[pydrake] Add function to make arbitrary ref cycles

This patch adds internal::make_arbitrary_ref_cycle(), which will prove
useful for awkward bindings like those involving multiple return values.

While in the neighborhood, it also adds a few previously missing tests.

bindings/pydrake/common/ref_cycle_pybind.cc

@@ -44,6 +44,32 @@ void make_ref_cycle(handle p0, handle p1) {
   make_link(p1, p0);
 }
 
+void check_and_make_ref_cycle(size_t peer0, handle p0, size_t peer1, handle p1,
+    std::function<std::string(size_t)> not_gc_message_function) {
+  // Returns false if the handle's value is None. Throws if the handle's value
+  // is not of a garbage-collectable type.
+  auto check_handle = [&](size_t n, handle p) -> bool {
+    if (p.is_none()) {
+      return false;
+    }
+    // Among the reasons the following check may fail is that one of the
+    // participating pybind11::class_ types does not declare
+    // pybind11::dynamic_attr().
+    if (!PyType_IS_GC(Py_TYPE(p.ptr()))) {
+      py::pybind11_fail(not_gc_message_function(n));
+    }
+    return true;
+  };
+  if (!check_handle(peer0, p0) || !check_handle(peer1, p1)) {
+    // At least one of the handles is None. We can't construct a ref-cycle, but
+    // neither should we complain. A None variable value could happen for any
+    // number of legitimate reasons, and does not mean that the ref_cycle call
+    // policy is defective.
+    return;
+  }
+  make_ref_cycle(p0, p1);
+}
+
 }  // namespace
 
 void ref_cycle_impl(
@@ -67,32 +93,24 @@ void ref_cycle_impl(
   handle p0 = get_arg(peer0);
   handle p1 = get_arg(peer1);
 
-  // Returns false if the handle's value is None. Throws if the handle's value
-  // is not of a garbage-collectable type.
-  auto check_handle = [&](size_t n, handle p) -> bool {
-    if (p.is_none()) {
-      return false;
-    }
-    // Among the reasons the following check may fail is that one of the
-    // participating pybind11::class_ types does not declare
-    // pybind11::dynamic_attr().
-    if (!PyType_IS_GC(Py_TYPE(p.ptr()))) {
-      py::pybind11_fail(fmt::format(
-          "Could not activate ref_cycle: object type at index {} for "
-          "function '{}' is not tracked by garbage collection. Was the object "
-          "defined with `pybind11::class_<...>(... pybind11::dynamic_attr())`?",
-          n, call.func.name));
-    }
-    return true;
+  auto not_gc_error = [&call](size_t n) -> std::string {
+    return fmt::format(
+        "Could not activate ref_cycle: object type at index {} for "
+        "function '{}' is not tracked by garbage collection.",
+        n, call.func.name);
   };
-  if (!check_handle(peer0, p0) || !check_handle(peer1, p1)) {
-    // At least one of the handles is None. We can't construct a ref-cycle, but
-    // neither should we complain. A None variable value could happen for any
-    // number of legitimate reasons, and does not mean that the ref_cycle call
-    // policy is defective.
-    return;
-  }
-  make_ref_cycle(p0, p1);
+  check_and_make_ref_cycle(peer0, p0, peer1, p1, not_gc_error);
+}
+
+void make_arbitrary_ref_cycle(
+    handle p0, handle p1, const std::string& location_hint) {
+  auto not_gc_error = [&location_hint](size_t n) -> std::string {
+    return fmt::format(
+        "Could not activate arbitrary ref_cycle: object type at argument {} "
+        "for binding at '{}' is not tracked by garbage collection.",
+        n, location_hint);
+  };
+  check_and_make_ref_cycle(0, p0, 1, p1, not_gc_error);
 }
 
 }  // namespace internal

bindings/pydrake/common/ref_cycle_pybind.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <string>
+
 #include "drake/bindings/pydrake/pydrake_pybind.h"
 
 namespace drake {
@@ -51,6 +53,13 @@ struct ref_cycle {};
 void ref_cycle_impl(size_t peer0, size_t peer1,
     const py::detail::function_call& call, py::handle ret);
 
+/* This function constructs a reference cycle from arbitrary handles. It may be
+ needed in special cases where the ordinary call-policy annotations won't work.
+ The `location_hint` will appear in any exception messages; it should help
+ developers locate where and why this function was called. */
+void make_arbitrary_ref_cycle(
+    py::handle p0, py::handle p1, const std::string& location_hint);
+
 }  // namespace internal
 }  // namespace pydrake
 }  // namespace drake

bindings/pydrake/common/test/ref_cycle_test.py

@@ -7,7 +7,8 @@
 import weakref
 
 from pydrake.common.ref_cycle_test_util import (
-    NotDynamic, IsDynamic, invalid_arg_index, free_function, ouroboros)
+    arbitrary_bad, arbitrary_ok, free_function,  invalid_arg_index, IsDynamic,
+    NotDynamic, ouroboros)
 from pydrake.common.test_utilities.memory_test_util import actual_ref_count
 
 
@@ -56,12 +57,27 @@ def test_ouroboros(self):
         self.assertEqual(len(dut._pydrake_internal_ref_cycle_peers), 1)
         self.check_is_collectable_cycle(returned, dut)
 
+    def test_arbitrary_ok(self):
+        got = arbitrary_ok()
+        self.assertTrue(hasattr(got, '_pydrake_internal_ref_cycle_peers'))
+
+    def test_arbitrary_bad(self):
+        with self.assertRaisesRegex(RuntimeError, ".*from arbitrary_bad.*"):
+            arbitrary_bad()
+
     def test_free_function(self):
         p0 = IsDynamic()
         p1 = IsDynamic()
         free_function(p0, p1)
         self.check_is_collectable_cycle(p0, p1)
 
+    def test_init_cycle(self):
+        # Cover the case where index 1 refers to the `self` of a py::init<>()
+        # binding.
+        other = IsDynamic()
+        dut = IsDynamic(other)
+        self.check_is_collectable_cycle(dut, other)
+
     def test_not_dynamic_add(self):
         dut = NotDynamic()
         peer = IsDynamic()

bindings/pydrake/common/test/ref_cycle_test_util_py.cc

@@ -22,6 +22,7 @@ class TestDummyBase {
   TestDummyBase(const TestDummyBase&) = default;
   ~TestDummyBase() = default;
 
+  explicit TestDummyBase(IsDynamic*) {}
   void AddNot(NotDynamic*) {}
   NotDynamic* ReturnNot() { return new NotDynamic(); }
   NotDynamic* ReturnNullNot() { return nullptr; }
@@ -50,6 +51,7 @@ PYBIND11_MODULE(ref_cycle_test_util, m) {
     using Class = IsDynamic;
     py::class_<Class>(m, "IsDynamic", py::dynamic_attr())
         .def(py::init<>())
+        .def(py::init<IsDynamic*>(), py::arg("thing"), ref_cycle<1, 2>())
         .def("AddNot", &Class::AddNot)
         .def("AddNotCycle", &Class::AddNot, ref_cycle<1, 2>())
         .def("ReturnNot", &Class::ReturnNot)
@@ -71,6 +73,18 @@ PYBIND11_MODULE(ref_cycle_test_util, m) {
   // Returns its argument and creates a self-cycle.
   m.def(
       "ouroboros", [](IsDynamic* x) { return x; }, ref_cycle<0, 1>());
+  m.def("arbitrary_ok", []() {
+    auto d1 = py::cast(new IsDynamic);
+    auto d2 = py::cast(new IsDynamic);
+    internal::make_arbitrary_ref_cycle(d1, d2, "from arbitrary_ok");
+    return d1;
+  });
+  m.def("arbitrary_bad", []() {
+    auto dyn = py::cast(new IsDynamic);
+    auto bad = py::cast(new NotDynamic);
+    internal::make_arbitrary_ref_cycle(dyn, bad, "from arbitrary_bad");
+    return dyn;
+  });
 }
 
 }  // namespace pydrake