Add a scope guard call policy

```c++
m.def("foo", foo, py::call_guard<T>());
```

is equivalent to:

```c++
m.def("foo", [](args...) {
    T scope_guard;
    return foo(args...); // forwarded arguments
});
```

docs/advanced/functions.rst

@@ -162,14 +162,16 @@ Additional call policies
 ========================
 
 In addition to the above return value policies, further *call policies* can be
-specified to indicate dependencies between parameters. In general, call policies
-are required when the C++ object is any kind of container and another object is being
-added to the container.
+specified to indicate dependencies between parameters or ensure a certain state
+for the function call.
 
-There is currently just
-one policy named ``keep_alive<Nurse, Patient>``, which indicates that the
-argument with index ``Patient`` should be kept alive at least until the
-argument with index ``Nurse`` is freed by the garbage collector. Argument
+Keep alive
+----------
+
+In general, this policy is required when the C++ object is any kind of container
+and another object is being added to the container. ``keep_alive<Nurse, Patient>``
+indicates that the argument with index ``Patient`` should be kept alive at least
+until the argument with index ``Nurse`` is freed by the garbage collector. Argument
 indices start at one, while zero refers to the return value. For methods, index
 ``1`` refers to the implicit ``this`` pointer, while regular arguments begin at
 index ``2``. Arbitrarily many call policies can be specified. When a ``Nurse``
@@ -194,10 +196,36 @@ container:
     Patient != 0) and ``with_custodian_and_ward_postcall`` (if Nurse/Patient ==
     0) policies from Boost.Python.
 
+Call guard
+----------
+
+The ``call_guard<T>`` policy allows any scope guard type ``T`` to be placed
+around the function call. For example, this definition:
+
+.. code-block:: cpp
+
+    m.def("foo", foo, py::call_guard<T>());
+
+is equivalent to the following pseudocode:
+
+.. code-block:: cpp
+
+    m.def("foo", [](args...) {
+        T scope_guard;
+        return foo(args...); // forwarded arguments
+    });
+
+The only requirement is that ``T`` is default-constructible, but otherwise any
+scope guard will work. This is very useful in combination with `gil_scoped_release`.
+See :ref:`gil`.
+
+Multiple guards can also be specified as ``py::call_guard<T1, T2, T3...>``. The
+constructor order is left to right and destruction happens in reverse.
+
 .. seealso::
 
-    The file :file:`tests/test_keep_alive.cpp` contains a complete example
-    that demonstrates using :class:`keep_alive` in more detail.
+    The file :file:`tests/test_call_policies.cpp` contains a complete example
+    that demonstrates using `keep_alive` and `call_guard` in more detail.
 
 .. _python_objects_as_args:
 

docs/advanced/misc.rst

@@ -15,6 +15,7 @@ T2>, myFunc)``. In this case, the preprocessor assumes that the comma indicates
 the beginning of the next parameter. Use a ``typedef`` to bind the template to
 another name and use it in the macro to avoid this problem.
 
+.. _gil:
 
 Global Interpreter Lock (GIL)
 =============================
@@ -68,6 +69,13 @@ could be realized as follows (important changes highlighted):
         return m.ptr();
     }
 
+The ``call_go`` wrapper can also be simplified using the `call_guard` policy
+(see :ref:`call_policies`) which yields the same result:
+
+.. code-block:: cpp
+
+    m.def("call_go", &call_go, py::call_guard<py::gil_scoped_release>());
+
 
 Binding sequence data types, iterators, the slicing protocol, etc.
 ==================================================================

include/pybind11/attr.h

@@ -67,6 +67,44 @@ struct metaclass {
 /// Annotation to mark enums as an arithmetic type
 struct arithmetic { };
 
+/** \rst
+    A call policy which places one or more guard variables (``Ts...``) around the function call.
+
+    For example, this definition:
+
+    .. code-block:: cpp
+
+        m.def("foo", foo, py::call_guard<T>());
+
+    is equivalent to the following pseudocode:
+
+    .. code-block:: cpp
+
+        m.def("foo", [](args...) {
+            T scope_guard;
+            return foo(args...); // forwarded arguments
+        });
+ \endrst */
+template <typename... Ts> struct call_guard;
+
+template <> struct call_guard<> { using type = detail::void_type; };
+
+template <typename T>
+struct call_guard<T> {
+    static_assert(std::is_default_constructible<T>::value,
+                  "The guard type must be default constructible");
+
+    using type = T;
+};
+
+template <typename T, typename... Ts>
+struct call_guard<T, Ts...> {
+    struct type {
+        T guard{}; // Compose multiple guard types with left-to-right default-constructor order
+        typename call_guard<Ts...>::type next{};
+    };
+};
+
 /// @} annotations
 
 NAMESPACE_BEGIN(detail)
@@ -374,6 +412,9 @@ struct process_attribute<metaclass> : process_attribute_default<metaclass> {
 template <>
 struct process_attribute<arithmetic> : process_attribute_default<arithmetic> {};
 
+template <typename... Ts>
+struct process_attribute<call_guard<Ts...>> : process_attribute_default<call_guard<Ts...>> { };
+
 /***
  * Process a keep_alive call policy -- invokes keep_alive_impl during the
  * pre-call handler if both Nurse, Patient != 0 and use the post-call handler
@@ -410,6 +451,13 @@ template <typename... Args> struct process_attributes {
     }
 };
 
+template <typename T>
+using is_call_guard = is_instantiation<call_guard, T>;
+
+/// Extract the ``type`` from the first `call_guard` in `Extras...` (or `void_type` if none found)
+template <typename... Extra>
+using extract_guard_t = typename first_of_t<is_call_guard, call_guard<>, Extra...>::type;
+
 /// Check the number of named arguments at compile time
 template <typename... Extra,
           size_t named = constexpr_sum(std::is_base_of<arg, Extra>::value...),

include/pybind11/cast.h

@@ -1383,14 +1383,14 @@ public:
         return load_impl_sequence(call, indices{});
     }
 
-    template <typename Return, typename Func>
+    template <typename Return, typename Guard, typename Func>
     enable_if_t<!std::is_void<Return>::value, Return> call(Func &&f) {
-        return call_impl<Return>(std::forward<Func>(f), indices{});
+        return call_impl<Return>(std::forward<Func>(f), indices{}, Guard{});
     }
 
-    template <typename Return, typename Func>
+    template <typename Return, typename Guard, typename Func>
     enable_if_t<std::is_void<Return>::value, void_type> call(Func &&f) {
-        call_impl<Return>(std::forward<Func>(f), indices{});
+        call_impl<Return>(std::forward<Func>(f), indices{}, Guard{});
         return void_type();
     }
 
@@ -1406,8 +1406,8 @@ private:
         return true;
     }
 
-    template <typename Return, typename Func, size_t... Is>
-    Return call_impl(Func &&f, index_sequence<Is...>) {
+    template <typename Return, typename Func, size_t... Is, typename Guard>
+    Return call_impl(Func &&f, index_sequence<Is...>, Guard &&) {
         return std::forward<Func>(f)(cast_op<Args>(std::get<Is>(value))...);
     }
 

include/pybind11/common.h

@@ -536,9 +536,15 @@ using is_template_base_of = decltype(is_template_base_of_impl<Base>::check((remo
 struct is_template_base_of : decltype(is_template_base_of_impl<Base>::check((remove_cv_t<T>*)nullptr)) { };
 #endif
 
+/// Check if T is an instantiation of the template `Class`. For example:
+/// `is_instantiation<shared_ptr, T>` is true if `T == shared_ptr<U>` where U can be anything.
+template <template<typename...> class Class, typename T>
+struct is_instantiation : std::false_type { };
+template <template<typename...> class Class, typename... Us>
+struct is_instantiation<Class, Class<Us...>> : std::true_type { };
+
 /// Check if T is std::shared_ptr<U> where U can be anything
-template <typename T> struct is_shared_ptr : std::false_type { };
-template <typename U> struct is_shared_ptr<std::shared_ptr<U>> : std::true_type { };
+template <typename T> using is_shared_ptr = is_instantiation<std::shared_ptr, T>;
 
 /// Ignore that a variable is unused in compiler warnings
 inline void ignore_unused(const int *) { }

include/pybind11/pybind11.h

@@ -143,8 +143,11 @@ protected:
             /* Override policy for rvalues -- usually to enforce rvp::move on an rvalue */
             const auto policy = detail::return_value_policy_override<Return>::policy(call.func.policy);
 
+            /* Function scope guard -- defaults to the compile-to-nothing `void_type` */
+            using Guard = detail::extract_guard_t<Extra...>;
+
             /* Perform the function call */
-            handle result = cast_out::cast(args_converter.template call<Return>(cap->f),
+            handle result = cast_out::cast(args_converter.template call<Return, Guard>(cap->f),
                                            policy, call.parent);
 
             /* Invoke call policy post-call hook */

tests/CMakeLists.txt

@@ -28,6 +28,7 @@ endif()
 set(PYBIND11_TEST_FILES
   test_alias_initialization.cpp
   test_buffers.cpp
+  test_call_policies.cpp
   test_callbacks.cpp
   test_chrono.cpp
   test_class_args.cpp
@@ -40,7 +41,6 @@ set(PYBIND11_TEST_FILES
   test_exceptions.cpp
   test_inheritance.cpp
   test_issues.cpp
-  test_keep_alive.cpp
   test_kwargs_and_defaults.cpp
   test_methods_and_attributes.cpp
   test_modules.cpp

tests/test_call_policies.cpp

@@ -0,0 +1,91 @@
+/*
+    tests/test_call_policies.cpp -- keep_alive and call_guard
+
+    Copyright (c) 2016 Wenzel Jakob <wenzel.jakob@epfl.ch>
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#include "pybind11_tests.h"
+
+class Child {
+public:
+    Child() { py::print("Allocating child."); }
+    ~Child() { py::print("Releasing child."); }
+};
+
+class Parent {
+public:
+    Parent() { py::print("Allocating parent."); }
+    ~Parent() { py::print("Releasing parent."); }
+    void addChild(Child *) { }
+    Child *returnChild() { return new Child(); }
+    Child *returnNullChild() { return nullptr; }
+};
+
+test_initializer keep_alive([](py::module &m) {
+    py::class_<Parent>(m, "Parent")
+        .def(py::init<>())
+        .def("addChild", &Parent::addChild)
+        .def("addChildKeepAlive", &Parent::addChild, py::keep_alive<1, 2>())
+        .def("returnChild", &Parent::returnChild)
+        .def("returnChildKeepAlive", &Parent::returnChild, py::keep_alive<1, 0>())
+        .def("returnNullChildKeepAliveChild", &Parent::returnNullChild, py::keep_alive<1, 0>())
+        .def("returnNullChildKeepAliveParent", &Parent::returnNullChild, py::keep_alive<0, 1>());
+
+    py::class_<Child>(m, "Child")
+        .def(py::init<>());
+});
+
+struct CustomGuard {
+    static bool enabled;
+
+    CustomGuard() { enabled = true; }
+    ~CustomGuard() { enabled = false; }
+
+    static const char *report_status() { return enabled ? "guarded" : "unguarded"; }
+};
+
+bool CustomGuard::enabled = false;
+
+struct DependentGuard {
+    static bool enabled;
+
+    DependentGuard() { enabled = CustomGuard::enabled; }
+    ~DependentGuard() { enabled = false; }
+
+    static const char *report_status() { return enabled ? "guarded" : "unguarded"; }
+};
+
+bool DependentGuard::enabled = false;
+
+test_initializer call_guard([](py::module &pm) {
+    auto m = pm.def_submodule("call_policies");
+
+    m.def("unguarded_call", &CustomGuard::report_status);
+    m.def("guarded_call", &CustomGuard::report_status, py::call_guard<CustomGuard>());
+
+    m.def("multiple_guards_correct_order", []() {
+        return CustomGuard::report_status() + std::string(" & ") + DependentGuard::report_status();
+    }, py::call_guard<CustomGuard, DependentGuard>());
+
+    m.def("multiple_guards_wrong_order", []() {
+        return DependentGuard::report_status() + std::string(" & ") + CustomGuard::report_status();
+    }, py::call_guard<DependentGuard, CustomGuard>());
+
+#if defined(WITH_THREAD) && !defined(PYPY_VERSION)
+    // `py::call_guard<py::gil_scoped_release>()` should work in PyPy as well,
+    // but it's unclear how to test it without `PyGILState_GetThisThreadState`.
+    auto report_gil_status = []() {
+        auto is_gil_held = false;
+        if (auto tstate = py::detail::get_thread_state_unchecked())
+            is_gil_held = (tstate == PyGILState_GetThisThreadState());
+
+        return is_gil_held ? "GIL held" : "GIL released";
+    };
+
+    m.def("with_gil", report_gil_status);
+    m.def("without_gil", report_gil_status, py::call_guard<py::gil_scoped_release>());
+#endif
+});

tests/test_call_policies.py

@@ -95,3 +95,17 @@ def test_return_none(capture):
         del p
         pytest.gc_collect()
     assert capture == "Releasing parent."
+
+
+def test_call_guard():
+    from pybind11_tests import call_policies
+
+    assert call_policies.unguarded_call() == "unguarded"
+    assert call_policies.guarded_call() == "guarded"
+
+    assert call_policies.multiple_guards_correct_order() == "guarded & guarded"
+    assert call_policies.multiple_guards_wrong_order() == "unguarded & guarded"
+
+    if hasattr(call_policies, "with_gil"):
+        assert call_policies.with_gil() == "GIL held"
+        assert call_policies.without_gil() == "GIL released"

tests/test_keep_alive.cpp

@@ -1,40 +0,0 @@
-/*
-    tests/test_keep_alive.cpp -- keep_alive modifier (pybind11's version
-    of Boost.Python's with_custodian_and_ward / with_custodian_and_ward_postcall)
-
-    Copyright (c) 2016 Wenzel Jakob <wenzel.jakob@epfl.ch>
-
-    All rights reserved. Use of this source code is governed by a
-    BSD-style license that can be found in the LICENSE file.
-*/
-
-#include "pybind11_tests.h"
-
-class Child {
-public:
-    Child() { py::print("Allocating child."); }
-    ~Child() { py::print("Releasing child."); }
-};
-
-class Parent {
-public:
-    Parent() { py::print("Allocating parent."); }
-    ~Parent() { py::print("Releasing parent."); }
-    void addChild(Child *) { }
-    Child *returnChild() { return new Child(); }
-    Child *returnNullChild() { return nullptr; }
-};
-
-test_initializer keep_alive([](py::module &m) {
-    py::class_<Parent>(m, "Parent")
-        .def(py::init<>())
-        .def("addChild", &Parent::addChild)
-        .def("addChildKeepAlive", &Parent::addChild, py::keep_alive<1, 2>())
-        .def("returnChild", &Parent::returnChild)
-        .def("returnChildKeepAlive", &Parent::returnChild, py::keep_alive<1, 0>())
-        .def("returnNullChildKeepAliveChild", &Parent::returnNullChild, py::keep_alive<1, 0>())
-        .def("returnNullChildKeepAliveParent", &Parent::returnNullChild, py::keep_alive<0, 1>());
-
-    py::class_<Child>(m, "Child")
-        .def(py::init<>());
-});