Hard crash with copy.copy() copying proxy object

[pekkaklarck]

The following example crashes IronPython 2.7.5 for good. I first get a dialog telling "IronPython Console has stopped working" and after closing it "Process is terminated due to StackOverflowException." is printed to the command prompt.

import copy

class Proxy(object):

    def __init__(self, obj):
        self.obj = obj

    def __getattr__(self, attr):
        return getattr(self.obj, attr)

class Object(object):
    attr = 1

copy.copy(Proxy(Object()))

[pekkaklarck]

A workaround for this problem is creating a custom __copy__ method. For example this works fine:

import copy

class Proxy(object):

    def __init__(self, obj):
        self.obj = obj

    def __getattr__(self, attr):
        return getattr(self.obj, attr)

    def __copy__(self):
        return Proxy(copy.copy(self.obj))

class Object(object):
    attr = 1

copy.copy(Proxy(Object()))

[kunom]

The underlying issue is that a call to hasattr(clone, '__setstate__') leads to an infinite loop in Proxy.__getattr__ since the clone instance does not (yet) have an obj member.

CPython has identical behaviour, but somehow stops recursion without crashing after a nesting level of 331 (in my case).

In any case, the current implementation of the Proxy class seems to be suboptimal at least with regard to the copy module.

Test code:

o = Proxy(Object())

reductor = getattr(o, "__reduce_ex__", None)
print "reductor =", reductor
info = reductor(2)
print "info =", info
callable, args = info[:2]
print "callable, args =", callable, args
y = callable(*args)
print "inst =", y
hasattr(y, '__setstate__')

Output:

reductor = <built-in method __reduce_ex__ of Proxy object at 0x000000000000002B>
info = (<built-in function __newobj__>, (<class '__main__.Proxy'>,), {'obj': <Object object at 0x000000000000002C>}, None, None)
callable, args = <built-in function __newobj__> (<class '__main__.Proxy'>,)
inst = <Proxy object at 0x000000000000002D>

Process is terminated due to StackOverflowException.