test_keep_alive: Show that keep_alive cycle escapes GC

tests/CMakeLists.txt

@@ -124,6 +124,7 @@ set(PYBIND11_TEST_FILES
     # test_tagbased_polymorphic.cpp
     # test_union.cpp
     # test_virtual_functions.cpp)
+    test_keep_alive.cpp
     )
 # cmake-format: on
 

tests/test_keep_alive.cpp

@@ -0,0 +1,12 @@
+#include "pybind11_tests.h"
+
+class SimpleClass {};
+
+TEST_SUBMODULE(keep_alive, m) {
+    m.def("keep_alive_impl", [](py::handle nurse, py::handle patient) {
+        py::detail::keep_alive_impl(nurse, patient);
+    });
+
+    py::class_<SimpleClass>(m, "SimpleClass")
+        .def(py::init());
+}

tests/test_keep_alive.py

@@ -0,0 +1,29 @@
+# -*- coding: utf-8 -*-
+# N.B. This is only focused on CPython, so using gc directly, rather than
+# `pytest.gc_collect()`.
+import gc
+import weakref
+
+from pybind11_tests import keep_alive as m
+
+
+def test_keep_alive_cycle():
+    # See #2761.
+    o1 = m.SimpleClass()
+    wr1 = weakref.ref(o1)
+    o2 = m.SimpleClass()
+    wr2 = weakref.ref(o2)
+    assert wr1() is not None
+    assert wr2() is not None
+
+    # Add a direct cycle.
+    m.keep_alive_impl(o1, o2)
+    m.keep_alive_impl(o2, o1)
+
+    del o1
+    del o2
+    gc.collect()
+
+    # This shows that py::keep_alive will leak objects :(
+    assert wr1() is not None
+    assert wr2() is not None