se since the garbage collector will take care of this but I figured I'd open a PR to fix the issue anyway.

[coveralls]

Coverage increased (+0.09%) to 92.593% when pulling 4854876 on a-feld:request-fixture-reference-cycle into dd97c94 on pytest-dev:master.

[RonnyPfannschmidt]

overall great job in finding this and providing a working solution

[RonnyPfannschmidt]

please use a frozen attrs class here,

[a-feld]

Hi @RonnyPfannschmidt , thanks for the feedback! Can you elaborate on the use of a frozen attr class? Do you mean using a class with __slots__ attribute set?

[RonnyPfannschmidt]

im sorry for not being precise enough, by frozen attrs class i meant http://www.attrs.org/en/stable/api.html#influencing-initialization - the frozen=True example

[a-feld]

Ah, ok. Will do! Thanks for the clarification!

[a-feld]

@RonnyPfannschmidt Updated to use a frozen attr class! Thanks again for that pointer!

[RonnyPfannschmidt]

@a-feld thanks for quickly and swiftly following up with my comments, its always a extra pleasure to observe a quick and precise followup

[nicoddemus]

@a-feld thanks for the PR! I didn't get a chance to look at it in detail, but I wonder if this fixes #2798?

[a-feld]

Hi @nicoddemus , I took a look at #2798 and I don't believe this issue is related; however, I think I have a pretty good theory for what's going on in #2798 !

I believe #2798 is related to the lifetime of tracebacks. In CPython, tracebacks hold references to their frames so if a frame (at any level in the traceback) stores the traceback in a variable, it creates a reference cycle. Python also stores references to the last traceback in sys.last_traceback which can cause the frame's lifetime to extend past what would be expected.

Here's some example code illustrating this issue:

import sys
import traceback
import weakref

class Obj:
    pass

ref = None

def test1():
    obj = Obj()
    global ref
    ref = weakref.ref(obj)
    assert 0

def test2():
    assert ref() is None, ref


def run():
    TESTS = (test1, test2)

    for test in TESTS:
        try:
            test()
        except:
            print("FAIL: %s" % str(test))
            # A reference cycle is created here because the run frame holds a
            # reference to the traceback which holds a reference to run
            # (through test1)
            _, _, tb = sys.exc_info()
            traceback.print_tb(tb)


if __name__ == '__main__':
    run()

In the example code above, both tests will fail because of the reference to tb inside of run. To fix this issue, the tb variable must be explicitly reassigned to None from within the run frame prior to executing test2. The traceback is also implicitly held in sys.last_traceback so that can be cleared out by raising and catching a dummy exception (there has to be a better way for this, but it works for now)

Fixed code:

import sys
import traceback
import weakref

class Obj:
    pass

ref = None

def test1():
    obj = Obj()
    global ref
    ref = weakref.ref(obj)
    assert 0

def test2():
    assert ref() is None, ref


def run():
    TESTS = (test1, test2)

    for test in TESTS:
        try:
            test()
        except:
            print("FAIL: %s" % str(test))
            # A reference cycle is created here because the run frame holds a
            # reference to the traceback which holds a reference to run
            # (through test1)
            _, _, tb = sys.exc_info()
            traceback.print_tb(tb)

            # Remove the frame reference cycle
            tb = None
        
        # This removes the test1 traceback from sys.last_traceback
        try:
            raise Exception
        except:
            pass


if __name__ == '__main__':
    run()

I'd be happy to investigate and hopefully root cause the issue sometime this weekend! I'm happy to contribute in any way I can. I use PyTest a lot (it's awesome!) and I'm more than happy to devote some time to this issue!

[nicoddemus]

Great @a-feld, thanks a lot for taking the time to investigate this!

The traceback is also implicitly held in sys.last_traceback so that can be cleared out by raising and catching a dummy exception (there has to be a better way for this, but it works for now)

It seems sys.last_traceback is writable, perhaps just sys.last_traceback = None will do the trick?

I'd be happy to investigate and hopefully root cause the issue sometime this weekend! I'm happy to contribute in any way I can. I use PyTest a lot (it's awesome!) and I'm more than happy to devote some time to this issue!

That would be great, we really appreciate the help!