Change implementation of the `__init__() must be called when overridi…

…ng __init__` safety feature to work for any metaclass. (#30095)

* Also wrap with `py::metaclass((PyObject *) &PyType_Type)`

* Transfer additional tests from PyCLIF python_multiple_inheritance_test.py

* Expand tests to fully cover wrapping with alternative metaclasses.

* * Factor out `ensure_base_init_functions_were_called()`.

* Call from new `tp_init_intercepted()` (adopting mechanism first added in PyCLIF: google/clif@7cba87d).

* Remove `pybind11_meta_call()` (which was added with pybind/pybind11#2152).

* Bug fix (maybe actually two bugs?): simplify condition to `type->tp_init != tp_init_intercepted`

* Removing `Py_DECREF(self)` that leads to MSAN failure (Google toolchain).

```
==6380==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x5611589c9a58 in Py_DECREF third_party/python_runtime/v3_11/Include/object.h:537:9
...

  Uninitialized value was created by a heap deallocation
    #0 0x5611552757b0 in free third_party/llvm/llvm-project/compiler-rt/lib/msan/msan_interceptors.cpp:218:3
    #1 0x56115898e06b in _PyMem_RawFree third_party/python_runtime/v3_11/Objects/obmalloc.c:154:5
    #2 0x56115898f6ad in PyObject_Free third_party/python_runtime/v3_11/Objects/obmalloc.c:769:5
    #3 0x561158271bcc in PyObject_GC_Del third_party/python_runtime/v3_11/Modules/gcmodule.c:2407:5
    #4 0x7f21224b070c in pybind11_object_dealloc third_party/pybind11/include/pybind11/detail/class.h:483:5
    #5 0x5611589c2ed0 in subtype_dealloc third_party/python_runtime/v3_11/Objects/typeobject.c:1463:5
...
```

* IncludeCleaner fixes (Google toolchain).

* Restore `type->tp_call = pybind11_meta_call;` for PyPy only.

* pytest.skip("ensure_base_init_functions_were_called() does not work with PyPy and Python `type` as metaclass")

* Do not intercept our own `tp_init` function (`pybind11_object_init`).

* Add `derived_tp_init_registry` weakref-based cleanup.

* Replace `assert()` with `if` to resolve erroneous `lambda capture 'type' is not used` diagnostics (many CI jobs; seems to be a clang issue).

* Add `derived_tp_init_registry()->count(type) == 0` condition.

* Changes based on feedback from @rainwoodman

* Use PYBIND11_INIT_SAFETY_CHECKS_VIA_* macros, based on suggestion from @rainwoodman

include/pybind11/detail/class.h

@@ -12,6 +12,9 @@
 #include "../attr.h"
 #include "../options.h"
 
+#include <cassert>
+#include <unordered_map>
+
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
 
@@ -179,6 +182,36 @@ extern "C" inline PyObject *pybind11_meta_getattro(PyObject *obj, PyObject *name
     return PyType_Type.tp_getattro(obj, name);
 }
 
+// Ensure that the base __init__ function(s) were called.
+// Set TypeError and return false if not.
+// CALLER IS RESPONSIBLE for managing the self refcount appropriately.
+inline bool ensure_base_init_functions_were_called(PyObject *self) {
+    values_and_holders vhs(self);
+    for (const auto &vh : vhs) {
+        if (!vh.holder_constructed() && !vhs.is_redundant_value_and_holder(vh)) {
+            PyErr_Format(PyExc_TypeError,
+                         "%.200s.__init__() must be called when overriding __init__",
+                         get_fully_qualified_tp_name(vh.type->type).c_str());
+            return false;
+        }
+    }
+    return true;
+}
+
+// See google/pywrapcc#30095 for background.
+#if !defined(PYBIND11_INIT_SAFETY_CHECKS_VIA_INTERCEPTING_TP_INIT)                                \
+    && !defined(PYBIND11_INIT_SAFETY_CHECKS_VIA_DEFAULT_PYBIND11_METACLASS)
+#    if !defined(PYPY_VERSION)
+// With CPython the safety checks work for any metaclass.
+// However, with PyPy this implementation does not work at all.
+#        define PYBIND11_INIT_SAFETY_CHECKS_VIA_INTERCEPTING_TP_INIT
+#    else
+// With this the safety checks work only for the default `py::metaclass()`.
+#        define PYBIND11_INIT_SAFETY_CHECKS_VIA_DEFAULT_PYBIND11_METACLASS
+#    endif
+#endif
+
+#if defined(PYBIND11_INIT_SAFETY_CHECKS_VIA_DEFAULT_PYBIND11_METACLASS)
 /// metaclass `__call__` function that is used to create all pybind11 objects.
 extern "C" inline PyObject *pybind11_meta_call(PyObject *type, PyObject *args, PyObject *kwargs) {
 
@@ -188,20 +221,14 @@ extern "C" inline PyObject *pybind11_meta_call(PyObject *type, PyObject *args, P
         return nullptr;
     }
 
-    // Ensure that the base __init__ function(s) were called
-    values_and_holders vhs(self);
-    for (const auto &vh : vhs) {
-        if (!vh.holder_constructed() && !vhs.is_redundant_value_and_holder(vh)) {
-            PyErr_Format(PyExc_TypeError,
-                         "%.200s.__init__() must be called when overriding __init__",
-                         get_fully_qualified_tp_name(vh.type->type).c_str());
-            Py_DECREF(self);
-            return nullptr;
-        }
+    if (!ensure_base_init_functions_were_called(self)) {
+        Py_DECREF(self);
+        return nullptr;
     }
 
     return self;
 }
+#endif
 
 /// Cleanup the type-info for a pybind11-registered type.
 extern "C" inline void pybind11_meta_dealloc(PyObject *obj) {
@@ -268,7 +295,9 @@ inline PyTypeObject *make_default_metaclass() {
     type->tp_base = type_incref(&PyType_Type);
     type->tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HEAPTYPE;
 
+#if defined(PYBIND11_INIT_SAFETY_CHECKS_VIA_DEFAULT_PYBIND11_METACLASS)
     type->tp_call = pybind11_meta_call;
+#endif
 
     type->tp_setattro = pybind11_meta_setattro;
     type->tp_getattro = pybind11_meta_getattro;
@@ -340,6 +369,33 @@ inline bool deregister_instance(instance *self, void *valptr, const type_info *t
     return ret;
 }
 
+#if defined(PYBIND11_INIT_SAFETY_CHECKS_VIA_INTERCEPTING_TP_INIT)
+
+using derived_tp_init_registry_type = std::unordered_map<PyTypeObject *, initproc>;
+
+inline derived_tp_init_registry_type *derived_tp_init_registry() {
+    // Intentionally leak the unordered_map:
+    // https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables
+    static auto *singleton = new derived_tp_init_registry_type();
+    return singleton;
+}
+
+extern "C" inline int tp_init_with_safety_checks(PyObject *self, PyObject *args, PyObject *kw) {
+    assert(PyType_Check(self) == 0);
+    const auto derived_tp_init = derived_tp_init_registry()->find(Py_TYPE(self));
+    if (derived_tp_init == derived_tp_init_registry()->end()) {
+        pybind11_fail("FATAL: Internal consistency check failed at " __FILE__
+                      ":" PYBIND11_TOSTRING(__LINE__));
+    }
+    int status = (*derived_tp_init->second)(self, args, kw);
+    if (status == 0 && !ensure_base_init_functions_were_called(self)) {
+        return -1; // No Py_DECREF here.
+    }
+    return status;
+}
+
+#endif // PYBIND11_INIT_SAFETY_CHECKS_VIA_INTERCEPTING_TP_INIT
+
 /// Instance creation function for all pybind11 types. It allocates the internal instance layout
 /// for holding C++ objects and holders.  Allocation is done lazily (the first time the instance is
 /// cast to a reference or pointer), and initialization is done by an `__init__` function.
@@ -360,11 +416,7 @@ inline PyObject *make_new_instance(PyTypeObject *type) {
     return self;
 }
 
-/// Instance creation function for all pybind11 types. It only allocates space for the
-/// C++ object, but doesn't call the constructor -- an `__init__` function must do that.
-extern "C" inline PyObject *pybind11_object_new(PyTypeObject *type, PyObject *, PyObject *) {
-    return make_new_instance(type);
-}
+extern "C" inline PyObject *pybind11_object_new(PyTypeObject *type, PyObject *, PyObject *);
 
 /// An `__init__` function constructs the C++ object. Users should provide at least one
 /// of these using `py::init` or directly with `.def(__init__, ...)`. Otherwise, the

include/pybind11/pybind11.h

@@ -1207,6 +1207,33 @@ class cpp_function : public function {
     }
 };
 
+PYBIND11_NAMESPACE_BEGIN(detail)
+
+/// Instance creation function for all pybind11 types. It only allocates space for the
+/// C++ object, but doesn't call the constructor -- an `__init__` function must do that.
+extern "C" inline PyObject *pybind11_object_new(PyTypeObject *type, PyObject *, PyObject *) {
+#if defined(PYBIND11_INIT_SAFETY_CHECKS_VIA_INTERCEPTING_TP_INIT)
+    if (type->tp_init != pybind11_object_init && type->tp_init != tp_init_with_safety_checks
+        && derived_tp_init_registry()->count(type) == 0) {
+        weakref((PyObject *) type, cpp_function([type](handle wr) {
+                    auto num_erased = derived_tp_init_registry()->erase(type);
+                    if (num_erased != 1) {
+                        pybind11_fail("FATAL: Internal consistency check failed at " __FILE__
+                                      ":" PYBIND11_TOSTRING(__LINE__) ": num_erased="
+                                      + std::to_string(num_erased));
+                    }
+                    wr.dec_ref();
+                }))
+            .release();
+        (*derived_tp_init_registry())[type] = type->tp_init;
+        type->tp_init = tp_init_with_safety_checks;
+    }
+#endif // PYBIND11_INIT_SAFETY_CHECKS_VIA_INTERCEPTING_TP_INIT
+    return make_new_instance(type);
+}
+
+PYBIND11_NAMESPACE_END(detail)
+
 /// Wrapper for Python extension modules
 class module_ : public object {
 public:

tests/test_python_multiple_inheritance.cpp

@@ -5,6 +5,7 @@ namespace test_python_multiple_inheritance {
 // Copied from:
 // https://github.com/google/clif/blob/5718e4d0807fd3b6a8187dde140069120b81ecef/clif/testing/python_multiple_inheritance.h
 
+template <int> // Using int as a trick to easily generate a series of types.
 struct CppBase {
     explicit CppBase(int value) : base_value(value) {}
     int get_base_value() const { return base_value; }
@@ -14,32 +15,45 @@ struct CppBase {
     int base_value;
 };
 
-struct CppDrvd : CppBase {
-    explicit CppDrvd(int value) : CppBase(value), drvd_value(value * 3) {}
+template <int SerNo>
+struct CppDrvd : CppBase<SerNo> {
+    explicit CppDrvd(int value) : CppBase<SerNo>(value), drvd_value(value * 3) {}
     int get_drvd_value() const { return drvd_value; }
     void reset_drvd_value(int new_value) { drvd_value = new_value; }
 
-    int get_base_value_from_drvd() const { return get_base_value(); }
-    void reset_base_value_from_drvd(int new_value) { reset_base_value(new_value); }
+    int get_base_value_from_drvd() const { return CppBase<SerNo>::get_base_value(); }
+    void reset_base_value_from_drvd(int new_value) { CppBase<SerNo>::reset_base_value(new_value); }
 
 private:
     int drvd_value;
 };
 
+template <int SerNo, typename... Extra>
+void wrap_classes(py::module_ &m, const char *name_base, const char *name_drvd, Extra... extra) {
+    py::class_<CppBase<SerNo>>(m, name_base, std::forward<Extra>(extra)...)
+        .def(py::init<int>())
+        .def("get_base_value", &CppBase<SerNo>::get_base_value)
+        .def("reset_base_value", &CppBase<SerNo>::reset_base_value);
+
+    py::class_<CppDrvd<SerNo>, CppBase<SerNo>>(m, name_drvd, std::forward<Extra>(extra)...)
+        .def(py::init<int>())
+        .def("get_drvd_value", &CppDrvd<SerNo>::get_drvd_value)
+        .def("reset_drvd_value", &CppDrvd<SerNo>::reset_drvd_value)
+        .def("get_base_value_from_drvd", &CppDrvd<SerNo>::get_base_value_from_drvd)
+        .def("reset_base_value_from_drvd", &CppDrvd<SerNo>::reset_base_value_from_drvd);
+}
+
 } // namespace test_python_multiple_inheritance
 
 TEST_SUBMODULE(python_multiple_inheritance, m) {
     using namespace test_python_multiple_inheritance;
-
-    py::class_<CppBase>(m, "CppBase")
-        .def(py::init<int>())
-        .def("get_base_value", &CppBase::get_base_value)
-        .def("reset_base_value", &CppBase::reset_base_value);
-
-    py::class_<CppDrvd, CppBase>(m, "CppDrvd")
-        .def(py::init<int>())
-        .def("get_drvd_value", &CppDrvd::get_drvd_value)
-        .def("reset_drvd_value", &CppDrvd::reset_drvd_value)
-        .def("get_base_value_from_drvd", &CppDrvd::get_base_value_from_drvd)
-        .def("reset_base_value_from_drvd", &CppDrvd::reset_base_value_from_drvd);
+    wrap_classes<0>(m, "CppBase0", "CppDrvd0");
+    wrap_classes<1>(m, "CppBase1", "CppDrvd1", py::metaclass((PyObject *) &PyType_Type));
+
+    m.attr("if_defined_PYBIND11_INIT_SAFETY_CHECKS_VIA_DEFAULT_PYBIND11_METACLASS") =
+#if defined(PYBIND11_INIT_SAFETY_CHECKS_VIA_DEFAULT_PYBIND11_METACLASS)
+        true;
+#else
+        false;
+#endif
 }

tests/test_python_multiple_inheritance.py

@@ -1,26 +1,78 @@
 # Adapted from:
-# https://github.com/google/clif/blob/5718e4d0807fd3b6a8187dde140069120b81ecef/clif/testing/python/python_multiple_inheritance_test.py
+# https://github.com/google/clif/blob/7d388e1de7db5beeb3d7429c18a2776d8188f44f/clif/testing/python/python_multiple_inheritance_test.py
+
+import pytest
 
 from pybind11_tests import python_multiple_inheritance as m
 
+#
+# Using default py::metaclass() (used with py::class_<> for CppBase0, CppDrvd0):
+#
+
+
+class PC0(m.CppBase0):
+    pass
+
+
+class PPCC0(PC0, m.CppDrvd0):
+    pass
+
+
+class PCExplicitInitWithSuper0(m.CppBase0):
+    def __init__(self, value):
+        super().__init__(value + 1)
+
+
+class PCExplicitInitMissingSuper0(m.CppBase0):
+    def __init__(self, value):
+        del value
+
+
+class PCExplicitInitMissingSuperB0(m.CppBase0):
+    def __init__(self, value):
+        del value
+
+
+#
+# Using py::metaclass((PyObject *) &PyType_Type) (used with py::class_<> for CppBase1, CppDrvd1):
+# COPY-PASTE block from above, replace 0 with 1:
+#
 
-class PC(m.CppBase):
+
+class PC1(m.CppBase1):
     pass
 
 
-class PPCC(PC, m.CppDrvd):
+class PPCC1(PC1, m.CppDrvd1):
     pass
 
 
-def test_PC():
-    d = PC(11)
+class PCExplicitInitWithSuper1(m.CppBase1):
+    def __init__(self, value):
+        super().__init__(value + 1)
+
+
+class PCExplicitInitMissingSuper1(m.CppBase1):
+    def __init__(self, value):
+        del value
+
+
+class PCExplicitInitMissingSuperB1(m.CppBase1):
+    def __init__(self, value):
+        del value
+
+
+@pytest.mark.parametrize(("pc_type"), [PC0, PC1])
+def test_PC(pc_type):
+    d = pc_type(11)
     assert d.get_base_value() == 11
     d.reset_base_value(13)
     assert d.get_base_value() == 13
 
 
-def test_PPCC():
-    d = PPCC(11)
+@pytest.mark.parametrize(("ppcc_type"), [PPCC0, PPCC1])
+def test_PPCC(ppcc_type):
+    d = ppcc_type(11)
     assert d.get_drvd_value() == 33
     d.reset_drvd_value(55)
     assert d.get_drvd_value() == 55
@@ -33,3 +85,54 @@ def test_PPCC():
     d.reset_base_value_from_drvd(30)
     assert d.get_base_value() == 30
     assert d.get_base_value_from_drvd() == 30
+
+
+@pytest.mark.parametrize(
+    ("pc_type"), [PCExplicitInitWithSuper0, PCExplicitInitWithSuper1]
+)
+def testPCExplicitInitWithSuper(pc_type):
+    d = pc_type(14)
+    assert d.get_base_value() == 15
+
+
+@pytest.mark.parametrize(
+    ("derived_type"),
+    [
+        PCExplicitInitMissingSuper0,
+        PCExplicitInitMissingSuperB0,
+        PCExplicitInitMissingSuper1,
+        PCExplicitInitMissingSuperB1,
+    ],
+)
+def testPCExplicitInitMissingSuper(derived_type):
+    if (
+        m.if_defined_PYBIND11_INIT_SAFETY_CHECKS_VIA_DEFAULT_PYBIND11_METACLASS
+        and derived_type
+        in (
+            PCExplicitInitMissingSuper1,
+            PCExplicitInitMissingSuperB1,
+        )
+    ):
+        pytest.skip(
+            "PYBIND11_INIT_SAFETY_CHECKS_VIA_DEFAULT_PYBIND11_METACLASS is defined"
+        )
+    with pytest.raises(TypeError) as excinfo:
+        derived_type(0)
+    assert str(excinfo.value).endswith(
+        ".__init__() must be called when overriding __init__"
+    )
+
+
+def test_derived_tp_init_registry_weakref_based_cleanup():
+    def nested_function(i):
+        class NestedClass(m.CppBase0):
+            def __init__(self, value):
+                super().__init__(value + 3)
+
+        d1 = NestedClass(i + 7)
+        d2 = NestedClass(i + 8)
+        return (d1.get_base_value(), d2.get_base_value())
+
+    for _ in range(100):
+        assert nested_function(0) == (10, 11)
+        assert nested_function(3) == (13, 14)