Introduce Proto C API; based on cl/198113115 by amauryfa

BUILD

@@ -689,6 +689,7 @@ cc_binary(
     linkstatic = 1,
     deps = [
         ":protobuf",
+        ":proto_api",
     ] + select({
         "//conditions:default": [],
         ":use_fast_cpp_protos": ["//external:python_headers"],
@@ -831,6 +832,15 @@ internal_protobuf_py_tests(
     deps = [":python_tests"],
 )
 
+cc_library(
+    name = "proto_api",
+    hdrs = ["python/google/protobuf/proto_api.h"],
+    deps = [
+        ":protobuf_python",
+        "//external:python_headers",
+    ],
+)
+
 proto_lang_toolchain(
     name = "cc_toolchain",
     command_line = "--cpp_out=$(OUT)",

Makefile.am

@@ -804,6 +804,7 @@ python_EXTRA_DIST=                                                           \
   python/google/protobuf/message.py                                          \
   python/google/protobuf/message_factory.py                                  \
   python/google/protobuf/python_protobuf.h                                   \
+  python/google/protobuf/proto_api.h                                         \
   python/google/protobuf/proto_builder.py                                    \
   python/google/protobuf/pyext/README                                        \
   python/google/protobuf/pyext/__init__.py                                   \

python/google/protobuf/proto_api.h

@@ -0,0 +1,92 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// This file can be included by other C++ libraries, typically extension modules
+// which want to interact with the Python Messages coming from the "cpp"
+// implementation of protocol buffers.
+//
+// Usage:
+// Declare a (probably static) variable to hold the API:
+//    const PyProto_API* py_proto_api;
+// In some initialization function, write:
+//    py_proto_api = static_cast<const PyProto_API*>(PyCapsule_Import(
+//        PyProtoAPICapsuleName(), 0));
+//    if (!py_proto_api) { ...handle ImportError... }
+// Then use the methods of the returned class:
+//    py_proto_api->GetMessagePointer(...);
+
+#ifndef PYTHON_GOOGLE_PROTOBUF_PROTO_API_H__
+#define PYTHON_GOOGLE_PROTOBUF_PROTO_API_H__
+
+#include <Python.h>
+
+namespace google {
+namespace protobuf {
+
+class Message;
+
+namespace python {
+
+// Note on the implementation:
+// This API is designed after
+// https://docs.python.org/3/extending/extending.html#providing-a-c-api-for-an-extension-module
+// The class below contains no mutable state, and all methods are "const";
+// we use a C++ class instead of a C struct with functions pointers just because
+// the code looks more readable.
+struct PyProto_API {
+  // The API object is created at initialization time and never freed.
+  // This destructor is never called.
+  virtual ~PyProto_API() {}
+
+  // Operations on Messages.
+
+  // If the passed object is a Python Message, returns its internal pointer.
+  // Otherwise, returns NULL with an exception set.
+  virtual const Message* GetMessagePointer(PyObject* msg) const = 0;
+
+  // If the passed object is a Python Message, returns a mutable pointer.
+  // Otherwise, returns NULL with an exception set.
+  // This function will succeed only if there are no other Python objects
+  // pointing to the message, like submessages or repeated containers.
+  // With the current implementation, only empty messages are in this case.
+  virtual Message* GetMutableMessagePointer(PyObject* msg) const = 0;
+};
+
+inline const char* PyProtoAPICapsuleName() {
+  static const char kCapsuleName[] =
+      "protobuf.python.google.protobuf.cpp._message.proto_API";
+  return kCapsuleName;
+}
+
+}  // namespace python
+}  // namespace protobuf
+}  // namespace google
+
+#endif  // PYTHON_GOOGLE_PROTOBUF_PROTO_API_H__

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);

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

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()) {