Address PR feedback. Also add a dummy plugin.

bazel/python_rules.bzl

@@ -72,6 +72,7 @@ _generate_pb2_src = rule(
         "plugin": attr.label(
             mandatory = False,
             executable = True,
+            providers = ["files_to_run"],
             cfg = "host",
         ),
         "_protoc": attr.label(
@@ -186,6 +187,7 @@ _generate_pb2_grpc_src = rule(
         "plugin": attr.label(
             mandatory = False,
             executable = True,
+            providers = ["files_to_run"],
             cfg = "host",
         ),
         "_grpc_plugin": attr.label(

bazel/test/python_test_repo/BUILD

@@ -79,9 +79,11 @@ proto_library(
     strip_import_prefix = ""
 )
 
+# Also test the custom plugin execution parameter
 py_proto_library(
     name = "helloworld_moved_py_pb2",
     deps = [":helloworld_moved_proto"],
+    plugin = ":dummy_plugin"
 )
 
 py_grpc_library(
@@ -100,4 +102,13 @@ py2and3_test(
         ":duration_py_pb2",
         ":timestamp_py_pb2",
     ],
+)
+
+py_binary(
+    name = "dummy_plugin",
+    srcs = [":dummy_plugin.py"],
+    deps = [
+        "@com_github_grpc_grpc//src/python/grpcio/grpc/_cython:cygrpc",
+        "@com_google_protobuf//:protobuf_python",
+    ],
 )

bazel/test/python_test_repo/dummy_plugin.py

@@ -0,0 +1,37 @@
+# Copyright 2019 the gRPC authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""A dummy plugin for testing"""
+
+import sys
+
+from google.protobuf.compiler.plugin_pb2 import CodeGeneratorRequest
+from google.protobuf.compiler.plugin_pb2 import CodeGeneratorResponse
+
+
+def main(input_file=sys.stdin, output_file=sys.stdout):
+    request = CodeGeneratorRequest.FromString(input_file.buffer.read())
+    answer = []
+    for fname in request.file_to_generate:
+        answer.append(CodeGeneratorResponse.File(
+            name=fname.replace('.proto', '_pb2.py'),
+            insertion_point='module_scope',
+            content="# Hello {}, I'm a dummy plugin!".format(fname),
+        ))
+
+    cgr = CodeGeneratorResponse(file=answer)
+    output_file.buffer.write(cgr.SerializeToString())
+
+
+if __name__ == '__main__':
+    main()