Don't construct unique_ptr around unowned pointers (#478)

If we need to initialize a holder around an unowned instance, and the
holder type is non-copyable (i.e. a unique_ptr), we currently construct
the holder type around the value pointer, but then never actually
destruct the holder: the holder destructor is called only for the
instance that actually has `inst->owned = true` set.

This seems no pointer, however, in creating such a holder around an
unowned instance: we never actually intend to use anything that the
unique_ptr gives us: and, in fact, do not want the unique_ptr (because
if it ever actually got destroyed, it would cause destruction of the
wrapped pointer, despite the fact that that wrapped pointer isn't
owned).

This commit changes the logic to only create a unique_ptr holder if we
actually own the instance, and to destruct via the constructed holder
whenever we have a constructed holder--which will now only be the case
for owned-unique-holder or shared-holder types.

Other changes include:

* Added test for non-movable holder constructor/destructor counts

The three alive assertions now pass, before #478 they fail with counts
of 2/2/1 respectively, because of the unique_ptr that we don't want and
don't destroy (because we don't *want* its destructor to run).

* Return cstats reference; fix ConstructStats doc

Small cleanup to the #478 test code, and fix to the ConstructStats
documentation (the static method definition should use `reference` not
`reference_internal`).

* Rename inst->constructed to inst->holder_constructed

This makes it clearer exactly what it's referring to.

include/pybind11/common.h

@@ -311,7 +311,7 @@ template <typename type> struct instance_essentials {
     type *value;
     PyObject *weakrefs;
     bool owned : 1;
-    bool constructed : 1;
+    bool holder_constructed : 1;
 };
 
 /// PyObject wrapper around generic types, includes a special holder type that is responsible for lifetime management

include/pybind11/pybind11.h

@@ -821,7 +821,7 @@ protected:
         auto tinfo = detail::get_type_info(type);
         self->value = ::operator new(tinfo->type_size);
         self->owned = true;
-        self->constructed = false;
+        self->holder_constructed = false;
         detail::get_internals().registered_instances.emplace(self->value, (PyObject *) self);
         return (PyObject *) self;
     }
@@ -1134,7 +1134,7 @@ private:
         } catch (const std::bad_weak_ptr &) {
             new (&inst->holder) holder_type(inst->value);
         }
-        inst->owned = true;
+        inst->holder_constructed = true;
     }
 
     /// Initialize holder object, variant 2: try to construct from existing holder object, if possible
@@ -1145,31 +1145,32 @@ private:
             new (&inst->holder) holder_type(*holder_ptr);
         else
             new (&inst->holder) holder_type(inst->value);
-        inst->owned = true;
+        inst->holder_constructed = true;
     }
 
     /// Initialize holder object, variant 3: holder is not copy constructible (e.g. unique_ptr), always initialize from raw pointer
     template <typename T = holder_type,
               detail::enable_if_t<!std::is_copy_constructible<T>::value, int> = 0>
     static void init_holder_helper(instance_type *inst, const holder_type * /* unused */, const void * /* dummy */) {
+        if (inst->owned) {
             new (&inst->holder) holder_type(inst->value);
+            inst->holder_constructed = true;
+        }
     }
 
     /// Initialize holder object of an instance, possibly given a pointer to an existing holder
     static void init_holder(PyObject *inst_, const void *holder_ptr) {
         auto inst = (instance_type *) inst_;
         init_holder_helper(inst, (const holder_type *) holder_ptr, inst->value);
-        inst->constructed = true;
     }
 
     static void dealloc(PyObject *inst_) {
         instance_type *inst = (instance_type *) inst_;
-        if (inst->owned) {
-            if (inst->constructed)
+        if (inst->holder_constructed)
             inst->holder.~holder_type();
-            else
+        else if (inst->owned)
             ::operator delete(inst->value);
-        }
+
         generic_type::dealloc((detail::instance<void> *) inst);
     }
 

tests/constructor_stats.h

@@ -56,7 +56,7 @@ from the ConstructorStats instance `.values()` method.
 In some cases, when you need to track instances of a C++ class not registered with pybind11, you
 need to add a function returning the ConstructorStats for the C++ class; this can be done with:
 
-    m.def("get_special_cstats", &ConstructorStats::get<SpecialClass>, py::return_value_policy::reference_internal)
+    m.def("get_special_cstats", &ConstructorStats::get<SpecialClass>, py::return_value_policy::reference)
 
 Finally, you can suppress the output messages, but keep the constructor tracking (for
 inspection/testing in python) by using the functions with `print_` replaced with `track_` (e.g.

tests/test_issues.cpp

@@ -48,6 +48,24 @@ class Dupe2 {};
 class Dupe3 {};
 class DupeException : public std::runtime_error {};
 
+// #478
+template <typename T> class custom_unique_ptr {
+public:
+    custom_unique_ptr() { print_default_created(this); }
+    custom_unique_ptr(T *ptr) : _ptr{ptr} { print_created(this, ptr); }
+    custom_unique_ptr(custom_unique_ptr<T> &&move) : _ptr{move._ptr} { move._ptr = nullptr; print_move_created(this); }
+    custom_unique_ptr &operator=(custom_unique_ptr<T> &&move) { print_move_assigned(this); if (_ptr) destruct_ptr(); _ptr = move._ptr; move._ptr = nullptr; return *this; }
+    custom_unique_ptr(const custom_unique_ptr<T> &) = delete;
+    void operator=(const custom_unique_ptr<T> &copy) = delete;
+    ~custom_unique_ptr() { print_destroyed(this); if (_ptr) destruct_ptr(); }
+private:
+    T *_ptr = nullptr;
+    void destruct_ptr() { delete _ptr; }
+};
+PYBIND11_DECLARE_HOLDER_TYPE(T, custom_unique_ptr<T>);
+
+
+
 void init_issues(py::module &m) {
     py::module m2 = m.def_submodule("issues");
 
@@ -311,6 +329,25 @@ void init_issues(py::module &m) {
     py::class_<SharedParent, std::shared_ptr<SharedParent>>(m, "SharedParent")
         .def(py::init<>())
         .def("get_child", &SharedParent::get_child, py::return_value_policy::reference);
+
+    /// Issue/PR #478: unique ptrs constructed and freed without destruction
+    class SpecialHolderObj {
+    public:
+        int val = 0;
+        SpecialHolderObj *ch = nullptr;
+        SpecialHolderObj(int v, bool make_child = true) : val{v}, ch{make_child ? new SpecialHolderObj(val+1, false) : nullptr}
+        { print_created(this, val); }
+        ~SpecialHolderObj() { delete ch; print_destroyed(this); }
+        SpecialHolderObj *child() { return ch; }
+    };
+
+    py::class_<SpecialHolderObj, custom_unique_ptr<SpecialHolderObj>>(m, "SpecialHolderObj")
+        .def(py::init<int>())
+        .def("child", &SpecialHolderObj::child, pybind11::return_value_policy::reference_internal)
+        .def_readwrite("val", &SpecialHolderObj::val)
+        .def_static("holder_cstats", &ConstructorStats::get<custom_unique_ptr<SpecialHolderObj>>,
+                py::return_value_policy::reference)
+        ;
 };
 
 

tests/test_issues.py

@@ -201,3 +201,20 @@ def test_enable_shared_from_this_with_reference_rvp():
     assert cstats.alive() == 1
     del child, parent
     assert cstats.alive() == 0
+
+def test_non_destructed_holders():
+    """ Issue #478: unique ptrs constructed and freed without destruction """
+    from pybind11_tests import SpecialHolderObj
+
+    a = SpecialHolderObj(123)
+    b = a.child()
+
+    assert a.val == 123
+    assert b.val == 124
+
+    cstats = SpecialHolderObj.holder_cstats()
+    assert cstats.alive() == 1
+    del b
+    assert cstats.alive() == 1
+    del a
+    assert cstats.alive() == 0