Hold strong references to keep_alive patients

This fixes #856. Instead of the weakref trick, the internals structure
holds an unordered_map from PyObject* to a vector of references. To
avoid the cost of the unordered_map lookup for objects that don't have
any keep_alive patients, a flag is added to each instance to indicate
whether there is anything to do.
diff --git a/tests/test_call_policies.cpp b/tests/test_call_policies.cpp
index 33a3f15..1527592 100644
--- a/tests/test_call_policies.cpp
+++ b/tests/test_call_policies.cpp
@@ -24,6 +24,13 @@
     Child *returnNullChild() { return nullptr; }
 };
 
+#if !defined(PYPY_VERSION)
+class ParentGC : public Parent {
+public:
+    using Parent::Parent;
+};
+#endif
+
 test_initializer keep_alive([](py::module &m) {
     py::class_<Parent>(m, "Parent")
         .def(py::init<>())
@@ -34,6 +41,11 @@
         .def("returnNullChildKeepAliveChild", &Parent::returnNullChild, py::keep_alive<1, 0>())
         .def("returnNullChildKeepAliveParent", &Parent::returnNullChild, py::keep_alive<0, 1>());
 
+#if !defined(PYPY_VERSION)
+    py::class_<ParentGC, Parent>(m, "ParentGC", py::dynamic_attr())
+        .def(py::init<>());
+#endif
+
     py::class_<Child>(m, "Child")
         .def(py::init<>());
 });
diff --git a/tests/test_call_policies.py b/tests/test_call_policies.py
index 1cb11c2..de4ec96 100644
--- a/tests/test_call_policies.py
+++ b/tests/test_call_policies.py
@@ -2,21 +2,22 @@
 
 
 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:
@@ -24,11 +25,11 @@
     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.
@@ -36,21 +37,22 @@
 
 
 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:
@@ -58,11 +60,74 @@
     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.
+    """
+
+
+# https://bitbucket.org/pypy/pypy/issues/2447
+@pytest.unsupported_on_pypy
+def test_alive_gc(capture):
+    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
+        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, 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
+        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.
@@ -70,18 +135,19 @@
 
 
 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:
@@ -89,11 +155,11 @@
     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."