[Python-Dev] Reference cycle on the module dict (globals())

[Victor Stinner]

Hi,

While working on my FAT Python optimizer project, I found an annoying
bug in my code. When at least one guard is created with a reference to
the global namespace (globals(), the module dictionary), objects of
the module are no more removed at exit.

Example:
---
import sys

class MessageAtExit:
    def __del__(self):
        print('__del__ called')

# display a message at exit, when message_at_exit is removed
message_at_exit = MessageAtExit()

# create a reference cycle:
# module -> module dict -> Guard -> module dict
guard = sys.Guard(globals())
---
(the code is adapted from a test of test_gc)

Apply attached patch to Python 3.6 to get the sys.Guard object. It's a
minimalist object to keep a strong reference to an object.

I expected the garbage collector to break such (simple?) reference cycle.

The Guard object implements a traverse module, but it is never called.

Did I miss something obvious, or is it a known issue of the garbage
collector on modules?

Victor
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