Trivial refactoring to make the capsule API more user friendly. (#4720)

* Trivial refactoring to make the capsule API more user friendly.

* Use new API in production code. Thanks @Lalaland for pointing this out.

include/pybind11/pybind11.h

@@ -508,8 +508,8 @@ protected:
             rec->def->ml_flags = METH_VARARGS | METH_KEYWORDS;
 
             capsule rec_capsule(unique_rec.release(),
+                                detail::get_function_record_capsule_name(),
                                 [](void *ptr) { destruct((detail::function_record *) ptr); });
-            rec_capsule.set_name(detail::get_function_record_capsule_name());
             guarded_strdup.release();
 
             object scope_module;

include/pybind11/pytypes.h

@@ -1925,28 +1925,13 @@ public:
         }
     }
 
+    /// Capsule name is nullptr.
     capsule(const void *value, void (*destructor)(void *)) {
-        m_ptr = PyCapsule_New(const_cast<void *>(value), nullptr, [](PyObject *o) {
-            // guard if destructor called while err indicator is set
-            error_scope error_guard;
-            auto destructor = reinterpret_cast<void (*)(void *)>(PyCapsule_GetContext(o));
-            if (destructor == nullptr && PyErr_Occurred()) {
-                throw error_already_set();
-            }
-            const char *name = get_name_in_error_scope(o);
-            void *ptr = PyCapsule_GetPointer(o, name);
-            if (ptr == nullptr) {
-                throw error_already_set();
-            }
+        initialize_with_void_ptr_destructor(value, nullptr, destructor);
+    }
 
-            if (destructor != nullptr) {
-                destructor(ptr);
-            }
-        });
-
-        if (!m_ptr || PyCapsule_SetContext(m_ptr, reinterpret_cast<void *>(destructor)) != 0) {
-            throw error_already_set();
-        }
+    capsule(const void *value, const char *name, void (*destructor)(void *)) {
+        initialize_with_void_ptr_destructor(value, name, destructor);
     }
 
     explicit capsule(void (*destructor)()) {
@@ -2014,6 +1999,32 @@ private:
 
         return name;
     }
+
+    void initialize_with_void_ptr_destructor(const void *value,
+                                             const char *name,
+                                             void (*destructor)(void *)) {
+        m_ptr = PyCapsule_New(const_cast<void *>(value), name, [](PyObject *o) {
+            // guard if destructor called while err indicator is set
+            error_scope error_guard;
+            auto destructor = reinterpret_cast<void (*)(void *)>(PyCapsule_GetContext(o));
+            if (destructor == nullptr && PyErr_Occurred()) {
+                throw error_already_set();
+            }
+            const char *name = get_name_in_error_scope(o);
+            void *ptr = PyCapsule_GetPointer(o, name);
+            if (ptr == nullptr) {
+                throw error_already_set();
+            }
+
+            if (destructor != nullptr) {
+                destructor(ptr);
+            }
+        });
+
+        if (!m_ptr || PyCapsule_SetContext(m_ptr, reinterpret_cast<void *>(destructor)) != 0) {
+            throw error_already_set();
+        }
+    }
 };
 
 class tuple : public object {

tests/test_pytypes.cpp

@@ -260,6 +260,15 @@ TEST_SUBMODULE(pytypes, m) {
         });
     });
 
+    m.def("return_capsule_with_destructor_3", []() {
+        py::print("creating capsule");
+        auto cap = py::capsule((void *) 1233, "oname", [](void *ptr) {
+            py::print("destructing capsule: {}"_s.format((size_t) ptr));
+        });
+        py::print("original name: {}"_s.format(cap.name()));
+        return cap;
+    });
+
     m.def("return_renamed_capsule_with_destructor_2", []() {
         py::print("creating capsule");
         auto cap = py::capsule((void *) 1234, [](void *ptr) {

tests/test_pytypes.py

@@ -319,6 +319,19 @@ def test_capsule(capture):
     """
     )
 
+    with capture:
+        a = m.return_capsule_with_destructor_3()
+        del a
+        pytest.gc_collect()
+    assert (
+        capture.unordered
+        == """
+        creating capsule
+        destructing capsule: 1233
+        original name: oname
+    """
+    )
+
     with capture:
         a = m.return_renamed_capsule_with_destructor_2()
         del a