GC thread not synchronized with main thread when acessing shared variables

This is from test_scope.ScopeTests('testLeaks') in StdLib:

class Foo:
    count = 0

    def __init__(self):
        Foo.count += 1

    def __del__(self):
        Foo.count -= 1

def f1():
    x = Foo()
    def f2():
        return x
    f2()

for i in range(100):
    f1()

assert Foo.count == 0

Even with adding gc.collect(), Foo.count is not always 0 (it may be off by a small number in either direction). I could never reproduce it when running such a simple case standalone, but it happens occasionally when the whole test suite is run (with parallel jobs enabled).

I suspect the problem is because __init__ and __del__ are being run on different threads but both update a shared variable.

Start with the StdLib test_scope.ScopeTests('testLeaks') case and reproduce it with gc.collect() while the whole suite runs with parallel jobs enabled. Investigate the differing-thread updates from Foo.init and Foo.del; done means the test consistently reaches Foo.count == 0 without parallel-run failures.