182980c4-e4f4-4e83-8e3e-662d412bbf71
commented
22 years ago `get_referrers()' can return objects which are already garbage but which are in cycles and haven't been collected yet.
That in itself is a bug, but there is something weirder: in my experiments it only returns *some* of those objects. Perhaps this is indicative of a deeper bug in gc.get_referrers()?
Here is a transcript showing how get_referrers() returns not one referrer (which would be correct), nor three (which would be the behavior if it included all the cyclic garbage, but two:
Python 2.2+ (#6, Jan 17 2002, 12:47:08)
[GCC 3.0.3] on linux2
Type "help", "copyright", "credits" or "license" for
more information.
>>> class A:
... def __init__(self):
... self.x = 0
...
>>> a1=A()
>>> a1.x=1
>>> a2=A()
>>> a2.x=2
>>> a1.a=a2
>>> a2.a=a1
>>> a3=A()
>>> a3.x=3
>>> a1.a3=a3
>>> del a1, a2; refs = gc.get_referrers(a3)
>>> len(refs)
2
>>> refs[0]
{'a': <__main__.A instance at 0x8155d54>, 'x': 1, 'a3':
<__main__.A instance at 0x8156d6c>}
>>> refs[1]
{'A': <class __main__.A at 0x815759c>, 'a3':
<__main__.A instance at 0x8156d6c>, 'gc': <module 'gc'
(built-in)>, '__builtins__': <module '__builtin__'
(built-in)>, '__name__': '__main__', 'refs': [{'a':
<__main__.A instance at 0x8155d54>, 'x': 1, 'a3':
<__main__.A instance at 0x8156d6c>}, {...}], '__doc__':
None}Then in the same session I start again and this time
call gc.collect()' before calling gc.get_referrers()', yielding the expected results:
>>> del a3
>>> collect()
0
>>> a1=A()
>>> a1.x=1
>>> a2=A()
>>> a2.x=2
>>> a1.a=a2
>>> a2.a=a1
>>> a3=A()
>>> a3.x=3
>>> a1.a3=a3
>>> del a1, a2; gc.collect(); refs = gc.get_referrers(a3)
4
>>> len(refs)
1
>>> refs[0]
{'A': <class __main__.A at 0x815759c>, 'a3':
<__main__.A instance at 0x8155bec>, 'gc': <module 'gc'
(built-in)>, '__builtins__': <module '__builtin__'
(built-in)>, '__name__': '__main__', 'refs': [{...}],
'__doc__': None}If nobody else is more inclined to do it, then please let me know and I will investigate this bug.
Also note that I am submitting a patch for gcmodule.c
which calls collect_generations()' at the beginning of get_referrers()'. I do not believe that patch should
be allowed to close this bug, unless someone can
explain the above anomaly.
Regards,
Zooko
--- zooko.com Security and Distributed Systems Engineering ---
61337411-43fc-4a9c-b8d5-4060aede66d0
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
Show more details
GitHub fields: ```python assignee = 'https://github.com/loewis' closed_at =
created_at =
labels = ['interpreter-core']
title = 'bug in gc.get_referrers()'
updated_at =
user = 'https://bugs.python.org/zooko'
```
bugs.python.org fields:
```python
activity =
actor = 'loewis'
assignee = 'loewis'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation =
creator = 'zooko'
dependencies = []
files = []
hgrepos = []
issue_num = 505453
keywords = []
message_count = 7.0
messages = ['8861', '8862', '8863', '8864', '8865', '8866', '8867']
nosy_count = 2.0
nosy_names = ['loewis', 'zooko']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue505453'
versions = ['Python 2.2']
```