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Memory leak on interpreter shutdown when reference cycle exists between structseq type and its instance #157176
Description
Activity
- added a commit that references this issue
on Sep 8, 2026 Does this affect 3.14, 3.15 branches?
@sergey-miryanov It reproduces on 3.15 and 3.14, but 3.13, 3.12, 3.11, and 3.10 failed with ASan/UBSan errors during the build.
How about adding
Py_TPFLAGS_IMMUTABLETYPEtostruct_time?How about adding Py_TPFLAGS_IMMUTABLETYPE to struct_time?
IMO it's perfectly reasonable to make the type immutable. I don't see why someone would want to modify the type.
- state->struct_time_type = PyStructSequence_NewType(&struct_time_type_desc); + state->struct_time_type = _PyStructSequence_NewType(&struct_time_type_desc, Py_TPFLAGS_IMMUTABLETYPE);
I tested this change and it works as expected.
Reacted by Peter Bierma and Shamiltype(time.gmtime()) has been made immutable, so it's no longer possible to create circular dependencies:
$ ./python -c "import time; t = time.gmtime(); type(t).refcyle = t;" Traceback (most recent call last): File "<string>", line 1, in <module> import time; t = time.gmtime(); type(t).refcyle = t; ^^^^^^^^^^^^^^^ TypeError: cannot set 'refcyle' attribute of immutable type 'time.struct_time'I don't think that this issue is important enough to justify a backport. Making a type immutable is a backward incompatible change, so it's only done in Python 3.16 (main branch).
I close the issue.
Bug report
Bug description:
Environment
main(rev:2d9fb5f2f70b18ce9e5d9f7f76fd5ef3f118d838)Steps to reproduce
Run:
./python -c "import time; t = time.gmtime(); type(t).refcyle = t;"Note: Explicitly breaking the cycle avoids the leak:
./python -c "import time; t = time.gmtime(); type(t).refcyle = t; del type(t).refcyle"Expected behavior
The garbage collector or interpreter finalization breaks the reference cycle during shutdown; no memory leaks reported by LeakSanitizer.
Actual behavior
LeakSanitizer detects memory leaks (indirect leaks of the type, its dict, descriptors, and the instance):
Full LeakSanitizer output - logs.txt
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux
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