Improve readability

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

@@ -23,7 +23,7 @@ cdef class PollerCompletionQueue(BaseCompletionQueue):
     cdef object _poller
     cdef object _poller_running
 
-    cdef _polling(self)
+    cdef void _poll(self) except *
 
 
 cdef class CallbackCompletionQueue(BaseCompletionQueue):

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

@@ -36,11 +36,11 @@ cdef class PollerCompletionQueue(BaseCompletionQueue):
         self._shutdown_completed = asyncio.get_event_loop().create_future()
         self._poller = None
         self._poller_running = asyncio.get_event_loop().create_future()
-        self._poller = threading.Thread(target=self._polling_wrapper)
+        self._poller = threading.Thread(target=self._poll_wrapper)
         self._poller.daemon = True
         self._poller.start()
 
-    cdef _polling(self):
+    cdef void _poll(self) except *:
         cdef grpc_event event
         cdef CallbackContext *context
         cdef object waiter
@@ -53,7 +53,7 @@ cdef class PollerCompletionQueue(BaseCompletionQueue):
                                                 NULL)
 
             if event.type == GRPC_QUEUE_TIMEOUT:
-                raise NotImplementedError()
+                raise AssertionError("Core should not return timeout error!")
             elif event.type == GRPC_QUEUE_SHUTDOWN:
                 self._shutdown = True
                 grpc_call_soon_threadsafe(self._shutdown_completed.set_result, None)
@@ -64,8 +64,8 @@ cdef class PollerCompletionQueue(BaseCompletionQueue):
                     <CallbackWrapper>context.callback_wrapper,
                     event.success)
 
-    def _polling_wrapper(self):
-        self._polling()
+    def _poll_wrapper(self):
+        self._poll()
 
     async def shutdown(self):
         grpc_completion_queue_shutdown(self._cq)
@@ -103,4 +103,4 @@ cdef BaseCompletionQueue create_completion_queue():
     elif grpc_aio_engine is AsyncIOEngine.POLLER:
         return PollerCompletionQueue()
     else:
-        raise ValueError('Unexpected engine type [%s]' % grpc_aio_engine)
+        raise ValueError('Unsupported engine type [%s]' % grpc_aio_engine)

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

@@ -15,13 +15,14 @@
 
 cdef bint _grpc_aio_initialized = False
 # NOTE(lidiz) Theoretically, applications can run in multiple event loops as
-# long as they are in the same thread with same magic. However, I don't think
-# we should support this use case. So, the gRPC Python Async Stack should use
-# a single event loop picked by "init_grpc_aio".
-cdef object _grpc_aio_loop
-cdef object _event_loop_thread_ident
+# long as they are in the same thread with same magic. This is not a supported
+# use case. So, the gRPC Python Async Stack should use a single event loop
+# picked by "init_grpc_aio".
+cdef object _grpc_aio_loop  # asyncio.AbstractEventLoop
+cdef int64_t _event_loop_thread_ident
 cdef str _GRPC_ASYNCIO_ENGINE = os.environ.get('GRPC_ASYNCIO_ENGINE', 'default').lower()
 grpc_aio_engine = None
+cdef object _grpc_initialization_lock = threading.Lock()
 
 
 class AsyncIOEngine(enum.Enum):
@@ -36,51 +37,50 @@ def init_grpc_aio():
     global _event_loop_thread_ident
     global grpc_aio_engine
 
-    # Marks this function as called
-    if _grpc_aio_initialized:
-        return
-    else:
-        _grpc_aio_initialized = True
-
-    # Picks the engine for gRPC AsyncIO Stack
-    for engine_type in AsyncIOEngine:
-        if engine_type.value == _GRPC_ASYNCIO_ENGINE:
-            grpc_aio_engine = engine_type
-            break
-    if grpc_aio_engine is None or grpc_aio_engine is AsyncIOEngine.DEFAULT:
-        grpc_aio_engine = AsyncIOEngine.CUSTOM_IO_MANAGER
-
-    # Anchors the event loop that the gRPC library going to use.
-    _grpc_aio_loop = asyncio.get_event_loop()
-    _event_loop_thread_ident = threading.current_thread().ident
-
-    if grpc_aio_engine is AsyncIOEngine.CUSTOM_IO_MANAGER:
-        # Activates asyncio IO manager.
-        # NOTE(lidiz) Custom IO manager must be activated before the first
-        # `grpc_init()`. Otherwise, some special configurations in Core won't
-        # pick up the change, and resulted in SEGFAULT or ABORT.
-        install_asyncio_iomgr()
-
-        # TODO(https://github.com/grpc/grpc/issues/22244) we need a the
-        # grpc_shutdown_blocking() counterpart for this call. Otherwise, the gRPC
-        # library won't shutdown cleanly.
-        grpc_init()
-
-        # Timers are triggered by the Asyncio loop. We disable
-        # the background thread that is being used by the native
-        # gRPC iomgr.
-        grpc_timer_manager_set_threading(False)
-
-        # gRPC callbaks are executed within the same thread used by the Asyncio
-        # event loop, as it is being done by the other Asyncio callbacks.
-        Executor.SetThreadingAll(False)
-    else:
-        # TODO(https://github.com/grpc/grpc/issues/22244) we need a the
-        # grpc_shutdown_blocking() counterpart for this call. Otherwise, the gRPC
-        # library won't shutdown cleanly.
-        grpc_init()
-
-    _grpc_aio_initialized = False
+    with _grpc_initialization_lock:
+        # Marks this function as called
+        if _grpc_aio_initialized:
+            return
+        else:
+            _grpc_aio_initialized = True
+
+        # Picks the engine for gRPC AsyncIO Stack
+        for engine_type in AsyncIOEngine:
+            if engine_type.value == _GRPC_ASYNCIO_ENGINE:
+                grpc_aio_engine = engine_type
+                break
+        if grpc_aio_engine is None or grpc_aio_engine is AsyncIOEngine.DEFAULT:
+            grpc_aio_engine = AsyncIOEngine.CUSTOM_IO_MANAGER
+
+        # Anchors the event loop that the gRPC library going to use.
+        _grpc_aio_loop = asyncio.get_event_loop()
+        _event_loop_thread_ident = threading.current_thread().ident
+
+        if grpc_aio_engine is AsyncIOEngine.CUSTOM_IO_MANAGER:
+            # Activates asyncio IO manager.
+            # NOTE(lidiz) Custom IO manager must be activated before the first
+            # `grpc_init()`. Otherwise, some special configurations in Core won't
+            # pick up the change, and resulted in SEGFAULT or ABORT.
+            install_asyncio_iomgr()
+
+            # TODO(https://github.com/grpc/grpc/issues/22244) we need a the
+            # grpc_shutdown_blocking() counterpart for this call. Otherwise, the gRPC
+            # library won't shutdown cleanly.
+            grpc_init()
+
+            # Timers are triggered by the Asyncio loop. We disable
+            # the background thread that is being used by the native
+            # gRPC iomgr.
+            grpc_timer_manager_set_threading(False)
+
+            # gRPC callbaks are executed within the same thread used by the Asyncio
+            # event loop, as it is being done by the other Asyncio callbacks.
+            Executor.SetThreadingAll(False)
+        else:
+            # TODO(https://github.com/grpc/grpc/issues/22244) we need a the
+            # grpc_shutdown_blocking() counterpart for this call. Otherwise, the gRPC
+            # library won't shutdown cleanly.
+            grpc_init()
 
 
 def grpc_aio_loop():