Patch cloudpickle to not reset dynamic class each time it is unpickled - #7388

Fixes a problem with cloudpickle when dynamic class is unpickled and the class is reset to the state when it is pickled.

A short reproducer for the cloudpickle problem is available here:

# Tested with cloudpickle 1.6.0
from cloudpickle import dumps, loads


class Klass:
    classvar = None

def mutator():
    Klass.classvar = 100

def check():
    print("checking....")
    print(f"   Klass.classvar [{hex(id(Klass))}] = {Klass.classvar}")


def failing_case():
    print("Klass", hex(id(Klass)))
    saved = dumps(Klass)
    mutator()
    check()
    loads(saved)
    check()
    loads(saved)
    check()



if __name__ == '__main__':
    failing_case()

Note, cloudpickle treats any class created in __main__ as dynamic.

The patch makes cloudpickle remember whether a dynamic class is being reused. Reuse can come from the class being unpickled in the same process as it being pickled; or, a previously unpickled class is still around.

Reference an existing issue

Fixes #7356