path: root/src/python/grpcio/grpc/_channel.py

# Copyright 2016, Google Inc.
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#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are
# met:
#
#     * Redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer.
#     * Redistributions in binary form must reproduce the above
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# in the documentation and/or other materials provided with the
# distribution.
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# contributors may be used to endorse or promote products derived from
# this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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"""Invocation-side implementation of gRPC Python."""

import sys
import threading
import time

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

_USER_AGENT = 'Python-gRPC-{}'.format(_grpcio_metadata.__version__)

_EMPTY_FLAGS = 0
_INFINITE_FUTURE = cygrpc.Timespec(float('+inf'))
_EMPTY_METADATA = cygrpc.Metadata(())

_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!')


def _deadline(timeout):
  if timeout is None:
    return None, _INFINITE_FUTURE
  else:
    deadline = time.time() + timeout
    return deadline, cygrpc.Timespec(deadline)


def _unknown_code_details(unknown_cygrpc_code, details):
  return b'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
    # 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 = _EMPTY_METADATA
    state.trailing_metadata = _EMPTY_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 is cygrpc.OperationType.receive_initial_metadata:
      state.initial_metadata = batch_operation.received_metadata
    elif operation_type is cygrpc.OperationType.receive_message:
      serialized_response = batch_operation.received_message.bytes()
      if serialized_response is not None:
        response = _common.deserialize(
            serialized_response, response_deserializer)
        if response is None:
          details = b'Exception deserializing response!'
          _abort(state, grpc.StatusCode.INTERNAL, details)
        else:
          state.response = response
    elif operation_type is cygrpc.OperationType.receive_status_on_client:
      state.trailing_metadata = batch_operation.received_metadata
      if state.code is None:
        code = _common.CYGRPC_STATUS_CODE_TO_STATUS_CODE.get(
            batch_operation.received_status_code)
        if code is None:
          state.code = grpc.StatusCode.UNKNOWN
          state.details = _unknown_code_details(
              batch_operation.received_status_code,
              batch_operation.received_status_details)
        else:
          state.code = code
          state.details = batch_operation.received_status_details
      callbacks.extend(state.callbacks)
      state.callbacks = None
  return callbacks


def _event_handler(state, call, 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 call if done else None
  return handle_event


def _consume_request_iterator(
    request_iterator, state, call, request_serializer):
  event_handler = _event_handler(state, call, None)
  def consume_request_iterator():
    for request in request_iterator:
      serialized_request = _common.serialize(request, request_serializer)
      with state.condition:
        if state.code is None and not state.cancelled:
          if serialized_request is None:
            call.cancel()
            details = b'Exception serializing request!'
            _abort(state, grpc.StatusCode.INTERNAL, details)
            return
          else:
            operations = (
                cygrpc.operation_send_message(
                    serialized_request, _EMPTY_FLAGS),
            )
            call.start_batch(cygrpc.Operations(operations), event_handler)
            state.due.add(cygrpc.OperationType.send_message)
            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.operation_send_close_from_client(_EMPTY_FLAGS),
        )
        call.start_batch(cygrpc.Operations(operations), event_handler)
        state.due.add(cygrpc.OperationType.send_close_from_client)
  thread = threading.Thread(target=consume_request_iterator)
  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:
        self._call.cancel()
        self._state.cancelled = True
        _abort(self._state, grpc.StatusCode.CANCELLED, b'Cancelled!')
        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._call, self._response_deserializer)
        self._call.start_batch(
            cygrpc.Operations(
                (cygrpc.operation_receive_message(_EMPTY_FLAGS),)),
            event_handler)
        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_cancellation_callback(self, callback):
    with self._state.condition:
      if self._state.callbacks is None:
        return False
      else:
        self._state.callbacks.append(lambda unused_future: 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 self._state.details

  def _repr(self):
    with self._state.condition:
      if self._state.code is None:
        return '<_Rendezvous object of in-flight RPC>'
      else:
        return '<_Rendezvous of RPC that terminated with ({}, {})>'.format(
            self._state.code, self._state.details)

  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._call.cancel()
        self._state.cancelled = True
        self._state.code = grpc.StatusCode.CANCELLED
        self._state.condition.notify_all()


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


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


class _UnaryUnaryMultiCallable(grpc.UnaryUnaryMultiCallable):

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

  def _prepare(self, request, timeout, metadata):
    deadline, deadline_timespec, serialized_request, rendezvous = (
        _start_unary_request(request, timeout, self._request_serializer))
    if serialized_request is None:
      return None, None, None, None, rendezvous
    else:
      state = _RPCState(_UNARY_UNARY_INITIAL_DUE, None, None, None, None)
      operations = (
          cygrpc.operation_send_initial_metadata(
              _common.metadata(metadata), _EMPTY_FLAGS),
          cygrpc.operation_send_message(serialized_request, _EMPTY_FLAGS),
          cygrpc.operation_send_close_from_client(_EMPTY_FLAGS),
          cygrpc.operation_receive_initial_metadata(_EMPTY_FLAGS),
          cygrpc.operation_receive_message(_EMPTY_FLAGS),
          cygrpc.operation_receive_status_on_client(_EMPTY_FLAGS),
      )
      return state, operations, deadline, deadline_timespec, None

  def __call__(
      self, request, timeout=None, metadata=None, credentials=None,
      with_call=False):
    state, operations, deadline, deadline_timespec, rendezvous = self._prepare(
        request, timeout, metadata)
    if rendezvous:
      raise rendezvous
    else:
      completion_queue = cygrpc.CompletionQueue()
      call = self._channel.create_call(
          None, 0, completion_queue, self._method, None, deadline_timespec)
      if credentials is not None:
        call.set_credentials(credentials._credentials)
      call.start_batch(cygrpc.Operations(operations), None)
      _handle_event(completion_queue.poll(), state, self._response_deserializer)
      return _end_unary_response_blocking(state, with_call, deadline)

  def future(self, request, timeout=None, metadata=None, credentials=None):
    state, operations, deadline, deadline_timespec, rendezvous = self._prepare(
        request, timeout, metadata)
    if rendezvous:
      return rendezvous
    else:
      call = self._create_managed_call(
          None, 0, self._method, None, deadline_timespec)
      if credentials is not None:
        call.set_credentials(credentials._credentials)
      event_handler = _event_handler(state, call, self._response_deserializer)
      with state.condition:
        call.start_batch(cygrpc.Operations(operations), event_handler)
      return _Rendezvous(state, call, self._response_deserializer, deadline)


class _UnaryStreamMultiCallable(grpc.UnaryStreamMultiCallable):

  def __init__(
      self, channel, create_managed_call, method, request_serializer,
      response_deserializer):
    self._channel = channel
    self._create_managed_call = create_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, deadline_timespec, 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)
      call = self._create_managed_call(
          None, 0, self._method, None, deadline_timespec)
      if credentials is not None:
        call.set_credentials(credentials._credentials)
      event_handler = _event_handler(state, call, self._response_deserializer)
      with state.condition:
        call.start_batch(
            cygrpc.Operations(
                (cygrpc.operation_receive_initial_metadata(_EMPTY_FLAGS),)),
            event_handler)
        operations = (
            cygrpc.operation_send_initial_metadata(
                _common.metadata(metadata), _EMPTY_FLAGS),
            cygrpc.operation_send_message(serialized_request, _EMPTY_FLAGS),
            cygrpc.operation_send_close_from_client(_EMPTY_FLAGS),
            cygrpc.operation_receive_status_on_client(_EMPTY_FLAGS),
        )
        call.start_batch(cygrpc.Operations(operations), event_handler)
      return _Rendezvous(state, call, self._response_deserializer, deadline)


class _StreamUnaryMultiCallable(grpc.StreamUnaryMultiCallable):

  def __init__(
      self, channel, create_managed_call, method, request_serializer,
      response_deserializer):
    self._channel = channel
    self._create_managed_call = create_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,
      with_call=False):
    deadline, deadline_timespec = _deadline(timeout)
    state = _RPCState(_STREAM_UNARY_INITIAL_DUE, None, None, None, None)
    completion_queue = cygrpc.CompletionQueue()
    call = self._channel.create_call(
        None, 0, completion_queue, self._method, None, deadline_timespec)
    if credentials is not None:
      call.set_credentials(credentials._credentials)
    with state.condition:
      call.start_batch(
          cygrpc.Operations(
              (cygrpc.operation_receive_initial_metadata(_EMPTY_FLAGS),)),
          None)
      operations = (
          cygrpc.operation_send_initial_metadata(
              _common.metadata(metadata), _EMPTY_FLAGS),
          cygrpc.operation_receive_message(_EMPTY_FLAGS),
          cygrpc.operation_receive_status_on_client(_EMPTY_FLAGS),
      )
      call.start_batch(cygrpc.Operations(operations), None)
      _consume_request_iterator(
          request_iterator, state, call, self._request_serializer)
    while True:
      event = completion_queue.poll()
      with state.condition:
        _handle_event(event, state, self._response_deserializer)
        state.condition.notify_all()
        if not state.due:
          break
    return _end_unary_response_blocking(state, with_call, deadline)

  def future(
      self, request_iterator, timeout=None, metadata=None, credentials=None):
    deadline, deadline_timespec = _deadline(timeout)
    state = _RPCState(_STREAM_UNARY_INITIAL_DUE, None, None, None, None)
    call = self._create_managed_call(
        None, 0, self._method, None, deadline_timespec)
    if credentials is not None:
      call.set_credentials(credentials._credentials)
    event_handler = _event_handler(state, call, self._response_deserializer)
    with state.condition:
      call.start_batch(
          cygrpc.Operations(
              (cygrpc.operation_receive_initial_metadata(_EMPTY_FLAGS),)),
          event_handler)
      operations = (
          cygrpc.operation_send_initial_metadata(
              _common.metadata(metadata), _EMPTY_FLAGS),
          cygrpc.operation_receive_message(_EMPTY_FLAGS),
          cygrpc.operation_receive_status_on_client(_EMPTY_FLAGS),
      )
      call.start_batch(cygrpc.Operations(operations), event_handler)
      _consume_request_iterator(
          request_iterator, state, call, self._request_serializer)
    return _Rendezvous(state, call, self._response_deserializer, deadline)


class _StreamStreamMultiCallable(grpc.StreamStreamMultiCallable):

  def __init__(
      self, channel, create_managed_call, method, request_serializer,
      response_deserializer):
    self._channel = channel
    self._create_managed_call = create_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_timespec = _deadline(timeout)
    state = _RPCState(_STREAM_STREAM_INITIAL_DUE, None, None, None, None)
    call = self._create_managed_call(
        None, 0, self._method, None, deadline_timespec)
    if credentials is not None:
      call.set_credentials(credentials._credentials)
    event_handler = _event_handler(state, call, self._response_deserializer)
    with state.condition:
      call.start_batch(
          cygrpc.Operations(
              (cygrpc.operation_receive_initial_metadata(_EMPTY_FLAGS),)),
          event_handler)
      operations = (
          cygrpc.operation_send_initial_metadata(
              _common.metadata(metadata), _EMPTY_FLAGS),
          cygrpc.operation_receive_status_on_client(_EMPTY_FLAGS),
      )
      call.start_batch(cygrpc.Operations(operations), event_handler)
      _consume_request_iterator(
          request_iterator, state, call, self._request_serializer)
    return _Rendezvous(state, call, self._response_deserializer, deadline)


class _ChannelCallState(object):

  def __init__(self, channel):
    self.lock = threading.Lock()
    self.channel = channel
    self.completion_queue = cygrpc.CompletionQueue()
    self.managed_calls = None


def _call_spin(state):
  while True:
    event = state.completion_queue.poll()
    completed_call = event.tag(event)
    if completed_call is not None:
      with state.lock:
        state.managed_calls.remove(completed_call)
        if not state.managed_calls:
          state.managed_calls = None
          return


def _create_channel_managed_call(state):
  def create_channel_managed_call(parent, flags, method, host, deadline):
    """Creates a managed cygrpc.Call.

    Callers of this function must conduct at least one operation on the returned
    call. The tags associated with operations conducted on the returned call
    must be no-argument callables that return None to indicate that this channel
    should continue polling for events associated with the call and return the
    call itself to indicate that no more events associated with the call will be
    generated.

    Args:
      parent: A cygrpc.Call to be used as the parent of the created call.
      flags: An integer bitfield of call flags.
      method: The RPC method.
      host: A host string for the created call.
      deadline: A cygrpc.Timespec to be the deadline of the created call.

    Returns:
      A cygrpc.Call with which to conduct an RPC.
    """
    with state.lock:
      call = state.channel.create_call(
          parent, flags, state.completion_queue, method, host, deadline)
      if state.managed_calls is None:
        state.managed_calls = set((call,))
        spin_thread = threading.Thread(target=_call_spin, args=(state,))
        spin_thread.start()
      else:
        state.managed_calls.add(call)
      return call
  return create_channel_managed_call


class _ChannelConnectivityState(object):

  def __init__(self, channel):
    self.lock = threading.Lock()
    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)
  completion_queue = cygrpc.CompletionQueue()
  while True:
    channel.watch_connectivity_state(
        connectivity, cygrpc.Timespec(time.time() + 0.2),
        completion_queue, None)
    event = completion_queue.poll()
    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 _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.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 _moot(state):
  with state.lock:
    del state.callbacks_and_connectivities[:]


def _options(options):
  if options is None:
    pairs = ((cygrpc.ChannelArgKey.primary_user_agent_string, _USER_AGENT),)
  else:
    pairs = list(options) + [
        (cygrpc.ChannelArgKey.primary_user_agent_string, _USER_AGENT)]
  return cygrpc.ChannelArgs(
      cygrpc.ChannelArg(arg_name, arg_value) for arg_name, arg_value in pairs)


class Channel(grpc.Channel):

  def __init__(self, target, options, credentials):
    self._channel = cygrpc.Channel(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, _create_channel_managed_call(self._call_state), method,
        request_serializer, response_deserializer)

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

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

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

  def __del__(self):
    _moot(self._connectivity_state)