PyCLIF-C-API feature parity with this pybind11 base.__init__() must be called when overriding __init__ error condition:

* google3/third_party/pybind11/include/pybind11/detail/class.h;l=195-204;rcl=555209523

Motivation: Guard against backsliding (b/296065655).

Situation:

`CppBase` is a PyCLIF-C-API-wrapped C++ object.

What happens when the Python interpreter processes the following code (usually at import time)?

```
class PC(CppBase):
  pass
```

When the native Python `PC` class is built:

* `PC` `tp_new` is set to use `CppBase` `tp_new`, but

* `PC` `tp_init` does NOT in any way involve `CppBase` `tp_init`.

It is the responsibility of `PC.__init__` to call `CppBase.__init__`, but this is not checked.

**This CL adds the missing check.** The approach is:

* `PC` `tp_init` is replaced with an "intercept" function.

* The intercept function calls the original `PC` `tp_init`.

* After that call finishes (and if it was successful), the intercept function checks if the `CppBase` wrapped C++ object was initialized.

This approach makes the assumption that `PC` `tp_init` is not also modified elsewhere, validated via TGP testing. The practical benefit of guarding against backsliding (e.g. cl/558247087) is assumed to far outweight the very theoretical risk of that assumption not being true, and even if it is not true, the consequences are unlikely to be harmful. An additional consideration is that the switch to PyCLIF-pybind11 will eliminate this risk entirely.

For easy reviewing, this is the generated code implementing the new mechanism:

```
// Intentionally leak the unordered_map:
// https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables
static auto* derived_tp_init_registry = new std::unordered_map<PyTypeObject*, int(*)(PyObject*, PyObject*, PyObject*)>;

static int tp_init_intercepted(PyObject* self, PyObject* args, PyObject* kw) {
  DCHECK(PyType_Check(self) == 0);
  const auto derived_tp_init = derived_tp_init_registry->find(Py_TYPE(self));
  CHECK(derived_tp_init != derived_tp_init_registry->end());
  int status = (*derived_tp_init->second)(self, args, kw);
  if (status == 0 &&
      reinterpret_cast<wrapper*>(self)->cpp.get() == nullptr) {
    Py_DECREF(self);
    PyErr_Format(PyExc_TypeError,
                 "%s.__init__() must be called when"
                 " overriding __init__", wrapper_Type->tp_name);
    return -1;
  }
  return status;
}

static PyObject* tp_new_impl(PyTypeObject* type, PyObject* args, PyObject* kwds) {
  if (type->tp_init != tp_init_impl &&
      derived_tp_init_registry->count(type) == 0) {
    (*derived_tp_init_registry)[type] = type->tp_init;
    type->tp_init = tp_init_intercepted;
  }
  return PyType_GenericNew(type, args, kwds);
}
```

Background technical pointers:

What happens when a `PC` object is constructed (i.e. `PC()`) in Python?

* `_PyObject_MakeTpCall()`: google3/third_party/python_runtime/v3_10/Objects/call.c;l=215;rcl=491965220

* `type_call` `type->tp_new()`: google3/third_party/python_runtime/v3_10/Objects/typeobject.c;l=1123;rcl=491965220

* `type_call` `type->tp_init()`: google3/third_party/python_runtime/v3_10/Objects/typeobject.c;l=1135;rcl=491965220

Side note:

* `CppBase` `tp_alloc` is NOT called. Instead, `PyObject* self` is
  allocated and `\0`-initialized in `PyType_GenericAlloc()`. For
  the wrapped `clif::Instance<CppBase>` this is Undefined Behavior.
  We are just getting lucky that it works:

  * google3/third_party/python_runtime/v3_10/Objects/typeobject.c;l=1166;rcl=491965220

  This was noted already here:

  * google3/third_party/clif/python/gen.py;l=504-506;rcl=551297023

Additional manual testing (go/py311-upgrade):

```
blaze test --python3_version=3.11 --python_mode=unstable third_party/clif/... devtools/clif/... -k
```

* http://sponge2/7f8c0ecd-48c4-4269-8d71-e2e08802d40a

The only failure is known and unrelated: b/288436695

PiperOrigin-RevId: 559501787

Author: rwgk

clif/python/gen.py

@@ -398,14 +398,24 @@ def TypeObject(ht_qualname, tracked_slot_groups,
     yield (
         'static int tp_init_impl(PyObject* self, PyObject* args, PyObject* kw);'
     )
+    yield (
+        'static int tp_init_intercepted('
+        'PyObject* self, PyObject* args, PyObject* kw);'
+    )
   if not iterator:
     yield ''
-    yield '// %s tp_alloc' % pyname
+    yield '// %s tp_alloc_impl' % pyname
     yield (
         'static PyObject* tp_alloc_impl(PyTypeObject* type, Py_ssize_t nitems);'
     )
     tp_slots['tp_alloc'] = 'tp_alloc_impl'
-    tp_slots['tp_new'] = 'PyType_GenericNew'
+    yield ''
+    yield '// %s tp_new_impl' % pyname
+    yield (
+        'static PyObject* tp_new_impl(PyTypeObject* type, PyObject* args,'
+        ' PyObject* kwds);'
+    )
+    tp_slots['tp_new'] = 'tp_new_impl'
   yield ''
   # Use dtor for dynamic types (derived) to wind down malloc'ed C++ obj, so
   # the C++ dtors are run.
@@ -433,6 +443,7 @@ def TypeObject(ht_qualname, tracked_slot_groups,
     # Use delete for static types (not derived), allocated with tp_alloc_impl.
     tp_slots['tp_free'] = 'tp_free_impl'
     yield ''
+    yield '// %s tp_free_impl' % pyname
     yield 'static void tp_free_impl(void* self) {'
     yield I+'delete %s(self);' % _Cast(wname)
     yield '}'
@@ -479,6 +490,16 @@ def TypeObject(ht_qualname, tracked_slot_groups,
   yield I+'return ty;'
   yield '}'
   if ctor:
+    yield ''
+    yield '// Intentionally leak the unordered_map:'
+    yield (
+        '// https://google.github.io/styleguide/cppguide.html'
+        '#Static_and_Global_Variables'
+    )
+    yield (
+        'static auto* derived_tp_init_registry = new std::unordered_map<'
+        'PyTypeObject*, int(*)(PyObject*, PyObject*, PyObject*)>;'
+    )
     yield ''
     yield (
         'static int tp_init_impl('
@@ -530,6 +551,28 @@ def TypeObject(ht_qualname, tracked_slot_groups,
         yield I+'Py_XDECREF(init);'
         yield I+'return init? 0: -1;'
     yield '}'
+    yield ''
+    yield (
+        'static int tp_init_intercepted('
+        'PyObject* self, PyObject* args, PyObject* kw) {'
+    )
+    yield I+'DCHECK(PyType_Check(self) == 0);'
+    yield (
+        I+'const auto derived_tp_init = '
+        'derived_tp_init_registry->find(Py_TYPE(self));'
+    )
+    yield I+'CHECK(derived_tp_init != derived_tp_init_registry->end());'
+    yield I+'int status = (*derived_tp_init->second)(self, args, kw);'
+    yield I+'if (status == 0 &&'
+    yield I+'    reinterpret_cast<wrapper*>(self)->cpp.get() == nullptr) {'
+    yield I+'  Py_DECREF(self);'
+    yield I+'  PyErr_Format(PyExc_TypeError,'
+    yield I+'               "%s.__init__() must be called when"'
+    yield I+'               " overriding __init__", wrapper_Type->tp_name);'
+    yield I+'  return -1;'
+    yield I+'}'
+    yield I+'return status;'
+    yield '}'
   if not iterator:
     yield ''
     yield (
@@ -543,6 +586,19 @@ def TypeObject(ht_qualname, tracked_slot_groups,
     yield I+'PyObject* self = %s(wobj);' % _Cast()
     yield I+'return PyObject_Init(self, %s);' % wtype
     yield '}'
+    yield ''
+    yield (
+        'static PyObject* tp_new_impl(PyTypeObject* type, PyObject* args,'
+        ' PyObject* kwds) {'
+    )
+    if ctor:
+      yield I+'if (type->tp_init != tp_init_impl &&'
+      yield I+'    derived_tp_init_registry->count(type) == 0) {'
+      yield I+I+'(*derived_tp_init_registry)[type] = type->tp_init;'
+      yield I+I+'type->tp_init = tp_init_intercepted;'
+      yield I+'}'
+    yield I+'return PyType_GenericNew(type, args, kwds);'
+    yield '}'
 
 
 def CreateInputParameter(func_name, ast_param, arg, args):

clif/testing/python/python_multiple_inheritance_test.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 from absl.testing import absltest
+from absl.testing import parameterized
 
 from clif.testing.python import python_multiple_inheritance as tm
 
@@ -32,7 +33,25 @@ def __init__(self, value):
     tm.CppDrvd.__init__(self, value + 1)
 
 
-class PythonMultipleInheritanceTest(absltest.TestCase):
+class PCExplicitInitWithSuper(tm.CppBase):
+
+  def __init__(self, value):
+    super().__init__(value + 1)
+
+
+class PCExplicitInitMissingSuper(tm.CppBase):
+
+  def __init__(self, value):
+    del value
+
+
+class PCExplicitInitMissingSuper2(tm.CppBase):
+
+  def __init__(self, value):
+    del value
+
+
+class PythonMultipleInheritanceTest(parameterized.TestCase):
 
   def testPC(self):
     d = PC(11)
@@ -80,6 +99,22 @@ def testPPCCInit(self):
     self.assertEqual(d.get_base_value(), (30, 20)[i])
     self.assertEqual(d.get_base_value_from_drvd(), 30)
 
+  def testPCExplicitInitWithSuper(self):
+    d = PCExplicitInitWithSuper(14)
+    self.assertEqual(d.get_base_value(), 15)
+
+  @parameterized.parameters(
+      PCExplicitInitMissingSuper, PCExplicitInitMissingSuper2
+  )
+  def testPCExplicitInitMissingSuper(self, derived_type):
+    with self.assertRaises(TypeError) as ctx:
+      derived_type(0)
+    self.assertEndsWith(
+        str(ctx.exception),
+        "python_multiple_inheritance.CppBase.__init__() must be called when"
+        " overriding __init__",
+    )
+
 
 if __name__ == "__main__":
   absltest.main()