Adds set_name method of pybind11::capsule class (#3866)

* Adds set_name method of pybind11::capsule class

This calls PyCapsule_SetName on the underlying capsule object.

modified destructors to query capsules's Name

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Handle possible exception thrown by PyCapsule_GetName

Also removed accidentally reintroduced use of `const char *&`.

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Fixed function name

* Introduced private static function to reuse get_name_or_throw

* added tests for capsule renaming

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* handle python error in flight

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* Initialized PyObject * variables to nullptr

* use write-unraisable if PyCapsule_GetName raises

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* get_name_or_throw->get_name_no_throw

If PyCapsule_GetName raises an error we should write as unraisable
to consume it and notify user, and then restore the error in flight if any.
This way this method called from destructor would not modify interpreter
error state.

* used error_scope struct

* Renamed get_name_no_throw->get_name_in_error_scope

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

include/pybind11/pytypes.h

@@ -1588,7 +1588,8 @@ class capsule : public object {
                 }
                 pybind11_fail("Unable to get capsule context");
             }
-            void *ptr = PyCapsule_GetPointer(o, nullptr);
+            const char *name = get_name_in_error_scope(o);
+            void *ptr = PyCapsule_GetPointer(o, name);
             if (ptr == nullptr) {
                 throw error_already_set();
             }
@@ -1602,7 +1603,8 @@ class capsule : public object {
 
     explicit capsule(void (*destructor)()) {
         m_ptr = PyCapsule_New(reinterpret_cast<void *>(destructor), nullptr, [](PyObject *o) {
-            auto destructor = reinterpret_cast<void (*)()>(PyCapsule_GetPointer(o, nullptr));
+            const char *name = get_name_in_error_scope(o);
+            auto destructor = reinterpret_cast<void (*)()>(PyCapsule_GetPointer(o, name));
             if (destructor == nullptr) {
                 throw error_already_set();
             }
@@ -1637,7 +1639,33 @@ class capsule : public object {
         }
     }
 
-    const char *name() const { return PyCapsule_GetName(m_ptr); }
+    const char *name() const {
+        const char *name = PyCapsule_GetName(m_ptr);
+        if ((name == nullptr) && PyErr_Occurred()) {
+            throw error_already_set();
+        }
+        return name;
+    }
+
+    /// Replaces a capsule's name *without* calling the destructor on the existing one.
+    void set_name(const char *new_name) {
+        if (PyCapsule_SetName(m_ptr, new_name) != 0) {
+            throw error_already_set();
+        }
+    }
+
+private:
+    static const char *get_name_in_error_scope(PyObject *o) {
+        error_scope error_guard;
+
+        const char *name = PyCapsule_GetName(o);
+        if ((name == nullptr) && PyErr_Occurred()) {
+            // write out and consume error raised by call to PyCapsule_GetName
+            PyErr_WriteUnraisable(o);
+        }
+
+        return name;
+    }
 };
 
 class tuple : public object {

tests/test_pytypes.cpp

@@ -159,13 +159,33 @@ TEST_SUBMODULE(pytypes, m) {
         return py::capsule([]() { py::print("destructing capsule"); });
     });
 
+    m.def("return_renamed_capsule_with_destructor", []() {
+        py::print("creating capsule");
+        auto cap = py::capsule([]() { py::print("destructing capsule"); });
+        static const char *capsule_name = "test_name1";
+        py::print("renaming capsule");
+        cap.set_name(capsule_name);
+        return cap;
+    });
+
     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));
         });
     });
 
+    m.def("return_renamed_capsule_with_destructor_2", []() {
+        py::print("creating capsule");
+        auto cap = py::capsule((void *) 1234, [](void *ptr) {
+            py::print("destructing capsule: {}"_s.format((size_t) ptr));
+        });
+        static const char *capsule_name = "test_name2";
+        py::print("renaming capsule");
+        cap.set_name(capsule_name);
+        return cap;
+    });
+
     m.def("return_capsule_with_name_and_destructor", []() {
         auto capsule = py::capsule((void *) 12345, "pointer type description", [](PyObject *ptr) {
             if (ptr) {

tests/test_pytypes.py

@@ -195,6 +195,19 @@ def test_capsule(capture):
     """
     )
 
+    with capture:
+        a = m.return_renamed_capsule_with_destructor()
+        del a
+        pytest.gc_collect()
+    assert (
+        capture.unordered
+        == """
+        creating capsule
+        renaming capsule
+        destructing capsule
+    """
+    )
+
     with capture:
         a = m.return_capsule_with_destructor_2()
         del a
@@ -207,6 +220,19 @@ def test_capsule(capture):
     """
     )
 
+    with capture:
+        a = m.return_renamed_capsule_with_destructor_2()
+        del a
+        pytest.gc_collect()
+    assert (
+        capture.unordered
+        == """
+        creating capsule
+        renaming capsule
+        destructing capsule: 1234
+    """
+    )
+
     with capture:
         a = m.return_capsule_with_name_and_destructor()
         del a