Add unit test for DM-21314.

The garbage collection bug is a significant restriction on Storable,
and providing a non-passing unit test makes this fact more
self-documenting.

tests/testGenericMapLib.cc

@@ -274,6 +274,25 @@ void addCppStorable(MutableGenericMap<std::string>& testmap) {
     testmap.insert("cppPointer", std::make_shared<CppStorable>("pointer"));
 }
 
+/**
+ * Store and retrieve a Storable in C++.
+ *
+ * To avoid cluttering testGenericMapLib with a special holder class, this
+ * function conflates two actions. When called with a Storable, it puts a
+ * pointer to it in internal storage. When called with no arguments, it
+ * returns the pointer it has been keeping.
+ *
+ * @param storable The Storable to assign to the pointer [optional].
+ * @returns The currently held pointer.
+ */
+std::shared_ptr<Storable> keepStaticStorable(std::shared_ptr<Storable> storable = nullptr) {
+    static std::shared_ptr<Storable> longLived = nullptr;
+    if (storable) {
+        longLived = storable;
+    }
+    return longLived;
+}
+
 }  // namespace
 
 namespace {
@@ -309,6 +328,7 @@ PYBIND11_MODULE(testGenericMapLib, mod) {
     mod.def("makeInitialMap", &makeInitialMap);
     mod.def("makeCppUpdates", &makeCppUpdates, "testmap"_a);
     mod.def("addCppStorable", &addCppStorable, "testmap"_a);
+    mod.def("keepStaticStorable", &keepStaticStorable, "storable"_a = nullptr);
 
     py::class_<CppStorable, std::shared_ptr<CppStorable>, Storable> cls(mod, "CppStorable");
     cls.def(py::init<std::string>());

tests/test_Storable.py

@@ -20,6 +20,7 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 import copy
+import gc
 import unittest
 
 import lsst.utils.tests
@@ -88,6 +89,17 @@ def testEq(self):
         self.assertEqual(self.testbed, DemoStorable([42]))
         self.assertNotEqual(self.testbed, DemoStorable(0))
 
+    @unittest.skip("TODO: Fix this bug in DM-21314")
+    def testGarbageCollection(self):
+        cppLib.keepStaticStorable(DemoStorable(3))
+
+        gc.collect()
+
+        retrieved = cppLib.keepStaticStorable()
+        self.assertIsInstance(retrieved, Storable)
+        self.assertIsInstance(retrieved, DemoStorable)
+        self.assertEqual(retrieved, DemoStorable(3))
+
 
 class CppStorableTestSuite(lsst.utils.tests.TestCase):