From f45bb585c32b761ded4e8ecd32897a5d205fcf72 Mon Sep 17 00:00:00 2001
From: Glen Walker <glen@walker.gen.nz>
Date: Tue, 16 Aug 2016 17:50:43 +1200
Subject: [PATCH] Support keep_alive where nurse may be None

For example keep_alive<0,1>() should work where the return value may sometimes be None. At present a "Could not allocate weak reference!" exception is thrown.
Update documentation to clarify behaviour of keep_alive when nurse is None or does not support weak references.
---
 docs/advanced.rst              | 11 +++++++----
 example/example-keep-alive.cpp |  5 ++++-
 example/example-keep-alive.py  | 21 +++++++++++++++++++++
 example/example-keep-alive.ref |  8 ++++++++
 include/pybind11/pybind11.h    |  4 ++--
 5 files changed, 42 insertions(+), 7 deletions(-)

diff --git a/docs/advanced.rst b/docs/advanced.rst
index 0a84a4554..a050dfbe6 100644
--- a/docs/advanced.rst
+++ b/docs/advanced.rst
@@ -663,10 +663,13 @@ In addition to the above return value policies, further `call policies` can be
 specified to indicate dependencies between parameters. There is currently just
 one policy named ``keep_alive<Nurse, Patient>``, which indicates that the
 argument with index ``Patient`` should be kept alive at least until the
-argument with index ``Nurse`` is freed by the garbage collector; argument
-indices start at one, while zero refers to the return value. For methods, index
-one refers to the implicit ``this`` pointer, while regular arguments begin at
-index two. Arbitrarily many call policies can be specified.
+argument with index ``Nurse`` is freed by the garbage collector, as long as the
+nurse object supports weak references (pybind11 extension classes all support
+weak references). If the nurse object does not support weak references and is
+not None an appropriate exception will be thrown. Argument indices start at
+one, while zero refers to the return value. For methods, index one refers to
+the implicit ``this`` pointer, while regular arguments begin at index two.
+Arbitrarily many call policies can be specified.
 
 Consider the following example: the binding code for a list append operation
 that ties the lifetime of the newly added element to the underlying container
diff --git a/example/example-keep-alive.cpp b/example/example-keep-alive.cpp
index c099aa81f..c2aeaeefa 100644
--- a/example/example-keep-alive.cpp
+++ b/example/example-keep-alive.cpp
@@ -22,6 +22,7 @@ public:
     ~Parent() { std::cout << "Releasing parent." << std::endl; }
     void addChild(Child *) { }
     Child *returnChild() { return new Child(); }
+    Child *returnNullChild() { return nullptr; }
 };
 
 void init_ex_keep_alive(py::module &m) {
@@ -30,7 +31,9 @@ void init_ex_keep_alive(py::module &m) {
         .def("addChild", &Parent::addChild)
         .def("addChildKeepAlive", &Parent::addChild, py::keep_alive<1, 2>())
         .def("returnChild", &Parent::returnChild)
-        .def("returnChildKeepAlive", &Parent::returnChild, py::keep_alive<1, 0>());
+        .def("returnChildKeepAlive", &Parent::returnChild, py::keep_alive<1, 0>())
+        .def("returnNullChildKeepAliveChild", &Parent::returnNullChild, py::keep_alive<1, 0>())
+        .def("returnNullChildKeepAliveParent", &Parent::returnNullChild, py::keep_alive<0, 1>());
 
     py::class_<Child>(m, "Child")
         .def(py::init<>());
diff --git a/example/example-keep-alive.py b/example/example-keep-alive.py
index ad0176ed4..187ad534f 100644
--- a/example/example-keep-alive.py
+++ b/example/example-keep-alive.py
@@ -31,6 +31,7 @@ if True:
     gc.collect()
     print(p)
     p = None
+
 gc.collect()
 print("")
 
@@ -41,6 +42,26 @@ if True:
     print(p)
     p = None
 
+gc.collect()
+print("")
+
+if True:
+    p = Parent()
+    p.returnNullChildKeepAliveChild()
+    gc.collect()
+    print(p)
+    p = None
+
+gc.collect()
+print("")
+
+if True:
+    p = Parent()
+    p.returnNullChildKeepAliveParent()
+    gc.collect()
+    print(p)
+    p = None
+
 gc.collect()
 print("")
 print("Terminating..")
diff --git a/example/example-keep-alive.ref b/example/example-keep-alive.ref
index 7eb02c51f..775aff418 100644
--- a/example/example-keep-alive.ref
+++ b/example/example-keep-alive.ref
@@ -22,4 +22,12 @@ Allocating child.
 Releasing parent.
 Releasing child.
 
+Allocating parent.
+<example.Parent object at 0x10eb726c0>
+Releasing parent.
+
+Allocating parent.
+<example.Parent object at 0x10eb726c0>
+Releasing parent.
+
 Terminating..
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 427b12c3e..d82ae926e 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1098,8 +1098,8 @@ inline void keep_alive_impl(handle nurse, handle patient) {
     if (!nurse || !patient)
         pybind11_fail("Could not activate keep_alive!");
 
-    if (patient.ptr() == Py_None)
-        return; /* Nothing to keep alive */
+    if (patient.ptr() == Py_None || nurse.ptr() == Py_None)
+        return; /* Nothing to keep alive or nothing to be kept alive by */
 
     cpp_function disable_lifesupport(
         [patient](handle weakref) { patient.dec_ref(); weakref.dec_ref(); });