Partially revert gh-6010: remove py_is_finalizing() workarounds

Now that the root cause (free of string literals in def_property_static,
gh-5976) is fixed in the previous commit, the py_is_finalizing() guards
introduced in gh-6010 are no longer needed:

- tp_dealloc_impl: remove early return during finalization (was leaking
  all function records instead of properly destroying them)
- destruct(): remove guard around arg.value.dec_ref()
- common.h: remove py_is_finalizing() helper (no remaining callers)

The genuine fix from gh-6010 (PyObject_Free + Py_DECREF ordering in
tp_dealloc_impl) is retained.

Made-with: Cursor

Author: rwgk

include/pybind11/detail/common.h

@@ -601,15 +601,6 @@ enum class return_value_policy : uint8_t {
 
 PYBIND11_NAMESPACE_BEGIN(detail)
 
-// Py_IsFinalizing() is a public API since 3.13; before that use _Py_IsFinalizing().
-inline bool py_is_finalizing() {
-#if PY_VERSION_HEX >= 0x030D0000
-    return Py_IsFinalizing() != 0;
-#else
-    return _Py_IsFinalizing() != 0;
-#endif
-}
-
 static constexpr int log2(size_t n, int k = 0) { return (n <= 1) ? k : log2(n >> 1, k + 1); }
 
 // Returns the size as a multiple of sizeof(void *), rounded up.

include/pybind11/pybind11.h

@@ -936,11 +936,8 @@ class cpp_function : public function {
                     std::free(const_cast<char *>(arg.descr));
                 }
             }
-            // During finalization, default arg values may already be freed by GC.
-            if (!detail::py_is_finalizing()) {
-                for (auto &arg : rec->args) {
-                    arg.value.dec_ref();
-                }
+            for (auto &arg : rec->args) {
+                arg.value.dec_ref();
             }
             if (rec->def) {
                 std::free(const_cast<char *>(rec->def->ml_doc));
@@ -1435,12 +1432,6 @@ PYBIND11_NAMESPACE_BEGIN(function_record_PyTypeObject_methods)
 
 // This implementation needs the definition of `class cpp_function`.
 inline void tp_dealloc_impl(PyObject *self) {
-    // Skip dealloc during finalization — GC may have already freed objects
-    // reachable from the function record (e.g. default arg values), causing
-    // use-after-free in destruct().
-    if (detail::py_is_finalizing()) {
-        return;
-    }
     // Save type before PyObject_Free invalidates self.
     auto *type = Py_TYPE(self);
     auto *py_func_rec = reinterpret_cast<function_record_PyObject *>(self);