Segfault in `examples/python/test_aw3.py`

[pbsds]

The examples/python/test_aw3.py test now fails with a segmentation fault:

$ PYTHONFAULTHANDLER=1 python test_aw3.py
Fatal Python error: Segmentation fault

Current thread 0x00007f43eaeaa780 (most recent call first):
  File "/nix/store/mhwrpvqpcaibcz4bb4zhapk8l63gwvlj-python3.13-cgal-6.0.1.post202410241521/lib/python3.13/site-packages/CGAL/CGAL_Alpha_wrap_3.py", line 442 in alpha_wrap_3
  File "/home/pbsds/repos/nixpkgs/tmp-fix/cgal-1772832119/results-src-writeable/python313Packages.cgal.src/examples/python/test_aw3.py", line 18 in <module>

Extension modules: CGAL._CGAL_Alpha_wrap_3, CGAL._CGAL_Kernel, CGAL._CGAL_Polyhedron_3 (total: 3)

Here is the gdb "backtrace full" output: https://gist.githubusercontent.com/pbsds/06b5444b48396a72a4db3db0fb1c40d6/raw/e9f781b75990a9797e24e8b6483bbf733d1a8bdb/gistfile0.txt

Here is the SWIG-generated CGAL_Alpha_wrap_3.py: https://gist.github.com/pbsds/10a3c29598fc8c6afc771e0b331a3a8d#file-cgal_alpha_wrap_3-py-L442

---

This error started occurring after the following batch of updates to nixpkgs: NixOS/nixpkgs@c0b0e0f...cad22e7

(A more readable git log --online: https://gist.githubusercontent.com/pbsds/ddc01914e5ab92810764b79f7db64c9b/raw/727b8b2eca7b23275f035e622c4bfc356f5f386a/gistfile0.txt)

[RajdeepKushwaha5]

Root Cause Analysis

This segfault is caused by a bug in CGAL's Compact_container infrastructure (used internally by the Triangulation_3 data structure that alpha_wrap_3 builds), not in the SWIG bindings themselves.

Reproduction

I built the SWIG bindings locally (MinGW GCC 15.2.0, Python 3.13, SWIG 4.4.1) against multiple CGAL versions and ran test_aw3.py:

CGAL Version	alpha_wrap_3(mesh)	alpha_wrap_3(soup)	alpha_wrap_3(points)
v6.0.1	Crash (segfault)	—	—
v6.0.2	Pass (209v, 414f)	Pass	Pass
v6.0.3	Pass (209v, 414f)	Pass	Pass
v6.1.1	Pass (209v, 414f)	Pass	Pass

The Fix

The crash is resolved by CGAL/cgal@2148806 ("workaround possible optimisation that would alter the value of the erase_counter"), which was backported to the 6.0.x-branch and first included in CGAL v6.0.2.

This commit fixes a race-like issue in Compact_container::emplace() / insert() where a compiler optimization could alter the erase_counter value during in-place construction, leading to memory corruption in the triangulation's internal data structures.

An additional related fix in the Alpha_wrap_3 triangulation cell base — CGAL/cgal#8714 ("Always init time_stamp to -2", merged Feb 2025) — initializes time_stamp_ to -2 instead of leaving it dependent on the constructor, which provides a further safety net. This fix is in CGAL v6.1+.

Recommendation

Upgrading to CGAL >= 6.0.2 resolves this crash. CGAL >= 6.1 is recommended as it includes both the Compact_container workaround and the time_stamp initialization fix.