Fix protobuf's Python runtime and imports.

third_party/protobuf/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//third_party/protobuf/proto_library.gni")
+
 config("protobuf_config") {
   include_dirs = [ "src" ]
   defines = [
     "GOOGLE_PROTOBUF_NO_RTTI",
     "GOOGLE_PROTOBUF_NO_STATIC_INITIALIZER",
   ]
   if (!is_win) {
     defines += [ "HAVE_PTHREAD" ]
   }
 }
@@ skipping 577 matching lines @@
 
     deps = [
       ":protoc_lib",
 
       # Default manifest on Windows (a no-op elsewhere).
       "//build/win:default_exe_manifest",
     ]
   }
 }
 
-google_python_dir = "$root_out_dir/pyproto/google"
-
-copy("copy_google") {
-  sources = [
-    "__init__.py",
-  ]
-  outputs = [
-    "$google_python_dir/{{source_file_part}}",
-  ]
-}
+pyproto_dir = "$root_out_dir/pyproto"
 
 copy("copy_six") {
   sources = [
     "third_party/six/six.py",
   ]
   outputs = [
-    "$google_python_dir/third_party/six/{{source_file_part}}",
+    "$pyproto_dir/{{source_file_part}}",
   ]
 }
 
 copy("copy_google_protobuf") {
   sources = [
-    "python/google/protobuf/__init__.py",
     "python/google/protobuf/descriptor.py",
     "python/google/protobuf/descriptor_database.py",
     "python/google/protobuf/descriptor_pool.py",
     "python/google/protobuf/json_format.py",
     "python/google/protobuf/message.py",
     "python/google/protobuf/message_factory.py",
     "python/google/protobuf/proto_builder.py",
     "python/google/protobuf/reflection.py",
     "python/google/protobuf/service.py",
     "python/google/protobuf/service_reflection.py",
     "python/google/protobuf/symbol_database.py",
     "python/google/protobuf/text_encoding.py",
     "python/google/protobuf/text_format.py",
-
-    # TODO(ncarter): protoc's python generator treats descriptor.proto
-    # specially, but only when the input path is exactly
-    # "google/protobuf/descriptor.proto".  I'm not sure how to execute a rule
-    # from a different directory.  For now, use a manually-generated copy of
-    # descriptor_pb2.py.
-    "python/google/protobuf/descriptor_pb2.py",
   ]
   outputs = [
-    "$google_python_dir/protobuf/{{source_file_part}}",
+    "$pyproto_dir/google/protobuf/{{source_file_part}}",
   ]
 }
 
 copy("copy_google_protobuf_internal") {
   sources = [
     "python/google/protobuf/internal/__init__.py",
     "python/google/protobuf/internal/_parameterized.py",
     "python/google/protobuf/internal/api_implementation.py",
     "python/google/protobuf/internal/containers.py",
     "python/google/protobuf/internal/decoder.py",
     "python/google/protobuf/internal/encoder.py",
     "python/google/protobuf/internal/enum_type_wrapper.py",
     "python/google/protobuf/internal/message_listener.py",
     "python/google/protobuf/internal/python_message.py",
     "python/google/protobuf/internal/type_checkers.py",
     "python/google/protobuf/internal/well_known_types.py",
     "python/google/protobuf/internal/wire_format.py",
   ]
   outputs = [
-    "$google_python_dir/protobuf/internal/{{source_file_part}}",
+    "$pyproto_dir/google/protobuf/internal/{{source_file_part}}",
   ]
 }
 
+proto_library("generate_google_protobuf_descriptor") {
+  sources = [
+    "src/google/protobuf/compiler/plugin.proto",
+    "src/google/protobuf/descriptor.proto",
+  ]
+  proto_in_dir = "src"
+  proto_out_dir = ""
+
+  generate_python = true
+  generate_cc = false
+}
+
 group("py_proto") {
   public_deps = [
-    ":copy_google",
     ":copy_google_protobuf",
     ":copy_google_protobuf_internal",
     ":copy_six",
+    ":generate_google_protobuf_descriptor",
   ]
 
   # Targets that depend on this should depend on the copied data files.
-  data = get_target_outputs(":copy_google")
-  data += get_target_outputs(":copy_six")
-  data += get_target_outputs(":copy_google_protobuf")
-  data += get_target_outputs(":copy_google_protobuf_internal")
+  data_deps = [
+    ":copy_google_protobuf",
+    ":copy_google_protobuf_internal",
+    ":copy_six",
+    ":generate_google_protobuf_descriptor",
+  ]
+
+  # Init files implicitly created by protoc_wrapper.
+  data = [
+    "$pyproto_dir/google/__init__.py",
+    "$pyproto_dir/google/protobuf/__init__.py",
+    "$pyproto_dir/google/protobuf/compiler/__init__.py",
+  ]
 }