Reference leak with custom tp_dealloc in PyType_FromSpec

[bfroehle]

BPO	16690
Nosy	@loewis, @ncoghlan, @pitrou, @encukou
Files	heaptype_refcnt_testcases.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-09-12.14:43:43.606>
created_at = <Date 2012-12-15.01:39:25.523>
labels = ['interpreter-core', 'performance']
title = 'Reference leak with custom tp_dealloc in PyType_FromSpec'
updated_at = <Date 2019-09-12.14:43:43.605>
user = 'https://bugs.python.org/bfroehle'

bugs.python.org fields:

activity = <Date 2019-09-12.14:43:43.605>
actor = 'petr.viktorin'
assignee = 'none'
closed = True
closed_date = <Date 2019-09-12.14:43:43.606>
closer = 'petr.viktorin'
components = ['Interpreter Core']
creation = <Date 2012-12-15.01:39:25.523>
creator = 'bfroehle'
dependencies = []
files = ['28372']
hgrepos = []
issue_num = 16690
keywords = []
message_count = 6.0
messages = ['177527', '177528', '177794', '244420', '244814', '352187']
nosy_count = 7.0
nosy_names = ['loewis', 'ncoghlan', 'pitrou', 'petr.viktorin', 'python-dev', 'Robin.Schreiber', 'bfroehle']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue16690'
versions = ['Python 3.2', 'Python 3.3', 'Python 3.4']

[bfroehle]

There is a reference leak when using PyType_FromSpec with custom tp_dealloc. This was first noted in issue bpo-15142, where a fix was given which only applies to types which do not override tp_dealloc.

For example, the xxlimited.Xxo type suffers from this:

Python 3.3.0 (default, Oct 26 2012, 11:06:17) 
[GCC 4.6.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import xxlimited
>>> import sys
>>> Xxo = type(xxlimited.new())
>>> e = Xxo()
>>> sys.getrefcount(Xxo)
7
>>> e = Xxo()
>>> sys.getrefcount(Xxo)
8
>>> e = Xxo()
>>> sys.getrefcount(Xxo)
9

[bfroehle]

I see this issue came up in the course of bpo-15653 as well.

[bfroehle]

The attached file heaptype_refcnt_testcases.py runs through several test cases (ssl.SSLError, a subclass of ssl.SSLError, and xxlimited.Xxo) seeing if references are leaked in each instance.

Unfortunately xxlimited.Xxo isn't set to be a base type and I don't know of any other types in the default install which use PyType_FromSpec with a custom tp_dealloc.

[ncoghlan]

The new test cases for PEP-489 (multi-phase extension module import) appear to be encountering this leak.

See bpo-24268 for context.

[python-dev]

New changeset 265eeb60443a by Nick Coghlan in branch '3.5':
Issue bpo-24373: Eliminate PEP-489 test refleaks
https://hg.python.org/cpython/rev/265eeb60443a

[encukou]

This is fixed in Python 3.5+.