[reflection] python: reflection response returns dependencies (#32909)

Fix #32899 - include all dependency-descriptors in the
ServerReflectionResponse

Using the C# Server-code as blue print:

6bf44820a2/src/Grpc.Reflection/ReflectionServiceImpl.cs (L148)

see also:
   - The cpp-server implementation of this:

d299f5ecce/src/cpp/ext/proto_server_reflection.cc (L208)
  - The go-server implementation of this:

2cd95c7514/reflection/serverreflection.go (L184)
`return s.fileDescWithDependencies(d.ParentFile(), sentFileDescriptors)`

---------

Co-authored-by: Xuan Wang <xuanwn@google.com>

src/python/grpcio_reflection/grpc_reflection/v1alpha/_base.py

@@ -30,13 +30,29 @@ def _not_found_error():
         ))
 
 
+def _collect_transitive_dependencies(descriptor, seen_files):
+    seen_files.update({descriptor.name: descriptor})
+    for dependency in descriptor.dependencies:
+        if not dependency.name in seen_files:
+            # descriptors cannot have circular dependencies
+            _collect_transitive_dependencies(dependency, seen_files)
+
+
 def _file_descriptor_response(descriptor):
-    proto = descriptor_pb2.FileDescriptorProto()
-    descriptor.CopyToProto(proto)
-    serialized_proto = proto.SerializeToString()
+    # collect all dependencies
+    descriptors = {}
+    _collect_transitive_dependencies(descriptor, descriptors)
+
+    # serialize all descriptors
+    serialized_proto_list = []
+    for d_key in descriptors:
+        proto = descriptor_pb2.FileDescriptorProto()
+        descriptors[d_key].CopyToProto(proto)
+        serialized_proto_list.append(proto.SerializeToString())
+
     return _reflection_pb2.ServerReflectionResponse(
         file_descriptor_response=_reflection_pb2.FileDescriptorResponse(
-            file_descriptor_proto=(serialized_proto,)),)
+            file_descriptor_proto=(serialized_proto_list)),)
 
 
 class BaseReflectionServicer(_reflection_pb2_grpc.ServerReflectionServicer):

src/python/grpcio_tests/tests/reflection/_reflection_servicer_test.py

@@ -25,6 +25,7 @@ from grpc_reflection.v1alpha import reflection_pb2_grpc
 
 from src.proto.grpc.testing import empty_pb2
 from src.proto.grpc.testing.proto2 import empty2_extensions_pb2
+from src.proto.grpc.testing.proto2 import empty2_pb2
 from tests.unit import test_common
 
 _EMPTY_PROTO_FILE_NAME = 'src/proto/grpc/testing/empty.proto'
@@ -128,9 +129,11 @@ class ReflectionServicerTest(unittest.TestCase):
         expected_responses = (
             reflection_pb2.ServerReflectionResponse(
                 valid_host='',
-                file_descriptor_response=reflection_pb2.FileDescriptorResponse(
-                    file_descriptor_proto=(_file_descriptor_to_proto(
-                        empty2_extensions_pb2.DESCRIPTOR),))),
+                file_descriptor_response=reflection_pb2.
+                FileDescriptorResponse(file_descriptor_proto=(
+                    _file_descriptor_to_proto(empty2_extensions_pb2.DESCRIPTOR),
+                    _file_descriptor_to_proto(empty2_pb2.DESCRIPTOR),
+                ))),
             reflection_pb2.ServerReflectionResponse(
                 valid_host='',
                 error_response=reflection_pb2.ErrorResponse(

src/python/grpcio_tests/tests_aio/reflection/reflection_servicer_test.py

@@ -25,6 +25,7 @@ from grpc_reflection.v1alpha import reflection_pb2_grpc
 
 from src.proto.grpc.testing import empty_pb2
 from src.proto.grpc.testing.proto2 import empty2_extensions_pb2
+from src.proto.grpc.testing.proto2 import empty2_pb2
 from tests_aio.unit._test_base import AioTestBase
 
 _EMPTY_PROTO_FILE_NAME = 'src/proto/grpc/testing/empty.proto'
@@ -132,9 +133,11 @@ class ReflectionServicerTest(AioTestBase):
         expected_responses = (
             reflection_pb2.ServerReflectionResponse(
                 valid_host='',
-                file_descriptor_response=reflection_pb2.FileDescriptorResponse(
-                    file_descriptor_proto=(_file_descriptor_to_proto(
-                        empty2_extensions_pb2.DESCRIPTOR),))),
+                file_descriptor_response=reflection_pb2.
+                FileDescriptorResponse(file_descriptor_proto=(
+                    _file_descriptor_to_proto(empty2_extensions_pb2.DESCRIPTOR),
+                    _file_descriptor_to_proto(empty2_pb2.DESCRIPTOR),
+                ))),
             reflection_pb2.ServerReflectionResponse(
                 valid_host='',
                 error_response=reflection_pb2.ErrorResponse(