Special handling of @__setstate__ to support alternative pickle mec…

…hanisms. (#30094)

* Use `py::metaclass((PyObject *) &PyType_Type)` and test compatibility with `metaclass=abc.ABCMeta`

* `@__setstate__` support (first stab)

* Add `test_abc_meta_incompatibility()`, `test_abc_meta_compatibility()` to test_methods_and_attributes.py

* Add `exercise_getinitargs_getstate_setstate` to test_pickling

* Improve `"@__setstate__"` related code in pybind11.h for readability.

* Resolve MSVC errors:  declaration of 'state' hides class member

* Change `@__setstate__` to `__setstate__[non-constructor]` based on feedback by @rainwoodman. Also revise comments.

include/pybind11/pybind11.h

@@ -394,6 +394,11 @@ class cpp_function : public function {
         // it alive.
         auto *rec = unique_rec.get();
 
+        // Note the "__setstate__" manipulation below.
+        rec->is_constructor = rec->name != nullptr
+                              && (std::strcmp(rec->name, "__init__") == 0
+                                  || std::strcmp(rec->name, "__setstate__") == 0);
+
         // Keep track of strdup'ed strings, and clean them up as long as the function's capsule
         // has not taken ownership yet (when `unique_rec.release()` is called).
         // Note: This cannot easily be fixed by a `unique_ptr` with custom deleter, because the
@@ -403,7 +408,14 @@ class cpp_function : public function {
         strdup_guard guarded_strdup;
 
         /* Create copies of all referenced C-style strings */
-        rec->name = guarded_strdup(rec->name ? rec->name : "");
+        if (rec->name == nullptr) {
+            rec->name = guarded_strdup("");
+        } else if (std::strcmp(rec->name, "__setstate__[non-constructor]") == 0) {
+            // See google/pywrapcc#30094 for background.
+            rec->name = guarded_strdup("__setstate__");
+        } else {
+            rec->name = guarded_strdup(rec->name);
+        }
         if (rec->doc) {
             rec->doc = guarded_strdup(rec->doc);
         }
@@ -418,9 +430,6 @@ class cpp_function : public function {
             }
         }
 
-        rec->is_constructor = (std::strcmp(rec->name, "__init__") == 0)
-                              || (std::strcmp(rec->name, "__setstate__") == 0);
-
 #if defined(PYBIND11_DETAILED_ERROR_MESSAGES) && !defined(PYBIND11_DISABLE_NEW_STYLE_INIT_WARNING)
         if (rec->is_constructor && !rec->is_new_style_constructor) {
             const auto class_name

tests/test_methods_and_attributes.py

@@ -1,3 +1,4 @@
+import abc
 import sys
 
 import pytest
@@ -227,6 +228,22 @@ def test_metaclass_override():
     assert isinstance(m.MetaclassOverride.__dict__["readonly"], int)
 
 
+def test_abc_meta_incompatibility():  # Mostly to clearly expose the behavior.
+    with pytest.raises(TypeError) as exc_info:
+
+        class ExampleMandAABC(m.ExampleMandA, metaclass=abc.ABCMeta):
+            pass
+
+    assert "metaclass conflict" in str(exc_info.value)
+
+
+def test_abc_meta_compatibility():
+    class MetaclassOverrideABC(m.MetaclassOverride, metaclass=abc.ABCMeta):
+        pass
+
+    assert type(MetaclassOverrideABC).__name__ == "ABCMeta"
+
+
 def test_no_mixed_overloads():
     from pybind11_tests import detailed_error_messages_enabled
 

tests/test_pickling.cpp

@@ -8,11 +8,14 @@
     BSD-style license that can be found in the LICENSE file.
 */
 
+#include <pybind11/stl.h>
+
 #include "pybind11_tests.h"
 
 #include <memory>
 #include <stdexcept>
 #include <utility>
+#include <vector>
 
 namespace exercise_trampoline {
 
@@ -60,6 +63,53 @@ void wrap(py::module m) {
 
 } // namespace exercise_trampoline
 
+namespace exercise_getinitargs_getstate_setstate {
+
+// Exercise `__setstate__[non-constructor]` (see google/pywrapcc#30094), which
+// was added to support a pickle protocol as established with Boost.Python
+// (in 2002):
+//   https://www.boost.org/doc/libs/1_31_0/libs/python/doc/v2/pickle.html
+// This mechanism was also adopted for PyCLIF:
+//   https://github.com/google/clif/blob/7d388e1de7db5beeb3d7429c18a2776d8188f44f/clif/testing/python/pickle_compatibility.clif
+// The code below exercises the pickle protocol intentionally exactly as used
+// in PyCLIF, to ensure that pybind11 stays fully compatible with existing
+// client code relying on the long-established protocol. (It is impractical to
+// change all such client code.)
+
+class StoreTwoWithState {
+public:
+    StoreTwoWithState(int v0, int v1) : values_{v0, v1}, state_{"blank"} {}
+    const std::vector<int> &GetInitArgs() const { return values_; }
+    const std::string &GetState() const { return state_; }
+    void SetState(const std::string &state) { state_ = state; }
+
+private:
+    std::vector<int> values_;
+    std::string state_;
+};
+
+void wrap(py::module m) {
+    py::class_<StoreTwoWithState>(std::move(m), "StoreTwoWithState")
+        .def(py::init<int, int>())
+        .def("__getinitargs__",
+             [](const StoreTwoWithState &self) { return py::tuple(py::cast(self.GetInitArgs())); })
+        .def("__getstate__", &StoreTwoWithState::GetState)
+        .def(
+            "__reduce_ex__",
+            [](py::handle self, int /*protocol*/) {
+                return py::make_tuple(self.attr("__class__"),
+                                      self.attr("__getinitargs__")(),
+                                      self.attr("__getstate__")());
+            },
+            py::arg("protocol") = -1)
+        .def(
+            "__setstate__[non-constructor]", // See google/pywrapcc#30094 for background.
+            [](StoreTwoWithState *self, const std::string &state) { self->SetState(state); },
+            py::arg("state"));
+}
+
+} // namespace exercise_getinitargs_getstate_setstate
+
 TEST_SUBMODULE(pickling, m) {
     m.def("simple_callable", []() { return 20220426; });
 
@@ -191,4 +241,5 @@ TEST_SUBMODULE(pickling, m) {
 #endif
 
     exercise_trampoline::wrap(m);
+    exercise_getinitargs_getstate_setstate::wrap(m);
 }

tests/test_pickling.py

@@ -1,3 +1,4 @@
+import copy
 import pickle
 import re
 
@@ -91,3 +92,22 @@ def test_roundtrip_simple_cpp_derived():
     # Issue #3062: pickleable base C++ classes can incur object slicing
     #              if derived typeid is not registered with pybind11
     assert not m.check_dynamic_cast_SimpleCppDerived(p2)
+
+
+#
+# exercise_getinitargs_getstate_setstate
+#
+def test_StoreTwoWithState():
+    obj = m.StoreTwoWithState(-38, 27)
+    assert obj.__getinitargs__() == (-38, 27)
+    assert obj.__getstate__() == "blank"
+    obj.__setstate__("blue")
+    reduced = obj.__reduce_ex__()
+    assert reduced == (m.StoreTwoWithState, (-38, 27), "blue")
+    cpy = copy.deepcopy(obj)
+    assert cpy.__reduce_ex__() == reduced
+    assert pickle.HIGHEST_PROTOCOL > 0  # To be sure the loop below makes sense.
+    for protocol in range(-1, pickle.HIGHEST_PROTOCOL + 1):
+        serialized = pickle.dumps(obj, protocol)
+        restored = pickle.loads(serialized)
+        assert restored.__reduce_ex__() == reduced