Enforce import order to make protobuf cpp implementation in python work (#18560)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18560

We have to import python protobuf here **before** we load cpp extension.
Otherwise it breaks under certain build conditions if cpp implementation of
protobuf is used. Presumably there's some registry in protobuf library and
python side has to initialize the dictionary first, before static
initialization in python extension does so. Otherwise, duplicated protobuf
descriptors will be created and it can lead to obscure errors like

  Parameter to MergeFrom() must be instance of same class: expected caffe2.NetDef got caffe2.NetDef.

I think it also fixes https://github.com/facebookarchive/caffe2/issues/1573

Reviewed By: ezyang, iroot900

Differential Revision: D14622054

fbshipit-source-id: 2499eb88ecdee85ff8d845859048f7ae5da2a480

caffe2/contrib/tensorboard/tensorboard_test.py

@@ -91,9 +91,11 @@ class TensorboardTest(unittest.TestCase):
         for i, (event, net) in enumerate(zip(events, nets), start=1):
             self.assertEqual(event.step, i)
             self.assertEqual(event.wall_time, i)
-            self.assertEqual(
-                event.graph_def,
-                tb_exporter.nets_to_graph_def([net]).SerializeToString())
+            g = tf.GraphDef()
+            g.ParseFromString(event.graph_def)
+            self.assertMultiLineEqual(
+                str(g),
+                str(tb_exporter.nets_to_graph_def([net])))
 
 
 if __name__ == "__main__":

caffe2/proto/__init__.py

@@ -0,0 +1,15 @@
+# NOTE: we have to import python protobuf here **before** we load cpp extension.
+# Otherwise it breaks under certain build conditions if cpp implementation of
+# protobuf is used. Presumably there's some registry in protobuf library and
+# python side has to initialize the dictionary first, before static
+# initialization in python extension does so. Otherwise, duplicated protobuf
+# descriptors will be created and it can lead to obscure errors like
+#   "Parameter to MergeFrom() must be instance of same class:
+#    expected caffe2.NetDef got caffe2.NetDef."
+#
+# This has to be done for all python targets, so listing them here
+from caffe2.proto import caffe2_pb2, metanet_pb2, torch_pb2
+try:
+    from caffe2.caffe2.fb.session.proto import session_pb2
+except ImportError:
+    pass

caffe2/python/_import_c_extension.py

@@ -5,6 +5,16 @@ import logging
 import sys
 from caffe2.python import extension_loader
 
+# NOTE: we have to import python protobuf here **before** we load cpp extension.
+# Otherwise it breaks under certain build conditions if cpp implementation of
+# protobuf is used. Presumably there's some registry in protobuf library and
+# python side has to initialize the dictionary first, before static
+# initialization in python extension does so. Otherwise, duplicated protobuf
+# descriptors will be created and it can lead to obscure errors like
+#   "Parameter to MergeFrom() must be instance of same class:
+#    expected caffe2.NetDef got caffe2.NetDef."
+import caffe2.proto
+
 # We will first try to load the gpu-enabled caffe2. If it fails, we will then
 # attempt to load the cpu version. The cpu backend is the minimum required, so
 # if that still fails, we will exit loud.

caffe2/python/test/python_protobuf_test.py

@@ -0,0 +1,24 @@
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+# make sure we use cpp implementation of protobuf
+import os
+os.environ["PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION"] = "cpp"
+
+# import cpp extension first
+from caffe2.python import core
+# then import protobuf
+from caffe2.proto import caffe2_pb2, metanet_pb2
+
+import unittest
+
+
+class TestCrossProtoCalls(unittest.TestCase):
+    def testSimple(self):
+        net = caffe2_pb2.NetDef()
+        meta = metanet_pb2.MetaNetDef()
+        # if metanet_pb2 wasn't initialized properly the following fails with a
+        # cryptic message: "Parameter to MergeFrom() must be instance of same
+        # class: expected caffe2.NetDef got caffe2.NetDef."
+        meta.nets.add(key="foo", value=net)