fix: PyCapsule_GetDestructor is allowed to return a nullptr destructor (#4221)

* fix: PyCapsule_GetDestructor is allowed to return a nullptr destructor

Previously, this code would error out if the destructor happened to be
a nullptr. This is incorrect. nullptrs are allowed for capsule
destructors.

"It is legal for a capsule to have a NULL destructor. This makes a
NULL return code somewhat ambiguous; use PyCapsule_IsValid() or
PyErr_Occurred() to disambiguate."

See:

https://docs.python.org/3/c-api/capsule.html#c.PyCapsule_GetDestructor

I noticed this while working on a type caster related to #3858 DLPack
happens to allow the destructor not to be defined on a capsule, and I
encountered such a case. See:

e2bdd3bee8/include/dlpack/dlpack.h (L219)

* Add test for the fix.

* Update tests/test_pytypes.cpp

I tried this locally and it works!
I never knew that there are cases where `reinterpret_cast` does not work but `static_cast` does. Let's see if all compilers are happy with this.

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

* style: pre-commit fixes

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgkio@gmail.com>
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

include/pybind11/pytypes.h

@@ -1829,18 +1829,18 @@ public:
             // 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) {
-                if (PyErr_Occurred()) {
-                    throw error_already_set();
-                }
-                pybind11_fail("Unable to get capsule context");
+            if (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();
             }
-            destructor(ptr);
+
+            if (destructor != nullptr) {
+                destructor(ptr);
+            }
         });
 
         if (!m_ptr || PyCapsule_SetContext(m_ptr, (void *) destructor) != 0) {

tests/test_pytypes.cpp

@@ -289,6 +289,12 @@ TEST_SUBMODULE(pytypes, m) {
         return capsule;
     });
 
+    m.def("return_capsule_with_explicit_nullptr_dtor", []() {
+        py::print("creating capsule with explicit nullptr dtor");
+        return py::capsule(reinterpret_cast<void *>(1234),
+                           static_cast<void (*)(void *)>(nullptr)); // PR #4221
+    });
+
     // test_accessors
     m.def("accessor_api", [](const py::object &o) {
         auto d = py::dict();

tests/test_pytypes.py

@@ -299,6 +299,17 @@ def test_capsule(capture):
     """
     )
 
+    with capture:
+        a = m.return_capsule_with_explicit_nullptr_dtor()
+        del a
+        pytest.gc_collect()
+    assert (
+        capture.unordered
+        == """
+        creating capsule with explicit nullptr dtor
+    """
+    )
+
 
 def test_accessors():
     class SubTestObject: