_server.py

# Copyright 2016 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.
"""Service-side implementation of gRPC Python."""

import collections
from concurrent import futures
import enum
import logging
import threading
import time

import grpc
from grpc import _common
from grpc import _compression
from grpc import _interceptor
from grpc._cython import cygrpc
import six

_LOGGER = logging.getLogger(__name__)

_SHUTDOWN_TAG = 'shutdown'
_REQUEST_CALL_TAG = 'request_call'

_RECEIVE_CLOSE_ON_SERVER_TOKEN = 'receive_close_on_server'
_SEND_INITIAL_METADATA_TOKEN = 'send_initial_metadata'
_RECEIVE_MESSAGE_TOKEN = 'receive_message'
_SEND_MESSAGE_TOKEN = 'send_message'
_SEND_INITIAL_METADATA_AND_SEND_MESSAGE_TOKEN = (
    'send_initial_metadata * send_message')
_SEND_STATUS_FROM_SERVER_TOKEN = 'send_status_from_server'
_SEND_INITIAL_METADATA_AND_SEND_STATUS_FROM_SERVER_TOKEN = (
    'send_initial_metadata * send_status_from_server')

_OPEN = 'open'
_CLOSED = 'closed'
_CANCELLED = 'cancelled'

_EMPTY_FLAGS = 0

_DEALLOCATED_SERVER_CHECK_PERIOD_S = 1.0
_INF_TIMEOUT = 1e9


def _serialized_request(request_event):
    return request_event.batch_operations[0].message()


def _application_code(code):
    cygrpc_code = _common.STATUS_CODE_TO_CYGRPC_STATUS_CODE.get(code)
    return cygrpc.StatusCode.unknown if cygrpc_code is None else cygrpc_code


def _completion_code(state):
    if state.code is None:
        return cygrpc.StatusCode.ok
    else:
        return _application_code(state.code)


def _abortion_code(state, code):
    if state.code is None:
        return code
    else:
        return _application_code(state.code)


def _details(state):
    return b'' if state.details is None else state.details


class _HandlerCallDetails(
        collections.namedtuple('_HandlerCallDetails', (
            'method',
            'invocation_metadata',
        )), grpc.HandlerCallDetails):
    pass


class _RPCState(object):

    def __init__(self):
        self.condition = threading.Condition()
        self.due = set()
        self.request = None
        self.client = _OPEN
        self.initial_metadata_allowed = True
        self.compression_algorithm = None
        self.disable_next_compression = False
        self.trailing_metadata = None
        self.code = None
        self.details = None
        self.statused = False
        self.rpc_errors = []
        self.callbacks = []
        self.aborted = False


def _raise_rpc_error(state):
    rpc_error = grpc.RpcError()
    state.rpc_errors.append(rpc_error)
    raise rpc_error


def _possibly_finish_call(state, token):
    state.due.remove(token)
    if not _is_rpc_state_active(state) and not state.due:
        callbacks = state.callbacks
        state.callbacks = None
        return state, callbacks
    else:
        return None, ()


def _send_status_from_server(state, token):

    def send_status_from_server(unused_send_status_from_server_event):
        with state.condition:
            return _possibly_finish_call(state, token)

    return send_status_from_server


def _get_initial_metadata(state, metadata):
    with state.condition:
        if state.compression_algorithm:
            compression_metadata = (
                _compression.compression_algorithm_to_metadata(
                    state.compression_algorithm),)
            if metadata is None:
                return compression_metadata
            else:
                return compression_metadata + tuple(metadata)
        else:
            return metadata


def _get_initial_metadata_operation(state, metadata):
    operation = cygrpc.SendInitialMetadataOperation(
        _get_initial_metadata(state, metadata), _EMPTY_FLAGS)
    return operation


def _abort(state, call, code, details):
    if state.client is not _CANCELLED:
        effective_code = _abortion_code(state, code)
        effective_details = details if state.details is None else state.details
        if state.initial_metadata_allowed:
            operations = (
                _get_initial_metadata_operation(state, None),
                cygrpc.SendStatusFromServerOperation(state.trailing_metadata,
                                                     effective_code,
                                                     effective_details,
                                                     _EMPTY_FLAGS),
            )
            token = _SEND_INITIAL_METADATA_AND_SEND_STATUS_FROM_SERVER_TOKEN
        else:
            operations = (cygrpc.SendStatusFromServerOperation(
                state.trailing_metadata, effective_code, effective_details,
                _EMPTY_FLAGS),)
            token = _SEND_STATUS_FROM_SERVER_TOKEN
        call.start_server_batch(operations,
                                _send_status_from_server(state, token))
        state.statused = True
        state.due.add(token)


def _receive_close_on_server(state):

    def receive_close_on_server(receive_close_on_server_event):
        with state.condition:
            if receive_close_on_server_event.batch_operations[0].cancelled():
                state.client = _CANCELLED
            elif state.client is _OPEN:
                state.client = _CLOSED
            state.condition.notify_all()
            return _possibly_finish_call(state, _RECEIVE_CLOSE_ON_SERVER_TOKEN)

    return receive_close_on_server


def _receive_message(state, call, request_deserializer):

    def receive_message(receive_message_event):
        serialized_request = _serialized_request(receive_message_event)
        if serialized_request is None:
            with state.condition:
                if state.client is _OPEN:
                    state.client = _CLOSED
                state.condition.notify_all()
                return _possibly_finish_call(state, _RECEIVE_MESSAGE_TOKEN)
        else:
            request = _common.deserialize(serialized_request,
                                          request_deserializer)
            with state.condition:
                if request is None:
                    _abort(state, call, cygrpc.StatusCode.internal,
                           b'Exception deserializing request!')
                else:
                    state.request = request
                state.condition.notify_all()
                return _possibly_finish_call(state, _RECEIVE_MESSAGE_TOKEN)

    return receive_message


def _send_initial_metadata(state):

    def send_initial_metadata(unused_send_initial_metadata_event):
        with state.condition:
            return _possibly_finish_call(state, _SEND_INITIAL_METADATA_TOKEN)

    return send_initial_metadata


def _send_message(state, token):

    def send_message(unused_send_message_event):
        with state.condition:
            state.condition.notify_all()
            return _possibly_finish_call(state, token)

    return send_message


class _Context(grpc.ServicerContext):

    def __init__(self, rpc_event, state, request_deserializer):
        self._rpc_event = rpc_event
        self._state = state
        self._request_deserializer = request_deserializer

    def is_active(self):
        with self._state.condition:
            return _is_rpc_state_active(self._state)

    def time_remaining(self):
        return max(self._rpc_event.call_details.deadline - time.time(), 0)

    def cancel(self):
        self._rpc_event.call.cancel()

    def add_callback(self, callback):
        with self._state.condition:
            if self._state.callbacks is None:
                return False
            else:
                self._state.callbacks.append(callback)
                return True

    def disable_next_message_compression(self):
        with self._state.condition:
            self._state.disable_next_compression = True

    def invocation_metadata(self):
        return self._rpc_event.invocation_metadata

    def peer(self):
        return _common.decode(self._rpc_event.call.peer())

    def peer_identities(self):
        return cygrpc.peer_identities(self._rpc_event.call)

    def peer_identity_key(self):
        id_key = cygrpc.peer_identity_key(self._rpc_event.call)
        return id_key if id_key is None else _common.decode(id_key)

    def auth_context(self):
        return {
            _common.decode(key): value for key, value in six.iteritems(
                cygrpc.auth_context(self._rpc_event.call))
        }

    def set_compression(self, compression):
        with self._state.condition:
            self._state.compression_algorithm = compression

    def send_initial_metadata(self, initial_metadata):
        with self._state.condition:
            if self._state.client is _CANCELLED:
                _raise_rpc_error(self._state)
            else:
                if self._state.initial_metadata_allowed:
                    operation = _get_initial_metadata_operation(
                        self._state, initial_metadata)
                    self._rpc_event.call.start_server_batch(
                        (operation,), _send_initial_metadata(self._state))
                    self._state.initial_metadata_allowed = False
                    self._state.due.add(_SEND_INITIAL_METADATA_TOKEN)
                else:
                    raise ValueError('Initial metadata no longer allowed!')

    def set_trailing_metadata(self, trailing_metadata):
        with self._state.condition:
            self._state.trailing_metadata = trailing_metadata

    def trailing_metadata(self):
        return self._state.trailing_metadata

    def abort(self, code, details):
        # treat OK like other invalid arguments: fail the RPC
        if code == grpc.StatusCode.OK:
            _LOGGER.error(
                'abort() called with StatusCode.OK; returning UNKNOWN')
            code = grpc.StatusCode.UNKNOWN
            details = ''
        with self._state.condition:
            self._state.code = code
            self._state.details = _common.encode(details)
            self._state.aborted = True
            raise Exception()

    def abort_with_status(self, status):
        self._state.trailing_metadata = status.trailing_metadata
        self.abort(status.code, status.details)

    def set_code(self, code):
        with self._state.condition:
            self._state.code = code

    def code(self):
        return self._state.code

    def set_details(self, details):
        with self._state.condition:
            self._state.details = _common.encode(details)

    def details(self):
        return self._state.details

    def _finalize_state(self):
        pass


class _RequestIterator(object):

    def __init__(self, state, call, request_deserializer):
        self._state = state
        self._call = call
        self._request_deserializer = request_deserializer

    def _raise_or_start_receive_message(self):
        if self._state.client is _CANCELLED:
            _raise_rpc_error(self._state)
        elif not _is_rpc_state_active(self._state):
            raise StopIteration()
        else:
            self._call.start_server_batch(
                (cygrpc.ReceiveMessageOperation(_EMPTY_FLAGS),),
                _receive_message(self._state, self._call,
                                 self._request_deserializer))
            self._state.due.add(_RECEIVE_MESSAGE_TOKEN)

    def _look_for_request(self):
        if self._state.client is _CANCELLED:
            _raise_rpc_error(self._state)
        elif (self._state.request is None and
              _RECEIVE_MESSAGE_TOKEN not in self._state.due):
            raise StopIteration()
        else:
            request = self._state.request
            self._state.request = None
            return request

        raise AssertionError()  # should never run

    def _next(self):
        with self._state.condition:
            self._raise_or_start_receive_message()
            while True:
                self._state.condition.wait()
                request = self._look_for_request()
                if request is not None:
                    return request

    def __iter__(self):
        return self

    def __next__(self):
        return self._next()

    def next(self):
        return self._next()


def _unary_request(rpc_event, state, request_deserializer):

    def unary_request():
        with state.condition:
            if not _is_rpc_state_active(state):
                return None
            else:
                rpc_event.call.start_server_batch(
                    (cygrpc.ReceiveMessageOperation(_EMPTY_FLAGS),),
                    _receive_message(state, rpc_event.call,
                                     request_deserializer))
                state.due.add(_RECEIVE_MESSAGE_TOKEN)
                while True:
                    state.condition.wait()
                    if state.request is None:
                        if state.client is _CLOSED:
                            details = '"{}" requires exactly one request message.'.format(
                                rpc_event.call_details.method)
                            _abort(state, rpc_event.call,
                                   cygrpc.StatusCode.unimplemented,
                                   _common.encode(details))
                            return None
                        elif state.client is _CANCELLED:
                            return None
                    else:
                        request = state.request
                        state.request = None
                        return request

    return unary_request


def _call_behavior(rpc_event,
                   state,
                   behavior,
                   argument,
                   request_deserializer,
                   send_response_callback=None):
    from grpc import _create_servicer_context
    with _create_servicer_context(rpc_event, state,
                                  request_deserializer) as context:
        try:
            response_or_iterator = None
            if send_response_callback is not None:
                response_or_iterator = behavior(argument, context,
                                                send_response_callback)
            else:
                response_or_iterator = behavior(argument, context)
            return response_or_iterator, True
        except Exception as exception:  # pylint: disable=broad-except
            with state.condition:
                if state.aborted:
                    _abort(state, rpc_event.call, cygrpc.StatusCode.unknown,
                           b'RPC Aborted')
                elif exception not in state.rpc_errors:
                    details = 'Exception calling application: {}'.format(
                        exception)
                    _LOGGER.exception(details)
                    _abort(state, rpc_event.call, cygrpc.StatusCode.unknown,
                           _common.encode(details))
            return None, False


def _take_response_from_response_iterator(rpc_event, state, response_iterator):
    try:
        return next(response_iterator), True
    except StopIteration:
        return None, True
    except Exception as exception:  # pylint: disable=broad-except
        with state.condition:
            if state.aborted:
                _abort(state, rpc_event.call, cygrpc.StatusCode.unknown,
                       b'RPC Aborted')
            elif exception not in state.rpc_errors:
                details = 'Exception iterating responses: {}'.format(exception)
                _LOGGER.exception(details)
                _abort(state, rpc_event.call, cygrpc.StatusCode.unknown,
                       _common.encode(details))
        return None, False


def _serialize_response(rpc_event, state, response, response_serializer):
    serialized_response = _common.serialize(response, response_serializer)
    if serialized_response is None:
        with state.condition:
            _abort(state, rpc_event.call, cygrpc.StatusCode.internal,
                   b'Failed to serialize response!')
        return None
    else:
        return serialized_response


def _get_send_message_op_flags_from_state(state):
    if state.disable_next_compression:
        return cygrpc.WriteFlag.no_compress
    else:
        return _EMPTY_FLAGS


def _reset_per_message_state(state):
    with state.condition:
        state.disable_next_compression = False


def _send_response(rpc_event, state, serialized_response):
    with state.condition:
        if not _is_rpc_state_active(state):
            return False
        else:
            if state.initial_metadata_allowed:
                operations = (
                    _get_initial_metadata_operation(state, None),
                    cygrpc.SendMessageOperation(
                        serialized_response,
                        _get_send_message_op_flags_from_state(state)),
                )
                state.initial_metadata_allowed = False
                token = _SEND_INITIAL_METADATA_AND_SEND_MESSAGE_TOKEN
            else:
                operations = (cygrpc.SendMessageOperation(
                    serialized_response,
                    _get_send_message_op_flags_from_state(state)),)
                token = _SEND_MESSAGE_TOKEN
            rpc_event.call.start_server_batch(operations,
                                              _send_message(state, token))
            state.due.add(token)
            _reset_per_message_state(state)
            while True:
                state.condition.wait()
                if token not in state.due:
                    return _is_rpc_state_active(state)


def _status(rpc_event, state, serialized_response):
    with state.condition:
        if state.client is not _CANCELLED:
            code = _completion_code(state)
            details = _details(state)
            operations = [
                cygrpc.SendStatusFromServerOperation(state.trailing_metadata,
                                                     code, details,
                                                     _EMPTY_FLAGS),
            ]
            if state.initial_metadata_allowed:
                operations.append(_get_initial_metadata_operation(state, None))
            if serialized_response is not None:
                operations.append(
                    cygrpc.SendMessageOperation(
                        serialized_response,
                        _get_send_message_op_flags_from_state(state)))
            rpc_event.call.start_server_batch(
                operations,
                _send_status_from_server(state, _SEND_STATUS_FROM_SERVER_TOKEN))
            state.statused = True
            _reset_per_message_state(state)
            state.due.add(_SEND_STATUS_FROM_SERVER_TOKEN)


def _unary_response_in_pool(rpc_event, state, behavior, argument_thunk,
                            request_deserializer, response_serializer):
    cygrpc.install_context_from_request_call_event(rpc_event)
    try:
        argument = argument_thunk()
        if argument is not None:
            response, proceed = _call_behavior(rpc_event, state, behavior,
                                               argument, request_deserializer)
            if proceed:
                serialized_response = _serialize_response(
                    rpc_event, state, response, response_serializer)
                if serialized_response is not None:
                    _status(rpc_event, state, serialized_response)
    finally:
        cygrpc.uninstall_context()


def _stream_response_in_pool(rpc_event, state, behavior, argument_thunk,
                             request_deserializer, response_serializer):
    cygrpc.install_context_from_request_call_event(rpc_event)

    def send_response(response):
        if response is None:
            _status(rpc_event, state, None)
        else:
            serialized_response = _serialize_response(rpc_event, state,
                                                      response,
                                                      response_serializer)
            if serialized_response is not None:
                _send_response(rpc_event, state, serialized_response)

    try:
        argument = argument_thunk()
        if argument is not None:
            if hasattr(behavior, 'experimental_non_blocking'
                      ) and behavior.experimental_non_blocking:
                _call_behavior(rpc_event,
                               state,
                               behavior,
                               argument,
                               request_deserializer,
                               send_response_callback=send_response)
            else:
                response_iterator, proceed = _call_behavior(
                    rpc_event, state, behavior, argument, request_deserializer)
                if proceed:
                    _send_message_callback_to_blocking_iterator_adapter(
                        rpc_event, state, send_response, response_iterator)
    finally:
        cygrpc.uninstall_context()


def _is_rpc_state_active(state):
    return state.client is not _CANCELLED and not state.statused


def _send_message_callback_to_blocking_iterator_adapter(rpc_event, state,
                                                        send_response_callback,
                                                        response_iterator):
    while True:
        response, proceed = _take_response_from_response_iterator(
            rpc_event, state, response_iterator)
        if proceed:
            send_response_callback(response)
            if not _is_rpc_state_active(state):
                break
        else:
            break


def _select_thread_pool_for_behavior(behavior, default_thread_pool):
    if hasattr(behavior, 'experimental_thread_pool') and isinstance(
            behavior.experimental_thread_pool, futures.ThreadPoolExecutor):
        return behavior.experimental_thread_pool
    else:
        return default_thread_pool


def _handle_unary_unary(rpc_event, state, method_handler, default_thread_pool):
    unary_request = _unary_request(rpc_event, state,
                                   method_handler.request_deserializer)
    thread_pool = _select_thread_pool_for_behavior(method_handler.unary_unary,
                                                   default_thread_pool)
    return thread_pool.submit(_unary_response_in_pool, rpc_event, state,
                              method_handler.unary_unary, unary_request,
                              method_handler.request_deserializer,
                              method_handler.response_serializer)


def _handle_unary_stream(rpc_event, state, method_handler, default_thread_pool):
    unary_request = _unary_request(rpc_event, state,
                                   method_handler.request_deserializer)
    thread_pool = _select_thread_pool_for_behavior(method_handler.unary_stream,
                                                   default_thread_pool)
    return thread_pool.submit(_stream_response_in_pool, rpc_event, state,
                              method_handler.unary_stream, unary_request,
                              method_handler.request_deserializer,
                              method_handler.response_serializer)


def _handle_stream_unary(rpc_event, state, method_handler, default_thread_pool):
    request_iterator = _RequestIterator(state, rpc_event.call,
                                        method_handler.request_deserializer)
    thread_pool = _select_thread_pool_for_behavior(method_handler.stream_unary,
                                                   default_thread_pool)
    return thread_pool.submit(_unary_response_in_pool, rpc_event, state,
                              method_handler.stream_unary,
                              lambda: request_iterator,
                              method_handler.request_deserializer,
                              method_handler.response_serializer)


def _handle_stream_stream(rpc_event, state, method_handler,
                          default_thread_pool):
    request_iterator = _RequestIterator(state, rpc_event.call,
                                        method_handler.request_deserializer)
    thread_pool = _select_thread_pool_for_behavior(method_handler.stream_stream,
                                                   default_thread_pool)
    return thread_pool.submit(_stream_response_in_pool, rpc_event, state,
                              method_handler.stream_stream,
                              lambda: request_iterator,
                              method_handler.request_deserializer,
                              method_handler.response_serializer)


def _find_method_handler(rpc_event, generic_handlers, interceptor_pipeline):

    def query_handlers(handler_call_details):
        for generic_handler in generic_handlers:
            method_handler = generic_handler.service(handler_call_details)
            if method_handler is not None:
                return method_handler
        return None

    handler_call_details = _HandlerCallDetails(
        _common.decode(rpc_event.call_details.method),
        rpc_event.invocation_metadata)

    if interceptor_pipeline is not None:
        return interceptor_pipeline.execute(query_handlers,
                                            handler_call_details)
    else:
        return query_handlers(handler_call_details)


def _reject_rpc(rpc_event, status, details):
    rpc_state = _RPCState()
    operations = (
        _get_initial_metadata_operation(rpc_state, None),
        cygrpc.ReceiveCloseOnServerOperation(_EMPTY_FLAGS),
        cygrpc.SendStatusFromServerOperation(None, status, details,
                                             _EMPTY_FLAGS),
    )
    rpc_event.call.start_server_batch(operations, lambda ignored_event: (
        rpc_state,
        (),
    ))
    return rpc_state


def _handle_with_method_handler(rpc_event, method_handler, thread_pool):
    state = _RPCState()
    with state.condition:
        rpc_event.call.start_server_batch(
            (cygrpc.ReceiveCloseOnServerOperation(_EMPTY_FLAGS),),
            _receive_close_on_server(state))
        state.due.add(_RECEIVE_CLOSE_ON_SERVER_TOKEN)
        if method_handler.request_streaming:
            if method_handler.response_streaming:
                return state, _handle_stream_stream(rpc_event, state,
                                                    method_handler, thread_pool)
            else:
                return state, _handle_stream_unary(rpc_event, state,
                                                   method_handler, thread_pool)
        else:
            if method_handler.response_streaming:
                return state, _handle_unary_stream(rpc_event, state,
                                                   method_handler, thread_pool)
            else:
                return state, _handle_unary_unary(rpc_event, state,
                                                  method_handler, thread_pool)


def _handle_call(rpc_event, generic_handlers, interceptor_pipeline, thread_pool,
                 concurrency_exceeded):
    if not rpc_event.success:
        return None, None
    if rpc_event.call_details.method is not None:
        try:
            method_handler = _find_method_handler(rpc_event, generic_handlers,
                                                  interceptor_pipeline)
        except Exception as exception:  # pylint: disable=broad-except
            details = 'Exception servicing handler: {}'.format(exception)
            _LOGGER.exception(details)
            return _reject_rpc(rpc_event, cygrpc.StatusCode.unknown,
                               b'Error in service handler!'), None
        if method_handler is None:
            return _reject_rpc(rpc_event, cygrpc.StatusCode.unimplemented,
                               b'Method not found!'), None
        elif concurrency_exceeded:
            return _reject_rpc(rpc_event, cygrpc.StatusCode.resource_exhausted,
                               b'Concurrent RPC limit exceeded!'), None
        else:
            return _handle_with_method_handler(rpc_event, method_handler,
                                               thread_pool)
    else:
        return None, None


@enum.unique
class _ServerStage(enum.Enum):
    STOPPED = 'stopped'
    STARTED = 'started'
    GRACE = 'grace'


class _ServerState(object):

    # pylint: disable=too-many-arguments
    def __init__(self, completion_queue, server, generic_handlers,
                 interceptor_pipeline, thread_pool, maximum_concurrent_rpcs):
        self.lock = threading.RLock()
        self.completion_queue = completion_queue
        self.server = server
        self.generic_handlers = list(generic_handlers)
        self.interceptor_pipeline = interceptor_pipeline
        self.thread_pool = thread_pool
        self.stage = _ServerStage.STOPPED
        self.termination_event = threading.Event()
        self.shutdown_events = [self.termination_event]
        self.maximum_concurrent_rpcs = maximum_concurrent_rpcs
        self.active_rpc_count = 0

        # TODO(https://github.com/grpc/grpc/issues/6597): eliminate these fields.
        self.rpc_states = set()
        self.due = set()

        # A "volatile" flag to interrupt the daemon serving thread
        self.server_deallocated = False


def _add_generic_handlers(state, generic_handlers):
    with state.lock:
        state.generic_handlers.extend(generic_handlers)


def _add_insecure_port(state, address):
    with state.lock:
        return state.server.add_http2_port(address)


def _add_secure_port(state, address, server_credentials):
    with state.lock:
        return state.server.add_http2_port(address,
                                           server_credentials._credentials)


def _request_call(state):
    state.server.request_call(state.completion_queue, state.completion_queue,
                              _REQUEST_CALL_TAG)
    state.due.add(_REQUEST_CALL_TAG)


# TODO(https://github.com/grpc/grpc/issues/6597): delete this function.
def _stop_serving(state):
    if not state.rpc_states and not state.due:
        state.server.destroy()
        for shutdown_event in state.shutdown_events:
            shutdown_event.set()
        state.stage = _ServerStage.STOPPED
        return True
    else:
        return False


def _on_call_completed(state):
    with state.lock:
        state.active_rpc_count -= 1


def _process_event_and_continue(state, event):
    should_continue = True
    if event.tag is _SHUTDOWN_TAG:
        with state.lock:
            state.due.remove(_SHUTDOWN_TAG)
            if _stop_serving(state):
                should_continue = False
    elif event.tag is _REQUEST_CALL_TAG:
        with state.lock:
            state.due.remove(_REQUEST_CALL_TAG)
            concurrency_exceeded = (
                state.maximum_concurrent_rpcs is not None and
                state.active_rpc_count >= state.maximum_concurrent_rpcs)
            rpc_state, rpc_future = _handle_call(event, state.generic_handlers,
                                                 state.interceptor_pipeline,
                                                 state.thread_pool,
                                                 concurrency_exceeded)
            if rpc_state is not None:
                state.rpc_states.add(rpc_state)
            if rpc_future is not None:
                state.active_rpc_count += 1
                rpc_future.add_done_callback(
                    lambda unused_future: _on_call_completed(state))
            if state.stage is _ServerStage.STARTED:
                _request_call(state)
            elif _stop_serving(state):
                should_continue = False
    else:
        rpc_state, callbacks = event.tag(event)
        for callback in callbacks:
            try:
                callback()
            except Exception:  # pylint: disable=broad-except
                _LOGGER.exception('Exception calling callback!')
        if rpc_state is not None:
            with state.lock:
                state.rpc_states.remove(rpc_state)
                if _stop_serving(state):
                    should_continue = False
    return should_continue


def _serve(state):
    while True:
        timeout = time.time() + _DEALLOCATED_SERVER_CHECK_PERIOD_S
        event = state.completion_queue.poll(timeout)
        if state.server_deallocated:
            _begin_shutdown_once(state)
        if event.completion_type != cygrpc.CompletionType.queue_timeout:
            if not _process_event_and_continue(state, event):
                return
        # We want to force the deletion of the previous event
        # ~before~ we poll again; if the event has a reference
        # to a shutdown Call object, this can induce spinlock.
        event = None


def _begin_shutdown_once(state):
    with state.lock:
        if state.stage is _ServerStage.STARTED:
            state.server.shutdown(state.completion_queue, _SHUTDOWN_TAG)
            state.stage = _ServerStage.GRACE
            state.due.add(_SHUTDOWN_TAG)


def _stop(state, grace):
    with state.lock:
        if state.stage is _ServerStage.STOPPED:
            shutdown_event = threading.Event()
            shutdown_event.set()
            return shutdown_event
        else:
            _begin_shutdown_once(state)
            shutdown_event = threading.Event()
            state.shutdown_events.append(shutdown_event)
            if grace is None:
                state.server.cancel_all_calls()
            else:

                def cancel_all_calls_after_grace():
                    shutdown_event.wait(timeout=grace)
                    with state.lock:
                        state.server.cancel_all_calls()

                thread = threading.Thread(target=cancel_all_calls_after_grace)
                thread.start()
                return shutdown_event
    shutdown_event.wait()
    return shutdown_event


def _start(state):
    with state.lock:
        if state.stage is not _ServerStage.STOPPED:
            raise ValueError('Cannot start already-started server!')
        state.server.start()
        state.stage = _ServerStage.STARTED
        _request_call(state)

        thread = threading.Thread(target=_serve, args=(state,))
        thread.daemon = True
        thread.start()


def _validate_generic_rpc_handlers(generic_rpc_handlers):
    for generic_rpc_handler in generic_rpc_handlers:
        service_attribute = getattr(generic_rpc_handler, 'service', None)
        if service_attribute is None:
            raise AttributeError(
                '"{}" must conform to grpc.GenericRpcHandler type but does '
                'not have "service" method!'.format(generic_rpc_handler))


def _augment_options(base_options, compression):
    compression_option = _compression.create_channel_option(compression)
    return tuple(base_options) + compression_option


class _Server(grpc.Server):

    # pylint: disable=too-many-arguments
    def __init__(self, thread_pool, generic_handlers, interceptors, options,
                 maximum_concurrent_rpcs, compression, xds):
        completion_queue = cygrpc.CompletionQueue()
        server = cygrpc.Server(_augment_options(options, compression), xds)
        server.register_completion_queue(completion_queue)
        self._state = _ServerState(completion_queue, server, generic_handlers,
                                   _interceptor.service_pipeline(interceptors),
                                   thread_pool, maximum_concurrent_rpcs)

    def add_generic_rpc_handlers(self, generic_rpc_handlers):
        _validate_generic_rpc_handlers(generic_rpc_handlers)
        _add_generic_handlers(self._state, generic_rpc_handlers)

    def add_insecure_port(self, address):
        return _common.validate_port_binding_result(
            address, _add_insecure_port(self._state, _common.encode(address)))

    def add_secure_port(self, address, server_credentials):
        return _common.validate_port_binding_result(
            address,
            _add_secure_port(self._state, _common.encode(address),
                             server_credentials))

    def start(self):
        _start(self._state)

    def wait_for_termination(self, timeout=None):
        # NOTE(https://bugs.python.org/issue35935)
        # Remove this workaround once threading.Event.wait() is working with
        # CTRL+C across platforms.
        return _common.wait(self._state.termination_event.wait,
                            self._state.termination_event.is_set,
                            timeout=timeout)

    def stop(self, grace):
        return _stop(self._state, grace)

    def __del__(self):
        if hasattr(self, '_state'):
            # We can not grab a lock in __del__(), so set a flag to signal the
            # serving daemon thread (if it exists) to initiate shutdown.
            self._state.server_deallocated = True


def create_server(thread_pool, generic_rpc_handlers, interceptors, options,
                  maximum_concurrent_rpcs, compression, xds):
    _validate_generic_rpc_handlers(generic_rpc_handlers)
    return _Server(thread_pool, generic_rpc_handlers, interceptors, options,
                   maximum_concurrent_rpcs, compression, xds)