Nicer API to pass py::capsule destructor

docs/advanced/misc.rst

@@ -170,6 +170,20 @@ would be then able to access the data behind the same pointer.
 
 .. [#f6] https://docs.python.org/3/extending/extending.html#using-capsules
 
+Module Destructors
+==================
+
+pybind11 does not provide an explicit mechanism to invoke cleanup code at
+module destruction time. In rare cases where such functionality is required, it
+is possible to emulate it using Python capsules with a destruction callback.
+
+.. code-block:: cpp
+
+    auto cleanup_callback = []() {
+        // perform cleanup here -- this function is called with the GIL held
+    };
+
+    m.add_object("_cleanup", py::capsule(cleanup_callback));
 
 Generating documentation using Sphinx
 =====================================

include/pybind11/eigen.h

@@ -235,7 +235,7 @@ handle eigen_ref_array(Type &src, handle parent = none()) {
 // not the Type of the pointer given is const.
 template <typename props, typename Type, typename = enable_if_t<is_eigen_dense_plain<Type>::value>>
 handle eigen_encapsulate(Type *src) {
-    capsule base(src, [](PyObject *o) { delete static_cast<Type *>(PyCapsule_GetPointer(o, nullptr)); });
+    capsule base(src, [](void *o) { delete static_cast<Type *>(o); });
     return eigen_ref_array<props>(*src, base);
 }
 

include/pybind11/pybind11.h

@@ -294,8 +294,8 @@ class cpp_function : public function {
             rec->def->ml_meth = reinterpret_cast<PyCFunction>(*dispatcher);
             rec->def->ml_flags = METH_VARARGS | METH_KEYWORDS;
 
-            capsule rec_capsule(rec, [](PyObject *o) {
-                destruct((detail::function_record *) PyCapsule_GetPointer(o, nullptr));
+            capsule rec_capsule(rec, [](void *ptr) {
+                destruct((detail::function_record *) ptr);
             });
 
             object scope_module;

include/pybind11/pytypes.h

@@ -1004,10 +1004,44 @@ class capsule : public object {
     PYBIND11_OBJECT_DEFAULT(capsule, object, PyCapsule_CheckExact)
     PYBIND11_DEPRECATED("Use reinterpret_borrow<capsule>() or reinterpret_steal<capsule>()")
     capsule(PyObject *ptr, bool is_borrowed) : object(is_borrowed ? object(ptr, borrowed) : object(ptr, stolen)) { }
-    explicit capsule(const void *value, void (*destruct)(PyObject *) = nullptr)
+
+    explicit capsule(const void *value)
+        : object(PyCapsule_New(const_cast<void *>(value), nullptr, nullptr), stolen) {
+        if (!m_ptr)
+            pybind11_fail("Could not allocate capsule object!");
+    }
+
+    PYBIND11_DEPRECATED("Please pass a destructor that takes a void pointer as input")
+    capsule(const void *value, void (*destruct)(PyObject *))
         : object(PyCapsule_New(const_cast<void*>(value), nullptr, destruct), stolen) {
-        if (!m_ptr) pybind11_fail("Could not allocate capsule object!");
+        if (!m_ptr)
+            pybind11_fail("Could not allocate capsule object!");
     }
+
+    capsule(const void *value, void (*destructor)(void *)) {
+        m_ptr = PyCapsule_New(const_cast<void *>(value), nullptr, [](PyObject *o) {
+            auto destructor = reinterpret_cast<void (*)(void *)>(PyCapsule_GetContext(o));
+            void *ptr = PyCapsule_GetPointer(o, nullptr);
+            destructor(ptr);
+        });
+
+        if (!m_ptr)
+            pybind11_fail("Could not allocate capsule object!");
+
+        if (PyCapsule_SetContext(m_ptr, (void *) destructor) != 0)
+            pybind11_fail("Could not set capsule context!");
+    }
+
+    capsule(void (*destructor)()) {
+        m_ptr = PyCapsule_New(reinterpret_cast<void *>(destructor), nullptr, [](PyObject *o) {
+            auto destructor = reinterpret_cast<void (*)()>(PyCapsule_GetPointer(o, nullptr));
+            destructor();
+        });
+
+        if (!m_ptr)
+            pybind11_fail("Could not allocate capsule object!");
+    }
+
     template <typename T> operator T *() const {
         T * result = static_cast<T *>(PyCapsule_GetPointer(m_ptr, nullptr));
         if (!result) pybind11_fail("Unable to extract capsule contents!");

tests/test_python_types.cpp

@@ -470,6 +470,24 @@ test_initializer python_types([](py::module &m) {
     m.def("return_none_bool",   []() -> bool *        { return nullptr; });
     m.def("return_none_int",    []() -> int *         { return nullptr; });
     m.def("return_none_float",  []() -> float *       { return nullptr; });
+
+    m.def("return_capsule_with_destructor",
+        []() {
+            py::print("creating capsule");
+            return py::capsule([]() {
+                py::print("destructing capsule");
+            });
+        }
+    );
+
+    m.def("return_capsule_with_destructor_2",
+        []() {
+            py::print("creating capsule");
+            return py::capsule((void *) 1234, [](void *ptr) {
+                py::print("destructing capsule: {}"_s.format((size_t) ptr));
+            });
+        }
+    );
 });
 
 #if defined(_MSC_VER)

tests/test_python_types.py

@@ -512,3 +512,24 @@ def test_builtins_cast_return_none():
     assert m.return_none_bool() is None
     assert m.return_none_int() is None
     assert m.return_none_float() is None
+
+
+def test_capsule_with_destructor(capture):
+    import pybind11_tests as m
+    with capture:
+        a = m.return_capsule_with_destructor()
+        del a
+        pytest.gc_collect()
+    assert capture.unordered == """
+        creating capsule
+        destructing capsule
+    """
+
+    with capture:
+        a = m.return_capsule_with_destructor_2()
+        del a
+        pytest.gc_collect()
+    assert capture.unordered == """
+        creating capsule
+        destructing capsule: 1234
+    """