gh-105927: Add _PyWeakref_GET_REF() internal function (#105929)

Add new pycore_weakref.h internal header file.

Include/internal/pycore_weakref.h

@@ -0,0 +1,40 @@
+#ifndef Py_INTERNAL_WEAKREF_H
+#define Py_INTERNAL_WEAKREF_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+static inline PyObject* _PyWeakref_GET_REF(PyObject *ref_obj) {
+    assert(PyWeakref_Check(ref_obj));
+    PyWeakReference *ref = _Py_CAST(PyWeakReference*, ref_obj);
+    PyObject *obj = ref->wr_object;
+
+    if (obj == Py_None) {
+        // clear_weakref() was called
+        return NULL;
+    }
+
+    // Explanation for the Py_REFCNT() check: when a weakref's target is part
+    // of a long chain of deallocations which triggers the trashcan mechanism,
+    // clearing the weakrefs can be delayed long after the target's refcount
+    // has dropped to zero.  In the meantime, code accessing the weakref will
+    // be able to "see" the target object even though it is supposed to be
+    // unreachable.  See issue gh-60806.
+    Py_ssize_t refcnt = Py_REFCNT(obj);
+    if (refcnt == 0) {
+        return NULL;
+    }
+
+    assert(refcnt > 0);
+    return Py_NewRef(obj);
+}
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_WEAKREF_H */
+