Fix protobuf's Python runtime and imports.

third_party/protobuf/BUILD.gn

Index: third_party/protobuf/BUILD.gn
diff --git a/third_party/protobuf/BUILD.gn b/third_party/protobuf/BUILD.gn
index 2cb1de57adbd35f6f91538158849718abdd79f23..2ba14ec2a79a94f765fd6fc51406684fb5775fea 100644
--- a/third_party/protobuf/BUILD.gn
+++ b/third_party/protobuf/BUILD.gn
@@ -2,6 +2,8 @@
 # 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 = [
@@ -599,29 +601,19 @@ if (current_toolchain == host_toolchain) {
   }
 }
 
-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",
@@ -635,16 +627,9 @@ copy("copy_google_protobuf") {
     "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}}",
   ]
 }
 
@@ -664,21 +649,42 @@ copy("copy_google_protobuf_internal") {
     "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",
+  ]
 }