Address pull review feedback from @jagerman

- Clear has_patients in clear_instance
- Handle multiple inheritence
- Use ConstructorStats.detail_reg_inst to do deep garbage collection and
  verify the instance tracking counts

include/pybind11/class_support.h

@@ -314,6 +314,7 @@ inline void clear_instance(PyObject *self) {
         // from the unordered_map first.
         auto patients = std::move(pos->second);
         internals.patients.erase(pos);
+        instance->has_patients = false;
         for (PyObject *&patient : patients)
             Py_CLEAR(patient);
     }

include/pybind11/pybind11.h

@@ -1334,8 +1334,8 @@ inline void keep_alive_impl(handle nurse, handle patient) {
     if (patient.is_none() || nurse.is_none())
         return; /* Nothing to keep alive or nothing to be kept alive by */
 
-    auto tinfo = get_type_info(Py_TYPE(nurse.ptr()));
-    if (tinfo) {
+    auto tinfo = all_type_info(Py_TYPE(nurse.ptr()));
+    if (!tinfo.empty()) {
         /* It's a pybind-registered type, so we can store the patient in the
          * internal list. */
         add_patient(nurse.ptr(), patient.ptr());

tests/test_call_policies.py

@@ -2,67 +2,69 @@
 
 
 def test_keep_alive_argument(capture):
-    from pybind11_tests import Parent, Child
+    from pybind11_tests import Parent, Child, ConstructorStats
 
+    n_inst = ConstructorStats.detail_reg_inst()
     with capture:
         p = Parent()
     assert capture == "Allocating parent."
     with capture:
         p.addChild(Child())
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst + 1
     assert capture == """
         Allocating child.
         Releasing child.
     """
     with capture:
         del p
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst
     assert capture == "Releasing parent."
 
     with capture:
         p = Parent()
     assert capture == "Allocating parent."
     with capture:
         p.addChildKeepAlive(Child())
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst + 2
     assert capture == "Allocating child."
     with capture:
         del p
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst
     assert capture == """
         Releasing parent.
         Releasing child.
     """
 
 
 def test_keep_alive_return_value(capture):
-    from pybind11_tests import Parent
+    from pybind11_tests import Parent, ConstructorStats
 
+    n_inst = ConstructorStats.detail_reg_inst()
     with capture:
         p = Parent()
     assert capture == "Allocating parent."
     with capture:
         p.returnChild()
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst + 1
     assert capture == """
         Allocating child.
         Releasing child.
     """
     with capture:
         del p
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst
     assert capture == "Releasing parent."
 
     with capture:
         p = Parent()
     assert capture == "Allocating parent."
     with capture:
         p.returnChildKeepAlive()
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst + 2
     assert capture == "Allocating child."
     with capture:
         del p
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst
     assert capture == """
         Releasing parent.
         Releasing child.
@@ -72,66 +74,92 @@ def test_keep_alive_return_value(capture):
 # https://bitbucket.org/pypy/pypy/issues/2447
 @pytest.unsupported_on_pypy
 def test_alive_gc(capture):
-    from pybind11_tests import ParentGC, Child
+    from pybind11_tests import ParentGC, Child, ConstructorStats
 
+    n_inst = ConstructorStats.detail_reg_inst()
     p = ParentGC()
     p.addChildKeepAlive(Child())
+    assert ConstructorStats.detail_reg_inst() == n_inst + 2
     lst = [p]
     lst.append(lst)   # creates a circular reference
     with capture:
         del p, lst
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst
     assert capture == """
         Releasing parent.
         Releasing child.
     """
 
 
 def test_alive_gc_derived(capture):
-    from pybind11_tests import Parent, Child
+    from pybind11_tests import Parent, Child, ConstructorStats
 
     class Derived(Parent):
         pass
 
+    n_inst = ConstructorStats.detail_reg_inst()
     p = Derived()
     p.addChildKeepAlive(Child())
+    assert ConstructorStats.detail_reg_inst() == n_inst + 2
     lst = [p]
     lst.append(lst)   # creates a circular reference
     with capture:
         del p, lst
-        pytest.gc_collect()
-        pytest.gc_collect()    # Needed to make PyPy free the child
+        assert ConstructorStats.detail_reg_inst() == n_inst
+    assert capture == """
+        Releasing parent.
+        Releasing child.
+    """
+
+
+def test_alive_gc_multi_derived(capture):
+    from pybind11_tests import Parent, Child, ConstructorStats
+
+    class Derived(Parent, Child):
+        pass
+
+    n_inst = ConstructorStats.detail_reg_inst()
+    p = Derived()
+    p.addChildKeepAlive(Child())
+    # +3 rather than +2 because Derived corresponds to two registered instances
+    assert ConstructorStats.detail_reg_inst() == n_inst + 3
+    lst = [p]
+    lst.append(lst)   # creates a circular reference
+    with capture:
+        del p, lst
+        assert ConstructorStats.detail_reg_inst() == n_inst
     assert capture == """
         Releasing parent.
         Releasing child.
     """
 
 
 def test_return_none(capture):
-    from pybind11_tests import Parent
+    from pybind11_tests import Parent, ConstructorStats
 
+    n_inst = ConstructorStats.detail_reg_inst()
     with capture:
         p = Parent()
     assert capture == "Allocating parent."
     with capture:
         p.returnNullChildKeepAliveChild()
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst + 1
     assert capture == ""
     with capture:
         del p
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst
     assert capture == "Releasing parent."
 
     with capture:
         p = Parent()
     assert capture == "Allocating parent."
     with capture:
         p.returnNullChildKeepAliveParent()
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst + 1
     assert capture == ""
     with capture:
         del p
-        pytest.gc_collect()
+        assert ConstructorStats.detail_reg_inst() == n_inst
     assert capture == "Releasing parent."