# 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.
"""Invocation-side implementation of gRPC Python."""

import logging
import sys
import threading
import time

import grpc
from grpc import _common
from grpc import _grpcio_metadata
from grpc._cython import cygrpc
from grpc.framework.foundation import callable_util

_LOGGER = logging.getLogger(__name__)

_USER_AGENT = 'grpc-python/{}'.format(_grpcio_metadata.__version__)

_EMPTY_FLAGS = 0

_UNARY_UNARY_INITIAL_DUE = (
    cygrpc.OperationType.send_initial_metadata,
    cygrpc.OperationType.send_message,
    cygrpc.OperationType.send_close_from_client,
    cygrpc.OperationType.receive_initial_metadata,
    cygrpc.OperationType.receive_message,
    cygrpc.OperationType.receive_status_on_client,
)
_UNARY_STREAM_INITIAL_DUE = (
    cygrpc.OperationType.send_initial_metadata,
    cygrpc.OperationType.send_message,
    cygrpc.OperationType.send_close_from_client,
    cygrpc.OperationType.receive_initial_metadata,
    cygrpc.OperationType.receive_status_on_client,
)
_STREAM_UNARY_INITIAL_DUE = (
    cygrpc.OperationType.send_initial_metadata,
    cygrpc.OperationType.receive_initial_metadata,
    cygrpc.OperationType.receive_message,
    cygrpc.OperationType.receive_status_on_client,
)
_STREAM_STREAM_INITIAL_DUE = (
    cygrpc.OperationType.send_initial_metadata,
    cygrpc.OperationType.receive_initial_metadata,
    cygrpc.OperationType.receive_status_on_client,
)

_CHANNEL_SUBSCRIPTION_CALLBACK_ERROR_LOG_MESSAGE = (
    'Exception calling channel subscription callback!')

_OK_RENDEZVOUS_REPR_FORMAT = ('<_Rendezvous of RPC that terminated with:\n'
                              '\tstatus = {}\n'
                              '\tdetails = "{}"\n'
                              '>')

_NON_OK_RENDEZVOUS_REPR_FORMAT = ('<_Rendezvous of RPC that terminated with:\n'
                                  '\tstatus = {}\n'
                                  '\tdetails = "{}"\n'
                                  '\tdebug_error_string = "{}"\n'
                                  '>')


def _deadline(timeout):
    return None if timeout is None else time.time() + timeout


def _unknown_code_details(unknown_cygrpc_code, details):
    return 'Server sent unknown code {} and details "{}"'.format(
        unknown_cygrpc_code, details)


def _wait_once_until(condition, until):
    if until is None:
        condition.wait()
    else:
        remaining = until - time.time()
        if remaining < 0:
            raise grpc.FutureTimeoutError()
        else:
            condition.wait(timeout=remaining)


class _RPCState(object):

    def __init__(self, due, initial_metadata, trailing_metadata, code, details):
        self.condition = threading.Condition()
        # The cygrpc.OperationType objects representing events due from the RPC's
        # completion queue.
        self.due = set(due)
        self.initial_metadata = initial_metadata
        self.response = None
        self.trailing_metadata = trailing_metadata
        self.code = code
        self.details = details
        self.debug_error_string = None
        # The semantics of grpc.Future.cancel and grpc.Future.cancelled are
        # slightly wonky, so they have to be tracked separately from the rest of the
        # result of the RPC. This field tracks whether cancellation was requested
        # prior to termination of the RPC.
        self.cancelled = False
        self.callbacks = []


def _abort(state, code, details):
    if state.code is None:
        state.code = code
        state.details = details
        if state.initial_metadata is None:
            state.initial_metadata = ()
        state.trailing_metadata = ()


def _handle_event(event, state, response_deserializer):
    callbacks = []
    for batch_operation in event.batch_operations:
        operation_type = batch_operation.type()
        state.due.remove(operation_type)
        if operation_type == cygrpc.OperationType.receive_initial_metadata:
            state.initial_metadata = batch_operation.initial_metadata()
        elif operation_type == cygrpc.OperationType.receive_message:
            serialized_response = batch_operation.message()
            if serialized_response is not None:
                response = _common.deserialize(serialized_response,
                                               response_deserializer)
                if response is None:
                    details = 'Exception deserializing response!'
                    _abort(state, grpc.StatusCode.INTERNAL, details)
                else:
                    state.response = response
        elif operation_type == cygrpc.OperationType.receive_status_on_client:
            state.trailing_metadata = batch_operation.trailing_metadata()
            if state.code is None:
                code = _common.CYGRPC_STATUS_CODE_TO_STATUS_CODE.get(
                    batch_operation.code())
                if code is None:
                    state.code = grpc.StatusCode.UNKNOWN
                    state.details = _unknown_code_details(
                        code, batch_operation.details())
                else:
                    state.code = code
                    state.details = batch_operation.details()
                    state.debug_error_string = batch_operation.error_string()
            callbacks.extend(state.callbacks)
            state.callbacks = None
    return callbacks


def _event_handler(state, response_deserializer):

    def handle_event(event):
        with state.condition:
            callbacks = _handle_event(event, state, response_deserializer)
            state.condition.notify_all()
            done = not state.due
        for callback in callbacks:
            callback()
        return done

    return handle_event


def _consume_request_iterator(request_iterator, state, call, request_serializer,
                              event_handler):

    def consume_request_iterator():  # pylint: disable=too-many-branches
        while True:
            try:
                request = next(request_iterator)
            except StopIteration:
                break
            except Exception:  # pylint: disable=broad-except
                code = grpc.StatusCode.UNKNOWN
                details = 'Exception iterating requests!'
                _LOGGER.exception(details)
                call.cancel(_common.STATUS_CODE_TO_CYGRPC_STATUS_CODE[code],
                            details)
                _abort(state, code, details)
                return
            serialized_request = _common.serialize(request, request_serializer)
            with state.condition:
                if state.code is None and not state.cancelled:
                    if serialized_request is None:
                        code = grpc.StatusCode.INTERNAL
                        details = 'Exception serializing request!'
                        call.cancel(
                            _common.STATUS_CODE_TO_CYGRPC_STATUS_CODE[code],
                            details)
                        _abort(state, code, details)
                        return
                    else:
                        operations = (cygrpc.SendMessageOperation(
                            serialized_request, _EMPTY_FLAGS),)
                        operating = call.operate(operations, event_handler)
                        if operating:
                            state.due.add(cygrpc.OperationType.send_message)
                        else:
                            return
                        while True:
                            state.condition.wait()
                            if state.code is None:
                                if cygrpc.OperationType.send_message not in state.due:
                                    break
                            else:
                                return
                else:
                    return
        with state.condition:
            if state.code is None:
                operations = (
                    cygrpc.SendCloseFromClientOperation(_EMPTY_FLAGS),)
                operating = call.operate(operations, event_handler)
                if operating:
                    state.due.add(cygrpc.OperationType.send_close_from_client)

    consumption_thread = threading.Thread(target=consume_request_iterator)
    consumption_thread.daemon = True
    consumption_thread.start()


class _Rendezvous(grpc.RpcError, grpc.Future, grpc.Call):

    def __init__(self, state, call, response_deserializer, deadline):
        super(_Rendezvous, self).__init__()
        self._state = state
        self._call = call
        self._response_deserializer = response_deserializer
        self._deadline = deadline

    def cancel(self):
        with self._state.condition:
            if self._state.code is None:
                code = grpc.StatusCode.CANCELLED
                details = 'Locally cancelled by application!'
                self._call.cancel(
                    _common.STATUS_CODE_TO_CYGRPC_STATUS_CODE[code], details)
                self._state.cancelled = True
                _abort(self._state, code, details)
                self._state.condition.notify_all()
            return False

    def cancelled(self):
        with self._state.condition:
            return self._state.cancelled

    def running(self):
        with self._state.condition:
            return self._state.code is None

    def done(self):
        with self._state.condition:
            return self._state.code is not None

    def result(self, timeout=None):
        until = None if timeout is None else time.time() + timeout
        with self._state.condition:
            while True:
                if self._state.code is None:
                    _wait_once_until(self._state.condition, until)
                elif self._state.code is grpc.StatusCode.OK:
                    return self._state.response
                elif self._state.cancelled:
                    raise grpc.FutureCancelledError()
                else:
                    raise self

    def exception(self, timeout=None):
        until = None if timeout is None else time.time() + timeout
        with self._state.condition:
            while True:
                if self._state.code is None:
                    _wait_once_until(self._state.condition, until)
                elif self._state.code is grpc.StatusCode.OK:
                    return None
                elif self._state.cancelled:
                    raise grpc.FutureCancelledError()
                else:
                    return self

    def traceback(self, timeout=None):
        until = None if timeout is None else time.time() + timeout
        with self._state.condition:
            while True:
                if self._state.code is None:
                    _wait_once_until(self._state.condition, until)
                elif self._state.code is grpc.StatusCode.OK:
                    return None
                elif self._state.cancelled:
                    raise grpc.FutureCancelledError()
                else:
                    try:
                        raise self
                    except grpc.RpcError:
                        return sys.exc_info()[2]

    def add_done_callback(self, fn):
        with self._state.condition:
            if self._state.code is None:
                self._state.callbacks.append(lambda: fn(self))
                return

        fn(self)

    def _next(self):
        with self._state.condition:
            if self._state.code is None:
                event_handler = _event_handler(self._state,
                                               self._response_deserializer)
                operating = self._call.operate(
                    (cygrpc.ReceiveMessageOperation(_EMPTY_FLAGS),),
                    event_handler)
                if operating:
                    self._state.due.add(cygrpc.OperationType.receive_message)
            elif self._state.code is grpc.StatusCode.OK:
                raise StopIteration()
            else:
                raise self
            while True:
                self._state.condition.wait()
                if self._state.response is not None:
                    response = self._state.response
                    self._state.response = None
                    return response
                elif cygrpc.OperationType.receive_message not in self._state.due:
                    if self._state.code is grpc.StatusCode.OK:
                        raise StopIteration()
                    elif self._state.code is not None:
                        raise self

    def __iter__(self):
        return self

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

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

    def is_active(self):
        with self._state.condition:
            return self._state.code is None

    def time_remaining(self):
        if self._deadline is None:
            return None
        else:
            return max(self._deadline - time.time(), 0)

    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 initial_metadata(self):
        with self._state.condition:
            while self._state.initial_metadata is None:
                self._state.condition.wait()
            return self._state.initial_metadata

    def trailing_metadata(self):
        with self._state.condition:
            while self._state.trailing_metadata is None:
                self._state.condition.wait()
            return self._state.trailing_metadata

    def code(self):
        with self._state.condition:
            while self._state.code is None:
                self._state.condition.wait()
            return self._state.code

    def details(self):
        with self._state.condition:
            while self._state.details is None:
                self._state.condition.wait()
            return _common.decode(self._state.details)

    def debug_error_string(self):
        with self._state.condition:
            while self._state.debug_error_string is None:
                self._state.condition.wait()
            return _common.decode(self._state.debug_error_string)

    def _repr(self):
        with self._state.condition:
            if self._state.code is None:
                return '<_Rendezvous object of in-flight RPC>'
            elif self._state.code is grpc.StatusCode.OK:
                return _OK_RENDEZVOUS_REPR_FORMAT.format(
                    self._state.code, self._state.details)
            else:
                return _NON_OK_RENDEZVOUS_REPR_FORMAT.format(
                    self._state.code, self._state.details,
                    self._state.debug_error_string)

    def __repr__(self):
        return self._repr()

    def __str__(self):
        return self._repr()

    def __del__(self):
        with self._state.condition:
            if self._state.code is None:
                self._state.code = grpc.StatusCode.CANCELLED
                self._state.details = 'Cancelled upon garbage collection!'
                self._state.cancelled = True
                self._call.cancel(
                    _common.STATUS_CODE_TO_CYGRPC_STATUS_CODE[self._state.code],
                    self._state.details)
                self._state.condition.notify_all()


def _start_unary_request(request, timeout, request_serializer):
    deadline = _deadline(timeout)
    serialized_request = _common.serialize(request, request_serializer)
    if serialized_request is None:
        state = _RPCState((), (), (), grpc.StatusCode.INTERNAL,
                          'Exception serializing request!')
        rendezvous = _Rendezvous(state, None, None, deadline)
        return deadline, None, rendezvous
    else:
        return deadline, serialized_request, None


def _end_unary_response_blocking(state, call, with_call, deadline):
    if state.code is grpc.StatusCode.OK:
        if with_call:
            rendezvous = _Rendezvous(state, call, None, deadline)
            return state.response, rendezvous
        else:
            return state.response
    else:
        raise _Rendezvous(state, None, None, deadline)


def _stream_unary_invocation_operationses(metadata):
    return (
        (
            cygrpc.SendInitialMetadataOperation(metadata, _EMPTY_FLAGS),
            cygrpc.ReceiveMessageOperation(_EMPTY_FLAGS),
            cygrpc.ReceiveStatusOnClientOperation(_EMPTY_FLAGS),
        ),
        (cygrpc.ReceiveInitialMetadataOperation(_EMPTY_FLAGS),),
    )


def _stream_unary_invocation_operationses_and_tags(metadata):
    return tuple((
        operations,
        None,
    ) for operations in _stream_unary_invocation_operationses(metadata))


class _UnaryUnaryMultiCallable(grpc.UnaryUnaryMultiCallable):

    def __init__(self, channel, managed_call, method, request_serializer,
                 response_deserializer):
        self._channel = channel
        self._managed_call = managed_call
        self._method = method
        self._request_serializer = request_serializer
        self._response_deserializer = response_deserializer

    def _prepare(self, request, timeout, metadata):
        deadline, serialized_request, rendezvous = _start_unary_request(
            request, timeout, self._request_serializer)
        if serialized_request is None:
            return None, None, None, rendezvous
        else:
            state = _RPCState(_UNARY_UNARY_INITIAL_DUE, None, None, None, None)
            operations = (
                cygrpc.SendInitialMetadataOperation(metadata, _EMPTY_FLAGS),
                cygrpc.SendMessageOperation(serialized_request, _EMPTY_FLAGS),
                cygrpc.SendCloseFromClientOperation(_EMPTY_FLAGS),
                cygrpc.ReceiveInitialMetadataOperation(_EMPTY_FLAGS),
                cygrpc.ReceiveMessageOperation(_EMPTY_FLAGS),
                cygrpc.ReceiveStatusOnClientOperation(_EMPTY_FLAGS),
            )
            return state, operations, deadline, None

    def _blocking(self, request, timeout, metadata, credentials):
        state, operations, deadline, rendezvous = self._prepare(
            request, timeout, metadata)
        if state is None:
            raise rendezvous
        else:
            call = self._channel.segregated_call(
                0, self._method, None, deadline, metadata, None
                if credentials is None else credentials._credentials, ((
                    operations,
                    None,
                ),))
            event = call.next_event()
            _handle_event(event, state, self._response_deserializer)
            return state, call,

    def __call__(self, request, timeout=None, metadata=None, credentials=None):
        state, call, = self._blocking(request, timeout, metadata, credentials)
        return _end_unary_response_blocking(state, call, False, None)

    def with_call(self, request, timeout=None, metadata=None, credentials=None):
        state, call, = self._blocking(request, timeout, metadata, credentials)
        return _end_unary_response_blocking(state, call, True, None)

    def future(self, request, timeout=None, metadata=None, credentials=None):
        state, operations, deadline, rendezvous = self._prepare(
            request, timeout, metadata)
        if state is None:
            raise rendezvous
        else:
            event_handler = _event_handler(state, self._response_deserializer)
            call = self._managed_call(
                0, self._method, None, deadline, metadata, None
                if credentials is None else credentials._credentials,
                (operations,), event_handler)
            return _Rendezvous(state, call, self._response_deserializer,
                               deadline)


class _UnaryStreamMultiCallable(grpc.UnaryStreamMultiCallable):

    def __init__(self, channel, managed_call, method, request_serializer,
                 response_deserializer):
        self._channel = channel
        self._managed_call = managed_call
        self._method = method
        self._request_serializer = request_serializer
        self._response_deserializer = response_deserializer

    def __call__(self, request, timeout=None, metadata=None, credentials=None):
        deadline, serialized_request, rendezvous = _start_unary_request(
            request, timeout, self._request_serializer)
        if serialized_request is None:
            raise rendezvous
        else:
            state = _RPCState(_UNARY_STREAM_INITIAL_DUE, None, None, None, None)
            operationses = (
                (
                    cygrpc.SendInitialMetadataOperation(metadata, _EMPTY_FLAGS),
                    cygrpc.SendMessageOperation(serialized_request,
                                                _EMPTY_FLAGS),
                    cygrpc.SendCloseFromClientOperation(_EMPTY_FLAGS),
                    cygrpc.ReceiveStatusOnClientOperation(_EMPTY_FLAGS),
                ),
                (cygrpc.ReceiveInitialMetadataOperation(_EMPTY_FLAGS),),
            )
            event_handler = _event_handler(state, self._response_deserializer)
            call = self._managed_call(
                0, self._method, None, deadline, metadata, None
                if credentials is None else credentials._credentials,
                operationses, event_handler)
            return _Rendezvous(state, call, self._response_deserializer,
                               deadline)


class _StreamUnaryMultiCallable(grpc.StreamUnaryMultiCallable):

    def __init__(self, channel, managed_call, method, request_serializer,
                 response_deserializer):
        self._channel = channel
        self._managed_call = managed_call
        self._method = method
        self._request_serializer = request_serializer
        self._response_deserializer = response_deserializer

    def _blocking(self, request_iterator, timeout, metadata, credentials):
        deadline = _deadline(timeout)
        state = _RPCState(_STREAM_UNARY_INITIAL_DUE, None, None, None, None)
        call = self._channel.segregated_call(
            0, self._method, None, deadline, metadata, None
            if credentials is None else credentials._credentials,
            _stream_unary_invocation_operationses_and_tags(metadata))
        _consume_request_iterator(request_iterator, state, call,
                                  self._request_serializer, None)
        while True:
            event = call.next_event()
            with state.condition:
                _handle_event(event, state, self._response_deserializer)
                state.condition.notify_all()
                if not state.due:
                    break
        return state, call,

    def __call__(self,
                 request_iterator,
                 timeout=None,
                 metadata=None,
                 credentials=None):
        state, call, = self._blocking(request_iterator, timeout, metadata,
                                      credentials)
        return _end_unary_response_blocking(state, call, False, None)

    def with_call(self,
                  request_iterator,
                  timeout=None,
                  metadata=None,
                  credentials=None):
        state, call, = self._blocking(request_iterator, timeout, metadata,
                                      credentials)
        return _end_unary_response_blocking(state, call, True, None)

    def future(self,
               request_iterator,
               timeout=None,
               metadata=None,
               credentials=None):
        deadline = _deadline(timeout)
        state = _RPCState(_STREAM_UNARY_INITIAL_DUE, None, None, None, None)
        event_handler = _event_handler(state, self._response_deserializer)
        call = self._managed_call(
            0, self._method, None, deadline, metadata, None
            if credentials is None else credentials._credentials,
            _stream_unary_invocation_operationses(metadata), event_handler)
        _consume_request_iterator(request_iterator, state, call,
                                  self._request_serializer, event_handler)
        return _Rendezvous(state, call, self._response_deserializer, deadline)


class _StreamStreamMultiCallable(grpc.StreamStreamMultiCallable):

    def __init__(self, channel, managed_call, method, request_serializer,
                 response_deserializer):
        self._channel = channel
        self._managed_call = managed_call
        self._method = method
        self._request_serializer = request_serializer
        self._response_deserializer = response_deserializer

    def __call__(self,
                 request_iterator,
                 timeout=None,
                 metadata=None,
                 credentials=None):
        deadline = _deadline(timeout)
        state = _RPCState(_STREAM_STREAM_INITIAL_DUE, None, None, None, None)
        operationses = (
            (
                cygrpc.SendInitialMetadataOperation(metadata, _EMPTY_FLAGS),
                cygrpc.ReceiveStatusOnClientOperation(_EMPTY_FLAGS),
            ),
            (cygrpc.ReceiveInitialMetadataOperation(_EMPTY_FLAGS),),
        )
        event_handler = _event_handler(state, self._response_deserializer)
        call = self._managed_call(
            0, self._method, None, deadline, metadata, None
            if credentials is None else credentials._credentials, operationses,
            event_handler)
        _consume_request_iterator(request_iterator, state, call,
                                  self._request_serializer, event_handler)
        return _Rendezvous(state, call, self._response_deserializer, deadline)


class _ChannelCallState(object):

    def __init__(self, channel):
        self.lock = threading.Lock()
        self.channel = channel
        self.managed_calls = 0


def _run_channel_spin_thread(state):

    def channel_spin():
        while True:
            event = state.channel.next_call_event()
            call_completed = event.tag(event)
            if call_completed:
                with state.lock:
                    state.managed_calls -= 1
                    if state.managed_calls == 0:
                        return

    channel_spin_thread = threading.Thread(target=channel_spin)
    channel_spin_thread.daemon = True
    channel_spin_thread.start()


def _channel_managed_call_management(state):

    # pylint: disable=too-many-arguments
    def create(flags, method, host, deadline, metadata, credentials,
               operationses, event_handler):
        """Creates a cygrpc.IntegratedCall.

        Args:
          flags: An integer bitfield of call flags.
          method: The RPC method.
          host: A host string for the created call.
          deadline: A float to be the deadline of the created call or None if
            the call is to have an infinite deadline.
          metadata: The metadata for the call or None.
          credentials: A cygrpc.CallCredentials or None.
          operationses: An iterable of iterables of cygrpc.Operations to be
            started on the call.
          event_handler: A behavior to call to handle the events resultant from
            the operations on the call.

        Returns:
          A cygrpc.IntegratedCall with which to conduct an RPC.
        """
        operationses_and_tags = tuple((
            operations,
            event_handler,
        ) for operations in operationses)
        with state.lock:
            call = state.channel.integrated_call(flags, method, host, deadline,
                                                 metadata, credentials,
                                                 operationses_and_tags)
            if state.managed_calls == 0:
                state.managed_calls = 1
                _run_channel_spin_thread(state)
            else:
                state.managed_calls += 1
            return call

    return create


class _ChannelConnectivityState(object):

    def __init__(self, channel):
        self.lock = threading.RLock()
        self.channel = channel
        self.polling = False
        self.connectivity = None
        self.try_to_connect = False
        self.callbacks_and_connectivities = []
        self.delivering = False


def _deliveries(state):
    callbacks_needing_update = []
    for callback_and_connectivity in state.callbacks_and_connectivities:
        callback, callback_connectivity, = callback_and_connectivity
        if callback_connectivity is not state.connectivity:
            callbacks_needing_update.append(callback)
            callback_and_connectivity[1] = state.connectivity
    return callbacks_needing_update


def _deliver(state, initial_connectivity, initial_callbacks):
    connectivity = initial_connectivity
    callbacks = initial_callbacks
    while True:
        for callback in callbacks:
            callable_util.call_logging_exceptions(
                callback, _CHANNEL_SUBSCRIPTION_CALLBACK_ERROR_LOG_MESSAGE,
                connectivity)
        with state.lock:
            callbacks = _deliveries(state)
            if callbacks:
                connectivity = state.connectivity
            else:
                state.delivering = False
                return


def _spawn_delivery(state, callbacks):
    delivering_thread = threading.Thread(
        target=_deliver, args=(
            state,
            state.connectivity,
            callbacks,
        ))
    delivering_thread.start()
    state.delivering = True


# NOTE(https://github.com/grpc/grpc/issues/3064): We'd rather not poll.
def _poll_connectivity(state, channel, initial_try_to_connect):
    try_to_connect = initial_try_to_connect
    connectivity = channel.check_connectivity_state(try_to_connect)
    with state.lock:
        state.connectivity = (
            _common.CYGRPC_CONNECTIVITY_STATE_TO_CHANNEL_CONNECTIVITY[
                connectivity])
        callbacks = tuple(callback
                          for callback, unused_but_known_to_be_none_connectivity
                          in state.callbacks_and_connectivities)
        for callback_and_connectivity in state.callbacks_and_connectivities:
            callback_and_connectivity[1] = state.connectivity
        if callbacks:
            _spawn_delivery(state, callbacks)
    while True:
        event = channel.watch_connectivity_state(connectivity,
                                                 time.time() + 0.2)
        with state.lock:
            if not state.callbacks_and_connectivities and not state.try_to_connect:
                state.polling = False
                state.connectivity = None
                break
            try_to_connect = state.try_to_connect
            state.try_to_connect = False
        if event.success or try_to_connect:
            connectivity = channel.check_connectivity_state(try_to_connect)
            with state.lock:
                state.connectivity = (
                    _common.CYGRPC_CONNECTIVITY_STATE_TO_CHANNEL_CONNECTIVITY[
                        connectivity])
                if not state.delivering:
                    callbacks = _deliveries(state)
                    if callbacks:
                        _spawn_delivery(state, callbacks)


def _moot(state):
    with state.lock:
        del state.callbacks_and_connectivities[:]


def _subscribe(state, callback, try_to_connect):
    with state.lock:
        if not state.callbacks_and_connectivities and not state.polling:
            polling_thread = threading.Thread(
                target=_poll_connectivity,
                args=(state, state.channel, bool(try_to_connect)))
            polling_thread.daemon = True
            polling_thread.start()
            state.polling = True
            state.callbacks_and_connectivities.append([callback, None])
        elif not state.delivering and state.connectivity is not None:
            _spawn_delivery(state, (callback,))
            state.try_to_connect |= bool(try_to_connect)
            state.callbacks_and_connectivities.append(
                [callback, state.connectivity])
        else:
            state.try_to_connect |= bool(try_to_connect)
            state.callbacks_and_connectivities.append([callback, None])


def _unsubscribe(state, callback):
    with state.lock:
        for index, (subscribed_callback, unused_connectivity) in enumerate(
                state.callbacks_and_connectivities):
            if callback == subscribed_callback:
                state.callbacks_and_connectivities.pop(index)
                break


def _options(options):
    return list(options) + [
        (
            cygrpc.ChannelArgKey.primary_user_agent_string,
            _USER_AGENT,
        ),
    ]


class Channel(grpc.Channel):
    """A cygrpc.Channel-backed implementation of grpc.Channel."""

    def __init__(self, target, options, credentials):
        """Constructor.

        Args:
          target: The target to which to connect.
          options: Configuration options for the channel.
          credentials: A cygrpc.ChannelCredentials or None.
        """
        self._channel = cygrpc.Channel(
            _common.encode(target), _options(options), credentials)
        self._call_state = _ChannelCallState(self._channel)
        self._connectivity_state = _ChannelConnectivityState(self._channel)

    def subscribe(self, callback, try_to_connect=None):
        _subscribe(self._connectivity_state, callback, try_to_connect)

    def unsubscribe(self, callback):
        _unsubscribe(self._connectivity_state, callback)

    def unary_unary(self,
                    method,
                    request_serializer=None,
                    response_deserializer=None):
        return _UnaryUnaryMultiCallable(
            self._channel, _channel_managed_call_management(self._call_state),
            _common.encode(method), request_serializer, response_deserializer)

    def unary_stream(self,
                     method,
                     request_serializer=None,
                     response_deserializer=None):
        return _UnaryStreamMultiCallable(
            self._channel, _channel_managed_call_management(self._call_state),
            _common.encode(method), request_serializer, response_deserializer)

    def stream_unary(self,
                     method,
                     request_serializer=None,
                     response_deserializer=None):
        return _StreamUnaryMultiCallable(
            self._channel, _channel_managed_call_management(self._call_state),
            _common.encode(method), request_serializer, response_deserializer)

    def stream_stream(self,
                      method,
                      request_serializer=None,
                      response_deserializer=None):
        return _StreamStreamMultiCallable(
            self._channel, _channel_managed_call_management(self._call_state),
            _common.encode(method), request_serializer, response_deserializer)

    def _close(self):
        self._channel.close(cygrpc.StatusCode.cancelled, 'Channel closed!')
        _moot(self._connectivity_state)

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self._close()
        return False

    def close(self):
        self._close()

    def __del__(self):
        # TODO(https://github.com/grpc/grpc/issues/12531): Several releases
        # after 1.12 (1.16 or thereabouts?) add a "self._channel.close" call
        # here (or more likely, call self._close() here). We don't do this today
        # because many valid use cases today allow the channel to be deleted
        # immediately after stubs are created. After a sufficient period of time
        # has passed for all users to be trusted to hang out to their channels
        # for as long as they are in use and to close them after using them,
        # then deletion of this grpc._channel.Channel instance can be made to
        # effect closure of the underlying cygrpc.Channel instance.
        _moot(self._connectivity_state)
