Add additional test runner for single-threaded unary-stream code path

src/python/grpcio/grpc/_channel.py

@@ -16,6 +16,7 @@
 import copy
 import functools
 import logging
+import os
 import sys
 import threading
 import time
@@ -33,6 +34,11 @@ _USER_AGENT = 'grpc-python/{}'.format(_grpcio_metadata.__version__)
 
 _EMPTY_FLAGS = 0
 
+# NOTE(rbellevi): No guarantees are given about the maintenance of this
+# environment variable.
+_DEFAULT_SINGLE_THREADED_UNARY_STREAM = os.getenv(
+    "GRPC_SINGLE_THREADED_UNARY_STREAM") is not None
+
 _UNARY_UNARY_INITIAL_DUE = (
     cygrpc.OperationType.send_initial_metadata,
     cygrpc.OperationType.send_message,
@@ -1334,7 +1340,7 @@ class Channel(grpc.Channel):
             used over the lifetime of the channel.
         """
         python_options, core_options = _separate_channel_options(options)
-        self._single_threaded_unary_stream = False
+        self._single_threaded_unary_stream = _DEFAULT_SINGLE_THREADED_UNARY_STREAM
         self._process_python_options(python_options)
         self._channel = cygrpc.Channel(
             _common.encode(target), _augment_options(core_options, compression),

src/python/grpcio_tests/tests/unit/BUILD.bazel

@@ -84,29 +84,39 @@ py_library(
 )
 
 [
-    py2and3_test(
-        name=test_file_name[:-3],
-        size="small",
-        srcs=[test_file_name],
-        main=test_file_name,
-        deps=[
-            "//src/python/grpcio/grpc:grpcio",
-            ":resources",
-            ":test_common",
-            ":thread_pool",
-            ":_exit_scenarios",
-            ":_server_shutdown_scenarios",
-            ":_from_grpc_import_star",
-            ":_tcp_proxy",
-            ":_signal_client",
-            "//src/python/grpcio_tests/tests/unit/framework/common",
-            "//src/python/grpcio_tests/tests/testing",
-            "//external:six"
-        ],
-        imports=["../../",],
-        data=[
-            "//src/python/grpcio_tests/tests/unit/credentials",
-        ],
-    ) for test_file_name in GRPCIO_TESTS_UNIT
+    [
+        py2and3_test(
+            name=test_file_name[:-3],
+            size="small",
+            srcs=[test_file_name],
+            main=test_file_name,
+            deps=[
+                "//src/python/grpcio/grpc:grpcio",
+                ":resources",
+                ":test_common",
+                ":thread_pool",
+                ":_exit_scenarios",
+                ":_server_shutdown_scenarios",
+                ":_from_grpc_import_star",
+                ":_tcp_proxy",
+                ":_signal_client",
+                "//src/python/grpcio_tests/tests/unit/framework/common",
+                "//src/python/grpcio_tests/tests/testing",
+                "//external:six"
+            ],
+            imports=["../../",],
+            data=[
+                "//src/python/grpcio_tests/tests/unit/credentials",
+            ],
+        ),
+        sh_test(
+            name = test_file_name[:-3] + ".single_threaded_unary_stream",
+            data = ["{}.python3".format(test_file_name[:-3])],
+            args = ["{}.python3".format(test_file_name[:-3])],
+            size = "small",
+            srcs = ["_single_threaded_unary_stream_test_runner.sh"]
+        )
+    ]
+    for test_file_name in GRPCIO_TESTS_UNIT
 ]
 

src/python/grpcio_tests/tests/unit/_single_threaded_unary_stream_test_runner.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+export GRPC_SINGLE_THREADED_UNARY_STREAM=true
+
+find . -name "$1" -exec {} \;