[smart_holder] Unique ptr deleter roundtrip tests and fix (#4921)

* Roundtrip through unique pointer with custom deleter.

Currently failing.

* Ensure the custom deleter is copied back to the unique pointer.

Feels like there's still a gap around the raw pointer flavour, but this at least
makes the unit test of the previous commit succeed.

* Add deleter roundtrip for const atyp.

Currently failing, custom deleter is lost.

* Fix storing deleter for const unique ptr.

Unit test from the previous commit passes.

* Remove SFINEA deleter assignment.

At the construction of the smart holder, it is either a del_fun, or a default constructed deleter, so this complexity is unnecessary.

* Clang format.

* Fixes for ci.

Clang 3.6 requires the extra constructors in the custom_deleter.

* fix(smart_holder): Loosen requirement on deleter to be default constructible.

And some other PR feedback.

* fix(smart_holder): Custom deleter in unit tests traces constructions.

* fix(smart_holder): Use pybind11_fail instead of assert.

* fix(smart_holder): Add unit tests for the default constructible deleter.

* fix(smart_holder): Use regex matching for deleter constructors in unit tests.

include/pybind11/detail/smart_holder_type_casters.h

@@ -464,6 +464,27 @@ struct shared_ptr_trampoline_self_life_support {
     }
 };
 
+template <typename T,
+          typename D,
+          typename std::enable_if<std::is_default_constructible<D>::value, int>::type = 0>
+inline std::unique_ptr<T, D> unique_with_deleter(T *raw_ptr, std::unique_ptr<D> &&deleter) {
+    if (deleter == nullptr) {
+        return std::unique_ptr<T, D>(raw_ptr);
+    }
+    return std::unique_ptr<T, D>(raw_ptr, std::move(*deleter));
+}
+
+template <typename T,
+          typename D,
+          typename std::enable_if<!std::is_default_constructible<D>::value, int>::type = 0>
+inline std::unique_ptr<T, D> unique_with_deleter(T *raw_ptr, std::unique_ptr<D> &&deleter) {
+    if (deleter == nullptr) {
+        pybind11_fail("smart_holder_type_casters: deleter is not default constructible and no"
+                      " instance available to return.");
+    }
+    return std::unique_ptr<T, D>(raw_ptr, std::move(*deleter));
+}
+
 template <typename T>
 struct smart_holder_type_caster_load {
     using holder_type = pybindit::memory::smart_holder;
@@ -589,7 +610,7 @@ struct smart_holder_type_caster_load {
                              " std::unique_ptr.");
         }
         if (!have_holder()) {
-            return nullptr;
+            return unique_with_deleter<T, D>(nullptr, std::unique_ptr<D>());
         }
         throw_if_uninitialized_or_disowned_holder();
         throw_if_instance_is_currently_owned_by_shared_ptr();
@@ -616,13 +637,30 @@ struct smart_holder_type_caster_load {
                               "instance cannot safely be transferred to C++.");
         }
 
+        // Temporary variable to store the extracted deleter in.
+        std::unique_ptr<D> extracted_deleter;
+
+        auto *gd = std::get_deleter<pybindit::memory::guarded_delete>(holder().vptr);
+        if (gd && gd->use_del_fun) { // Note the ensure_compatible_rtti_uqp_del<T, D>() call above.
+            // In smart_holder_poc, a custom  deleter is always stored in a guarded delete.
+            // The guarded delete's std::function<void(void*)> actually points at the
+            // custom_deleter type, so we can verify it is of the custom deleter type and
+            // finally extract its deleter.
+            using custom_deleter_D = pybindit::memory::custom_deleter<T, D>;
+            const auto &custom_deleter_ptr = gd->del_fun.template target<custom_deleter_D>();
+            assert(custom_deleter_ptr != nullptr);
+            // Now that we have confirmed the type of the deleter matches the desired return
+            // value we can extract the function.
+            extracted_deleter = std::unique_ptr<D>(new D(std::move(custom_deleter_ptr->deleter)));
+        }
+
         // Critical transfer-of-ownership section. This must stay together.
         if (self_life_support != nullptr) {
             holder().disown();
         } else {
             holder().release_ownership();
         }
-        auto result = std::unique_ptr<T, D>(raw_type_ptr);
+        auto result = unique_with_deleter<T, D>(raw_type_ptr, std::move(extracted_deleter));
         if (self_life_support != nullptr) {
             self_life_support->activate_life_support(load_impl.loaded_v_h);
         } else {
@@ -1056,7 +1094,8 @@ struct smart_holder_type_caster<std::unique_ptr<T const, D>>
     static handle
     cast(std::unique_ptr<T const, D> &&src, return_value_policy policy, handle parent) {
         return smart_holder_type_caster<std::unique_ptr<T, D>>::cast(
-            std::unique_ptr<T, D>(const_cast<T *>(src.release())), // Const2Mutbl
+            std::unique_ptr<T, D>(const_cast<T *>(src.release()),
+                                  std::move(src.get_deleter())), // Const2Mutbl
             policy,
             parent);
     }

tests/test_class_sh_basic.cpp

@@ -28,6 +28,43 @@ struct uconsumer { // unique_ptr consumer
     const std::unique_ptr<atyp> &rtrn_cref() const { return held; }
 };
 
+/// Custom deleter that is default constructible.
+struct custom_deleter {
+    std::string trace_txt;
+
+    custom_deleter() = default;
+    explicit custom_deleter(const std::string &trace_txt_) : trace_txt(trace_txt_) {}
+
+    custom_deleter(const custom_deleter &other) { trace_txt = other.trace_txt + "_CpCtor"; }
+
+    custom_deleter &operator=(const custom_deleter &rhs) {
+        trace_txt = rhs.trace_txt + "_CpLhs";
+        return *this;
+    }
+
+    custom_deleter(custom_deleter &&other) noexcept {
+        trace_txt = other.trace_txt + "_MvCtorTo";
+        other.trace_txt += "_MvCtorFrom";
+    }
+
+    custom_deleter &operator=(custom_deleter &&rhs) noexcept {
+        trace_txt = rhs.trace_txt + "_MvLhs";
+        rhs.trace_txt += "_MvRhs";
+        return *this;
+    }
+
+    void operator()(atyp *p) const { std::default_delete<atyp>()(p); }
+    void operator()(const atyp *p) const { std::default_delete<const atyp>()(p); }
+};
+static_assert(std::is_default_constructible<custom_deleter>::value, "");
+
+/// Custom deleter that is not default constructible.
+struct custom_deleter_nd : custom_deleter {
+    custom_deleter_nd() = delete;
+    explicit custom_deleter_nd(const std::string &trace_txt_) : custom_deleter(trace_txt_) {}
+};
+static_assert(!std::is_default_constructible<custom_deleter_nd>::value, "");
+
 // clang-format off
 
 atyp        rtrn_valu() { atyp obj{"rtrn_valu"}; return obj; }
@@ -64,6 +101,18 @@ std::unique_ptr<atyp const, sddc> rtrn_udcp() { return std::unique_ptr<atyp cons
 std::string pass_udmp(std::unique_ptr<atyp,       sddm> obj) { return "pass_udmp:" + obj->mtxt; }
 std::string pass_udcp(std::unique_ptr<atyp const, sddc> obj) { return "pass_udcp:" + obj->mtxt; }
 
+std::unique_ptr<atyp,       custom_deleter> rtrn_udmp_del() { return std::unique_ptr<atyp,       custom_deleter>(new atyp{"rtrn_udmp_del"}, custom_deleter{"udmp_deleter"}); }
+std::unique_ptr<atyp const, custom_deleter> rtrn_udcp_del() { return std::unique_ptr<atyp const, custom_deleter>(new atyp{"rtrn_udcp_del"}, custom_deleter{"udcp_deleter"}); }
+
+std::string pass_udmp_del(std::unique_ptr<atyp,       custom_deleter> obj) { return "pass_udmp_del:" + obj->mtxt + "," + obj.get_deleter().trace_txt; }
+std::string pass_udcp_del(std::unique_ptr<atyp const, custom_deleter> obj) { return "pass_udcp_del:" + obj->mtxt + "," + obj.get_deleter().trace_txt; }
+
+std::unique_ptr<atyp,       custom_deleter_nd> rtrn_udmp_del_nd() { return std::unique_ptr<atyp,       custom_deleter_nd>(new atyp{"rtrn_udmp_del_nd"}, custom_deleter_nd{"udmp_deleter_nd"}); }
+std::unique_ptr<atyp const, custom_deleter_nd> rtrn_udcp_del_nd() { return std::unique_ptr<atyp const, custom_deleter_nd>(new atyp{"rtrn_udcp_del_nd"}, custom_deleter_nd{"udcp_deleter_nd"}); }
+
+std::string pass_udmp_del_nd(std::unique_ptr<atyp,       custom_deleter_nd> obj) { return "pass_udmp_del_nd:" + obj->mtxt + "," + obj.get_deleter().trace_txt; }
+std::string pass_udcp_del_nd(std::unique_ptr<atyp const, custom_deleter_nd> obj) { return "pass_udcp_del_nd:" + obj->mtxt + "," + obj.get_deleter().trace_txt; }
+
 // clang-format on
 
 // Helpers for testing.
@@ -130,6 +179,18 @@ TEST_SUBMODULE(class_sh_basic, m) {
     m.def("pass_udmp", pass_udmp);
     m.def("pass_udcp", pass_udcp);
 
+    m.def("rtrn_udmp_del", rtrn_udmp_del);
+    m.def("rtrn_udcp_del", rtrn_udcp_del);
+
+    m.def("pass_udmp_del", pass_udmp_del);
+    m.def("pass_udcp_del", pass_udcp_del);
+
+    m.def("rtrn_udmp_del_nd", rtrn_udmp_del_nd);
+    m.def("rtrn_udcp_del_nd", rtrn_udcp_del_nd);
+
+    m.def("pass_udmp_del_nd", pass_udmp_del_nd);
+    m.def("pass_udcp_del_nd", pass_udcp_del_nd);
+
     py::classh<uconsumer>(m, "uconsumer")
         .def(py::init<>())
         .def("valid", &uconsumer::valid)

tests/test_class_sh_basic.py

@@ -65,6 +65,35 @@ def test_load_with_rtrn_f(pass_f, rtrn_f, expected):
     assert pass_f(rtrn_f()) == expected
 
 
+@pytest.mark.parametrize(
+    ("pass_f", "rtrn_f", "regex_expected"),
+    [
+        (
+            m.pass_udmp_del,
+            m.rtrn_udmp_del,
+            "pass_udmp_del:rtrn_udmp_del,udmp_deleter(_MvCtorTo)*_MvCtorTo",
+        ),
+        (
+            m.pass_udcp_del,
+            m.rtrn_udcp_del,
+            "pass_udcp_del:rtrn_udcp_del,udcp_deleter(_MvCtorTo)*_MvCtorTo",
+        ),
+        (
+            m.pass_udmp_del_nd,
+            m.rtrn_udmp_del_nd,
+            "pass_udmp_del_nd:rtrn_udmp_del_nd,udmp_deleter_nd(_MvCtorTo)*_MvCtorTo",
+        ),
+        (
+            m.pass_udcp_del_nd,
+            m.rtrn_udcp_del_nd,
+            "pass_udcp_del_nd:rtrn_udcp_del_nd,udcp_deleter_nd(_MvCtorTo)*_MvCtorTo",
+        ),
+    ],
+)
+def test_deleter_roundtrip(pass_f, rtrn_f, regex_expected):
+    assert re.match(regex_expected, pass_f(rtrn_f()))
+
+
 @pytest.mark.parametrize(
     ("pass_f", "rtrn_f", "expected"),
     [