Hide init_grpc_aio and guard async API outside of AsyncIO context

doc/python/sphinx/grpc_asyncio.rst

@@ -24,27 +24,20 @@ gRPC Async API objects may only be used on the thread on which they were
 created. AsyncIO doesn't provide thread safety for most of its APIs.
 
 
-Module Contents
----------------
-
 Enable AsyncIO in gRPC
-^^^^^^^^^^^^^^^^^^^^^^
+----------------------
 
-.. function:: init_grpc_aio
+Enable AsyncIO in gRPC Python is automatic when instantiating gRPC AsyncIO
+objects (e.g., channels and servers). No additional function invocation is
+required.
 
-    Enable AsyncIO for gRPC Python.
+Making blocking function calls in coroutines or in the thread running event
+loop will block the event loop, potentially starving all RPCs in the process.
+Refer to the Python language documentation on AsyncIO for more details (`running-blocking-code <https://docs.python.org/3/library/asyncio-dev.html#running-blocking-code>`_).
 
-    This function is idempotent and it should be invoked before creation of
-    AsyncIO stack objects. Otherwise, the application might deadlock.
 
-    This function configurates the gRPC C-Core to invoke AsyncIO methods for IO
-    operations (e.g., socket read, write). The configuration applies to the
-    entire process.
-
-    After invoking this function, making blocking function calls in coroutines
-    or in the thread running event loop will block the event loop, potentially
-    starving all RPCs in the process. Refer to the Python language
-    documentation on AsyncIO for more details (`running-blocking-code <https://docs.python.org/3/library/asyncio-dev.html#running-blocking-code>`_).
+Module Contents
+---------------
 
 
 Create Channel

src/python/grpcio/grpc/_cython/_cygrpc/aio/completion_queue.pyx.pxi

@@ -41,7 +41,11 @@ cdef class BaseCompletionQueue:
 cdef class PollerCompletionQueue(BaseCompletionQueue):
 
     def __cinit__(self):
-        self._loop = asyncio.get_event_loop()
+        # NOTE(lidiz) Due to a defect of asyncio.get_event_loop, its returned
+        # event loop might not be set as the default event loop for the main
+        # thread. So, asyncio.get_running_loop() is needed to ensure the poller
+        # is bound to a working event loop.
+        self._loop = asyncio.get_running_loop()
         self._cq = grpc_completion_queue_create_for_next(NULL)
         self._shutdown = False
         self._poller_thread = threading.Thread(target=self._poll_wrapper, daemon=True)
@@ -119,7 +123,7 @@ cdef class PollerCompletionQueue(BaseCompletionQueue):
 cdef class CallbackCompletionQueue(BaseCompletionQueue):
 
     def __cinit__(self):
-        self._loop = asyncio.get_event_loop()
+        self._loop = asyncio.get_running_loop()
         self._shutdown_completed = self._loop.create_future()
         self._wrapper = CallbackWrapper(
             self._shutdown_completed,

src/python/grpcio/grpc/_cython/_cygrpc/aio/iomgr/iomgr.pyx.pxi

@@ -216,7 +216,7 @@ def _auth_plugin_callback_wrapper(object cb,
                                   str service_url,
                                   str method_name,
                                   object callback):
-    asyncio.get_event_loop().call_soon(cb, service_url, method_name, callback)
+    asyncio.get_running_loop().call_soon(cb, service_url, method_name, callback)
 
 
 def install_asyncio_iomgr():

src/python/grpcio/grpc/_cython/_cygrpc/aio/iomgr/resolver.pyx.pxi

@@ -15,7 +15,7 @@
 
 cdef class _AsyncioResolver:
     def __cinit__(self):
-        self._loop = asyncio.get_event_loop()
+        self._loop = asyncio.get_running_loop()
         self._grpc_resolver = NULL
         self._task_resolve = None
 

src/python/grpcio/grpc/_cython/_cygrpc/aio/iomgr/socket.pyx.pxi

@@ -37,7 +37,7 @@ cdef class _AsyncioSocket:
         self._py_socket = None
         self._peername = None
         self._closed = False
-        self._loop = asyncio.get_event_loop()
+        self._loop = asyncio.get_running_loop()
 
     @staticmethod
     cdef _AsyncioSocket create(grpc_custom_socket * grpc_socket,

src/python/grpcio/grpc/_cython/_cygrpc/aio/iomgr/timer.pyx.pxi

@@ -18,7 +18,7 @@ cdef class _AsyncioTimer:
         self._grpc_timer = NULL
         self._timer_future = None
         self._active = False
-        self._loop = asyncio.get_event_loop()
+        self._loop = asyncio.get_running_loop()
         cpython.Py_INCREF(self)
 
     @staticmethod

src/python/grpcio/grpc/experimental/aio/_channel.py

@@ -269,7 +269,7 @@ class Channel(_base_channel.Channel):
                         "{} or ".format(StreamUnaryClientInterceptor.__name__) +
                         "{}. ".format(StreamStreamClientInterceptor.__name__))
 
-        self._loop = asyncio.get_event_loop()
+        self._loop = asyncio.get_running_loop()
         self._channel = cygrpc.AioChannel(
             _common.encode(target),
             _augment_channel_arguments(options, compression), credentials,

src/python/grpcio/grpc/experimental/aio/_server.py

@@ -41,7 +41,7 @@ class Server(_base_server.Server):
                  options: ChannelArgumentType,
                  maximum_concurrent_rpcs: Optional[int],
                  compression: Optional[grpc.Compression]):
-        self._loop = asyncio.get_event_loop()
+        self._loop = asyncio.get_running_loop()
         if interceptors:
             invalid_interceptors = [
                 interceptor for interceptor in interceptors

src/python/grpcio_tests/tests_aio/unit/_test_base.py

@@ -64,6 +64,3 @@ class AioTestBase(unittest.TestCase):
                 return _async_to_sync_decorator(attr, self._TEST_LOOP)
         # For other attributes, let them pass.
         return attr
-
-
-aio.init_grpc_aio()

src/python/grpcio_tests/tests_aio/unit/outside_init_test.py

@@ -0,0 +1,46 @@
+# Copyright 2020 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.
+"""Tests behavior around the metadata mechanism."""
+
+import asyncio
+import logging
+import unittest
+from grpc.experimental import aio
+import grpc
+
+
+class TestOutsideInit(unittest.TestCase):
+
+    def test_behavior_outside_asyncio(self):
+        # Ensures non-AsyncIO object can be initiated
+        channel_creds = grpc.ssl_channel_credentials()
+
+        # Ensures AsyncIO API NOT working outside of AsyncIO
+        with self.assertRaises(RuntimeError):
+            aio.insecure_channel('')
+
+        with self.assertRaises(RuntimeError):
+            aio.secure_channel('', channel_creds)
+
+        with self.assertRaises(RuntimeError):
+            aio.server('', None)
+
+        # Ensures init_grpc_aio fail outside of AsyncIO
+        with self.assertRaises(RuntimeError):
+            aio.init_grpc_aio()
+
+
+if __name__ == "__main__":
+    logging.basicConfig(level=logging.DEBUG)
+    unittest.main(verbosity=2)