gh-151842: Make _PyXI_FreeExcInfo NULL-safe to fix crash on capture_exception() OOM (OOM-0031)

Author: amruthamodela06

Misc/NEWS.d/next/Core_and_Builtins/2026-06-30-13-00-00.gh-issue-151842.OOM31g.rst

@@ -0,0 +1,5 @@
+Fix a segfault in :func:`_interpreters.capture_exception` under memory
+pressure. When ``_PyXI_NewExcInfo()`` failed to allocate, the cleanup path
+called ``_PyXI_FreeExcInfo(NULL)``, which then dereferenced offset 0 in
+``_excinfo_clear_type()``. ``_PyXI_FreeExcInfo()`` now accepts ``NULL`` like
+the surrounding ``PyMem_RawFree`` idiom. Patch by Amrutha Modela.

Python/crossinterp.c

@@ -1709,6 +1709,11 @@ _PyXI_NewExcInfo(PyObject *exc)
 void
 _PyXI_FreeExcInfo(_PyXI_excinfo *info)
 {
+    if (info == NULL) {
+        // Matches the PyMem_RawFree(NULL) idiom: callers may pass NULL when
+        // _PyXI_NewExcInfo() failed (e.g. under OOM) before any allocation.
+        return;
+    }
     _PyXI_excinfo_clear(info);
     PyMem_RawFree(info);
 }