3 files changed, 54 insertions, 1 deletions
diff --git a/python/google/protobuf/pyext/message.cc b/python/google/protobuf/pyext/message.cc
index 5893533a..53736b9c 100644
--- a/python/google/protobuf/pyext/message.cc
+++ b/python/google/protobuf/pyext/message.cc
@@ -2866,17 +2866,38 @@ const Message* (*GetCProtoInsidePyProtoPtr)(PyObject* msg);
 Message* (*MutableCProtoInsidePyProtoPtr)(PyObject* msg);
 
 static const Message* GetCProtoInsidePyProtoImpl(PyObject* msg) {
+  const Message* message = PyMessage_GetMessagePointer(msg);
+  if (message == NULL) {
+    PyErr_Clear();
+    return NULL;
+  }
+  return message;
+}
+
+static Message* MutableCProtoInsidePyProtoImpl(PyObject* msg) {
+  Message* message = PyMessage_GetMutableMessagePointer(msg);
+  if (message == NULL) {
+    PyErr_Clear();
+    return NULL;
+  }
+  return message;
+}
+
+const Message* PyMessage_GetMessagePointer(PyObject* msg) {
   if (!PyObject_TypeCheck(msg, &CMessage_Type)) {
+    PyErr_SetString(PyExc_TypeError, "Not a Message instance");
     return NULL;
   }
   CMessage* cmsg = reinterpret_cast<CMessage*>(msg);
   return cmsg->message;
 }
 
-static Message* MutableCProtoInsidePyProtoImpl(PyObject* msg) {
+Message* PyMessage_GetMutableMessagePointer(PyObject* msg) {
   if (!PyObject_TypeCheck(msg, &CMessage_Type)) {
+    PyErr_SetString(PyExc_TypeError, "Not a Message instance");
     return NULL;
   }
+
   CMessage* cmsg = reinterpret_cast<CMessage*>(msg);
   if ((cmsg->composite_fields && PyDict_Size(cmsg->composite_fields) != 0) ||
       (cmsg->extensions != NULL &&
@@ -2885,6 +2906,9 @@ static Message* MutableCProtoInsidePyProtoImpl(PyObject* msg) {
     // the underlying C++ message back to the CMessage (e.g. removed repeated
     // composite containers). We only allow direct mutation of the underlying
     // C++ message if there is no child data in the CMessage.
+    PyErr_SetString(PyExc_ValueError,
+                    "Cannot reliably get a mutable pointer "
+                    "to a message with extra references");
     return NULL;
   }
   cmessage::AssureWritable(cmsg);
diff --git a/python/google/protobuf/pyext/message.h b/python/google/protobuf/pyext/message.h
index 72bcfa83..d754e62a 100644
--- a/python/google/protobuf/pyext/message.h
+++ b/python/google/protobuf/pyext/message.h
@@ -341,6 +341,9 @@ bool CheckFieldBelongsToMessage(const FieldDescriptor* field_descriptor,
 
 extern PyObject* PickleError_class;
 
+const Message* PyMessage_GetMessagePointer(PyObject* msg);
+Message* PyMessage_GetMutableMessagePointer(PyObject* msg);
+
 bool InitProto2MessageModule(PyObject *m);
 
 #if LANG_CXX11
diff --git a/python/google/protobuf/pyext/message_module.cc b/python/google/protobuf/pyext/message_module.cc
index 7c4df47f..8c933866 100644
--- a/python/google/protobuf/pyext/message_module.cc
+++ b/python/google/protobuf/pyext/message_module.cc
@@ -31,9 +31,26 @@
 #include <Python.h>
 
 #include <google/protobuf/pyext/message.h>
+#include <google/protobuf/proto_api.h>
 
 #include <google/protobuf/message_lite.h>
 
+namespace {
+
+// C++ API.  Clients get at this via proto_api.h
+struct ApiImplementation : google::protobuf::python::PyProto_API {
+  const google::protobuf::Message*
+      GetMessagePointer(PyObject* msg) const override {
+    return google::protobuf::python::PyMessage_GetMessagePointer(msg);
+  }
+  google::protobuf::Message*
+      GetMutableMessagePointer(PyObject* msg) const override {
+    return google::protobuf::python::PyMessage_GetMutableMessagePointer(msg);
+  }
+};
+
+}  // namespace
+
 static PyObject* GetPythonProto3PreserveUnknownsDefault(
     PyObject* /*m*/, PyObject* /*args*/) {
   if (google::protobuf::internal::GetProto3PreserveUnknownsDefault()) {
@@ -113,6 +130,15 @@ extern "C" {
       Py_DECREF(m);
       return INITFUNC_ERRORVAL;
     }
+    
+    // Adds the C++ API
+    if (PyObject* api =
+            PyCapsule_New(new ApiImplementation(),
+                          google::protobuf::python::PyProtoAPICapsuleName(), NULL)) {
+      PyModule_AddObject(m, "proto_API", api);
+    } else {
+      return INITFUNC_ERRORVAL;
+    }
 
 #if PY_MAJOR_VERSION >= 3
     return m;