Describe the bug
A Python subclass with a nonempty __slots__ tuple can expose one weak-reference offset through its Python type while its native type representation exposes a different offset. If an ordinary native object field occupies the native offset, cyclic garbage collection can treat that field as the weak-reference list and fail to complete.
The standalone reproducer uses only the Python C API. It defines a heap type with an ordinary integer field before a Python reference field and weak-reference field, then creates a Python subclass with __slots__ = ("extra",). The subclass reports:
Python layout: 48 32
Native layout: (48, 24)
After assigning the ordinary Python reference field to the instance itself, gc.collect() does not return within 15 seconds.
This can affect extension classes that support both subclassing and weak references. An unreachable cycle can hang collection rather than being reclaimed.
Operating system
Linux
CPU architecture
x86_64
GraalPy version
25.3.4.1, Python 3.13.14, sys.implementation.version == 25.3.4, source revision 6bd9d25023bf713ae3dd7e83ee620c897a2c51d0
JDK version
25.0.4.1
Context configuration
No response
Steps to reproduce
Place the attached gc_probe.c and reproduce_runtime_gc.py in one directory, then run:
graalpy reproduce_runtime_gc.py graal-subclass
The script compiles the extension using the selected interpreter's headers and runs collection in a subprocess with a 15-second timeout.
Observed output:
3.13.14 (Wed Aug 19 15:15:54 UTC 2026)
[Graal, GraalVM CE, Java 25.0.4.1 (amd64)]
Python layout: 48 32
Native layout: (48, 24)
collecting
Collection did not return within 15 seconds.
The command exits with status 124.
gc_probe.c
reproduce_runtime_gc.py
Expected behavior
The Python-visible and native weak-reference offsets should describe the same object layout, and collection should return after reclaiming the unreachable self-cycle.
Stack trace
Additional context
No response
Describe the bug
A Python subclass with a nonempty
__slots__tuple can expose one weak-reference offset through its Python type while its native type representation exposes a different offset. If an ordinary native object field occupies the native offset, cyclic garbage collection can treat that field as the weak-reference list and fail to complete.The standalone reproducer uses only the Python C API. It defines a heap type with an ordinary integer field before a Python reference field and weak-reference field, then creates a Python subclass with
__slots__ = ("extra",). The subclass reports:After assigning the ordinary Python reference field to the instance itself,
gc.collect()does not return within 15 seconds.This can affect extension classes that support both subclassing and weak references. An unreachable cycle can hang collection rather than being reclaimed.
Operating system
Linux
CPU architecture
x86_64
GraalPy version
25.3.4.1, Python 3.13.14,
sys.implementation.version == 25.3.4, source revision6bd9d25023bf713ae3dd7e83ee620c897a2c51d0JDK version
25.0.4.1
Context configuration
No response
Steps to reproduce
Place the attached
gc_probe.candreproduce_runtime_gc.pyin one directory, then run:The script compiles the extension using the selected interpreter's headers and runs collection in a subprocess with a 15-second timeout.
Observed output:
The command exits with status 124.
gc_probe.c
reproduce_runtime_gc.py
Expected behavior
The Python-visible and native weak-reference offsets should describe the same object layout, and collection should return after reclaiming the unreachable self-cycle.
Stack trace
Additional context
No response