Graceful streaming pull shutdown breaks PubSub Lite
#292 added an option to gracefully wait for any active message callbacks on streaming pull shutdown, but the addition of the new optional parameter `await_msg_callbacks` broke cancelling the PubSub Lite ([issue](https://github.com/googleapis/python-pubsublite/issues/95)).

As discussed offline, this parameter should be removed from the streaming pull future, and instead implemented as a subscriber option.
diff --git a/google/cloud/pubsub_v1/subscriber/_protocol/dispatcher.py b/google/cloud/pubsub_v1/subscriber/_protocol/dispatcher.py
index 7a89508..382c5c3 100644
--- a/google/cloud/pubsub_v1/subscriber/_protocol/dispatcher.py
+++ b/google/cloud/pubsub_v1/subscriber/_protocol/dispatcher.py
@@ -99,9 +99,6 @@ class Dispatcher(object):
             ValueError: If ``action`` isn't one of the expected actions
                 "ack", "drop", "lease", "modify_ack_deadline" or "nack".
         """
-        if not self._manager.is_active:
-            return
-
         batched_commands = collections.defaultdict(list)
 
         for item in items:
diff --git a/google/cloud/pubsub_v1/subscriber/_protocol/heartbeater.py b/google/cloud/pubsub_v1/subscriber/_protocol/heartbeater.py
index 9cd84a1..fef1589 100644
--- a/google/cloud/pubsub_v1/subscriber/_protocol/heartbeater.py
+++ b/google/cloud/pubsub_v1/subscriber/_protocol/heartbeater.py
@@ -35,10 +35,11 @@ class Heartbeater(object):
         self._period = period
 
     def heartbeat(self):
-        """Periodically send heartbeats."""
-        while self._manager.is_active and not self._stop_event.is_set():
-            self._manager.heartbeat()
-            _LOGGER.debug("Sent heartbeat.")
+        """Periodically send streaming pull heartbeats.
+        """
+        while not self._stop_event.is_set():
+            if self._manager.heartbeat():
+                _LOGGER.debug("Sent heartbeat.")
             self._stop_event.wait(timeout=self._period)
 
         _LOGGER.info("%s exiting.", _HEARTBEAT_WORKER_NAME)
diff --git a/google/cloud/pubsub_v1/subscriber/_protocol/leaser.py b/google/cloud/pubsub_v1/subscriber/_protocol/leaser.py
index 5830680..4a19792 100644
--- a/google/cloud/pubsub_v1/subscriber/_protocol/leaser.py
+++ b/google/cloud/pubsub_v1/subscriber/_protocol/leaser.py
@@ -126,7 +126,7 @@ class Leaser(object):
         ack IDs, then waits for most of that time (but with jitter), and
         repeats.
         """
-        while self._manager.is_active and not self._stop_event.is_set():
+        while not self._stop_event.is_set():
             # Determine the appropriate duration for the lease. This is
             # based off of how long previous messages have taken to ack, with
             # a sensible default and within the ranges allowed by Pub/Sub.
diff --git a/google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py b/google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py
index e8a4a8c..8a3ff0e 100644
--- a/google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py
+++ b/google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py
@@ -16,6 +16,7 @@ from __future__ import division
 
 import collections
 import functools
+import itertools
 import logging
 import threading
 import uuid
@@ -113,10 +114,6 @@ class StreamingPullManager(object):
             scheduler will be used.
     """
 
-    _UNARY_REQUESTS = True
-    """If set to True, this class will make requests over a separate unary
-    RPC instead of over the streaming RPC."""
-
     def __init__(
         self,
         client,
@@ -292,6 +289,9 @@ class StreamingPullManager(object):
                 activate. May be empty.
         """
         with self._pause_resume_lock:
+            if self._scheduler is None:
+                return  # We are shutting down, don't try to dispatch any more messages.
+
             self._messages_on_hold.activate_ordering_keys(
                 ordering_keys, self._schedule_message_on_hold
             )
@@ -421,37 +421,36 @@ class StreamingPullManager(object):
         If a RetryError occurs, the manager shutdown is triggered, and the
         error is re-raised.
         """
-        if self._UNARY_REQUESTS:
-            try:
-                self._send_unary_request(request)
-            except exceptions.GoogleAPICallError:
-                _LOGGER.debug(
-                    "Exception while sending unary RPC. This is typically "
-                    "non-fatal as stream requests are best-effort.",
-                    exc_info=True,
-                )
-            except exceptions.RetryError as exc:
-                _LOGGER.debug(
-                    "RetryError while sending unary RPC. Waiting on a transient "
-                    "error resolution for too long, will now trigger shutdown.",
-                    exc_info=False,
-                )
-                # The underlying channel has been suffering from a retryable error
-                # for too long, time to give up and shut the streaming pull down.
-                self._on_rpc_done(exc)
-                raise
-
-        else:
-            self._rpc.send(request)
+        try:
+            self._send_unary_request(request)
+        except exceptions.GoogleAPICallError:
+            _LOGGER.debug(
+                "Exception while sending unary RPC. This is typically "
+                "non-fatal as stream requests are best-effort.",
+                exc_info=True,
+            )
+        except exceptions.RetryError as exc:
+            _LOGGER.debug(
+                "RetryError while sending unary RPC. Waiting on a transient "
+                "error resolution for too long, will now trigger shutdown.",
+                exc_info=False,
+            )
+            # The underlying channel has been suffering from a retryable error
+            # for too long, time to give up and shut the streaming pull down.
+            self._on_rpc_done(exc)
+            raise
 
     def heartbeat(self):
         """Sends an empty request over the streaming pull RPC.
 
-        This always sends over the stream, regardless of if
-        ``self._UNARY_REQUESTS`` is set or not.
+        Returns:
+            bool: If a heartbeat request has actually been sent.
         """
         if self._rpc is not None and self._rpc.is_active:
             self._rpc.send(gapic_types.StreamingPullRequest())
+            return True
+
+        return False
 
     def open(self, callback, on_callback_error):
         """Begin consuming messages.
@@ -513,7 +512,7 @@ class StreamingPullManager(object):
         # Start the stream heartbeater thread.
         self._heartbeater.start()
 
-    def close(self, reason=None):
+    def close(self, reason=None, await_msg_callbacks=False):
         """Stop consuming messages and shutdown all helper threads.
 
         This method is idempotent. Additional calls will have no effect.
@@ -522,6 +521,15 @@ class StreamingPullManager(object):
             reason (Any): The reason to close this. If None, this is considered
                 an "intentional" shutdown. This is passed to the callbacks
                 specified via :meth:`add_close_callback`.
+
+            await_msg_callbacks (bool):
+                If ``True``, the method will wait until all scheduler threads terminate
+                and only then proceed with the shutdown with the remaining shutdown
+                tasks,
+
+                If ``False`` (default), the method will shut down the scheduler in a
+                non-blocking fashion, i.e. it will not wait for the currently executing
+                scheduler threads to terminate.
         """
         with self._closing:
             if self._closed:
@@ -535,7 +543,9 @@ class StreamingPullManager(object):
 
             # Shutdown all helper threads
             _LOGGER.debug("Stopping scheduler.")
-            self._scheduler.shutdown()
+            dropped_messages = self._scheduler.shutdown(
+                await_msg_callbacks=await_msg_callbacks
+            )
             self._scheduler = None
 
             # Leaser and dispatcher reference each other through the shared
@@ -549,11 +559,23 @@ class StreamingPullManager(object):
             # because the consumer gets shut down first.
             _LOGGER.debug("Stopping leaser.")
             self._leaser.stop()
+
+            total = len(dropped_messages) + len(
+                self._messages_on_hold._messages_on_hold
+            )
+            _LOGGER.debug(f"NACK-ing all not-yet-dispatched messages (total: {total}).")
+            messages_to_nack = itertools.chain(
+                dropped_messages, self._messages_on_hold._messages_on_hold
+            )
+            for msg in messages_to_nack:
+                msg.nack()
+
             _LOGGER.debug("Stopping dispatcher.")
             self._dispatcher.stop()
             self._dispatcher = None
             # dispatcher terminated, OK to dispose the leaser reference now
             self._leaser = None
+
             _LOGGER.debug("Stopping heartbeater.")
             self._heartbeater.stop()
             self._heartbeater = None
diff --git a/google/cloud/pubsub_v1/subscriber/futures.py b/google/cloud/pubsub_v1/subscriber/futures.py
index f9fdd76..cefe1aa 100644
--- a/google/cloud/pubsub_v1/subscriber/futures.py
+++ b/google/cloud/pubsub_v1/subscriber/futures.py
@@ -43,12 +43,23 @@ class StreamingPullFuture(futures.Future):
         else:
             self.set_exception(result)
 
-    def cancel(self):
+    def cancel(self, await_msg_callbacks=False):
         """Stops pulling messages and shutdowns the background thread consuming
         messages.
+
+        Args:
+            await_msg_callbacks (bool):
+                If ``True``, the method will block until the background stream and its
+                helper threads have has been terminated, as well as all currently
+                executing message callbacks are done processing.
+
+                If ``False`` (default), the method returns immediately after the
+                background stream and its helper threads have has been terminated, but
+                some of the message callback threads might still be running at that
+                point.
         """
         self._cancelled = True
-        return self._manager.close()
+        return self._manager.close(await_msg_callbacks=await_msg_callbacks)
 
     def cancelled(self):
         """
diff --git a/google/cloud/pubsub_v1/subscriber/scheduler.py b/google/cloud/pubsub_v1/subscriber/scheduler.py
index ef2ef59..2690c1f 100644
--- a/google/cloud/pubsub_v1/subscriber/scheduler.py
+++ b/google/cloud/pubsub_v1/subscriber/scheduler.py
@@ -20,7 +20,6 @@ each message.
 
 import abc
 import concurrent.futures
-import sys
 
 import six
 from six.moves import queue
@@ -58,19 +57,29 @@ class Scheduler(object):
         raise NotImplementedError
 
     @abc.abstractmethod
-    def shutdown(self):
+    def shutdown(self, await_msg_callbacks=False):
         """Shuts down the scheduler and immediately end all pending callbacks.
+
+        Args:
+            await_msg_callbacks (bool):
+                If ``True``, the method will block until all currently executing
+                callbacks are done processing. If ``False`` (default), the
+                method will not wait for the currently running callbacks to complete.
+
+        Returns:
+            List[pubsub_v1.subscriber.message.Message]:
+                The messages submitted to the scheduler that were not yet dispatched
+                to their callbacks.
+                It is assumed that each message was submitted to the scheduler as the
+                first positional argument to the provided callback.
         """
         raise NotImplementedError
 
 
 def _make_default_thread_pool_executor():
-    # Python 2.7 and 3.6+ have the thread_name_prefix argument, which is useful
-    # for debugging.
-    executor_kwargs = {}
-    if sys.version_info[:2] == (2, 7) or sys.version_info >= (3, 6):
-        executor_kwargs["thread_name_prefix"] = "ThreadPoolExecutor-ThreadScheduler"
-    return concurrent.futures.ThreadPoolExecutor(max_workers=10, **executor_kwargs)
+    return concurrent.futures.ThreadPoolExecutor(
+        max_workers=10, thread_name_prefix="ThreadPoolExecutor-ThreadScheduler"
+    )
 
 
 class ThreadScheduler(Scheduler):
@@ -110,15 +119,35 @@ class ThreadScheduler(Scheduler):
         """
         self._executor.submit(callback, *args, **kwargs)
 
-    def shutdown(self):
-        """Shuts down the scheduler and immediately end all pending callbacks.
+    def shutdown(self, await_msg_callbacks=False):
+        """Shut down the scheduler and immediately end all pending callbacks.
+
+        Args:
+            await_msg_callbacks (bool):
+                If ``True``, the method will block until all currently executing
+                executor threads are done processing. If ``False`` (default), the
+                method will not wait for the currently running threads to complete.
+
+        Returns:
+            List[pubsub_v1.subscriber.message.Message]:
+                The messages submitted to the scheduler that were not yet dispatched
+                to their callbacks.
+                It is assumed that each message was submitted to the scheduler as the
+                first positional argument to the provided callback.
         """
-        # Drop all pending item from the executor. Without this, the executor
-        # will block until all pending items are complete, which is
-        # undesirable.
+        dropped_messages = []
+
+        # Drop all pending item from the executor. Without this, the executor will also
+        # try to process any pending work items before termination, which is undesirable.
+        #
+        # TODO: Replace the logic below by passing `cancel_futures=True` to shutdown()
+        # once we only need to support Python 3.9+.
         try:
             while True:
-                self._executor._work_queue.get(block=False)
+                work_item = self._executor._work_queue.get(block=False)
+                dropped_messages.append(work_item.args[0])
         except queue.Empty:
             pass
-        self._executor.shutdown()
+
+        self._executor.shutdown(wait=await_msg_callbacks)
+        return dropped_messages
diff --git a/tests/system.py b/tests/system.py
index bbedd9a..05a91a4 100644
--- a/tests/system.py
+++ b/tests/system.py
@@ -14,6 +14,7 @@
 
 from __future__ import absolute_import
 
+import concurrent.futures
 import datetime
 import itertools
 import operator as op
@@ -609,6 +610,78 @@ class TestStreamingPull(object):
         finally:
             subscription_future.cancel()  # trigger clean shutdown
 
+    def test_streaming_pull_blocking_shutdown(
+        self, publisher, topic_path, subscriber, subscription_path, cleanup
+    ):
+        # Make sure the topic and subscription get deleted.
+        cleanup.append((publisher.delete_topic, (), {"topic": topic_path}))
+        cleanup.append(
+            (subscriber.delete_subscription, (), {"subscription": subscription_path})
+        )
+
+        # The ACK-s are only persisted if *all* messages published in the same batch
+        # are ACK-ed. We thus publish each message in its own batch so that the backend
+        # treats all messages' ACKs independently of each other.
+        publisher.create_topic(name=topic_path)
+        subscriber.create_subscription(name=subscription_path, topic=topic_path)
+        _publish_messages(publisher, topic_path, batch_sizes=[1] * 10)
+
+        # Artificially delay message processing, gracefully shutdown the streaming pull
+        # in the meantime, then verify that those messages were nevertheless processed.
+        processed_messages = []
+
+        def callback(message):
+            time.sleep(15)
+            processed_messages.append(message.data)
+            message.ack()
+
+        # Flow control limits should exceed the number of worker threads, so that some
+        # of the messages will be blocked on waiting for free scheduler threads.
+        flow_control = pubsub_v1.types.FlowControl(max_messages=5)
+        executor = concurrent.futures.ThreadPoolExecutor(max_workers=3)
+        scheduler = pubsub_v1.subscriber.scheduler.ThreadScheduler(executor=executor)
+        subscription_future = subscriber.subscribe(
+            subscription_path,
+            callback=callback,
+            flow_control=flow_control,
+            scheduler=scheduler,
+        )
+
+        try:
+            subscription_future.result(timeout=10)  # less than the sleep in callback
+        except exceptions.TimeoutError:
+            subscription_future.cancel(await_msg_callbacks=True)
+
+        # The shutdown should have waited for the already executing callbacks to finish.
+        assert len(processed_messages) == 3
+
+        # The messages that were not processed should have been NACK-ed and we should
+        # receive them again quite soon.
+        all_done = threading.Barrier(7 + 1, timeout=5)  # +1 because of the main thread
+        remaining = []
+
+        def callback2(message):
+            remaining.append(message.data)
+            message.ack()
+            all_done.wait()
+
+        subscription_future = subscriber.subscribe(
+            subscription_path, callback=callback2
+        )
+
+        try:
+            all_done.wait()
+        except threading.BrokenBarrierError:  # PRAGMA: no cover
+            pytest.fail("The remaining messages have not been re-delivered in time.")
+        finally:
+            subscription_future.cancel(await_msg_callbacks=False)
+
+        # There should be 7 messages left that were not yet processed and none of them
+        # should be a message that should have already been sucessfully processed in the
+        # first streaming pull.
+        assert len(remaining) == 7
+        assert not (set(processed_messages) & set(remaining))  # no re-delivery
+
 
 @pytest.mark.skipif(
     "KOKORO_GFILE_DIR" not in os.environ,
@@ -790,8 +863,8 @@ def _publish_messages(publisher, topic_path, batch_sizes):
     publish_futures = []
     msg_counter = itertools.count(start=1)
 
-    for batch_size in batch_sizes:
-        msg_batch = _make_messages(count=batch_size)
+    for batch_num, batch_size in enumerate(batch_sizes, start=1):
+        msg_batch = _make_messages(count=batch_size, batch_num=batch_num)
         for msg in msg_batch:
             future = publisher.publish(topic_path, msg, seq_num=str(next(msg_counter)))
             publish_futures.append(future)
@@ -802,9 +875,10 @@ def _publish_messages(publisher, topic_path, batch_sizes):
         future.result(timeout=30)
 
 
-def _make_messages(count):
+def _make_messages(count, batch_num):
     messages = [
-        "message {}/{}".format(i, count).encode("utf-8") for i in range(1, count + 1)
+        f"message {i}/{count} of batch {batch_num}".encode("utf-8")
+        for i in range(1, count + 1)
     ]
     return messages
 
diff --git a/tests/unit/pubsub_v1/subscriber/test_dispatcher.py b/tests/unit/pubsub_v1/subscriber/test_dispatcher.py
index 288e4bd..47c62ba 100644
--- a/tests/unit/pubsub_v1/subscriber/test_dispatcher.py
+++ b/tests/unit/pubsub_v1/subscriber/test_dispatcher.py
@@ -29,14 +29,14 @@ import pytest
 @pytest.mark.parametrize(
     "item,method_name",
     [
-        (requests.AckRequest(0, 0, 0, ""), "ack"),
-        (requests.DropRequest(0, 0, ""), "drop"),
-        (requests.LeaseRequest(0, 0, ""), "lease"),
-        (requests.ModAckRequest(0, 0), "modify_ack_deadline"),
-        (requests.NackRequest(0, 0, ""), "nack"),
+        (requests.AckRequest("0", 0, 0, ""), "ack"),
+        (requests.DropRequest("0", 0, ""), "drop"),
+        (requests.LeaseRequest("0", 0, ""), "lease"),
+        (requests.ModAckRequest("0", 0), "modify_ack_deadline"),
+        (requests.NackRequest("0", 0, ""), "nack"),
     ],
 )
-def test_dispatch_callback(item, method_name):
+def test_dispatch_callback_active_manager(item, method_name):
     manager = mock.create_autospec(
         streaming_pull_manager.StreamingPullManager, instance=True
     )
@@ -50,16 +50,29 @@ def test_dispatch_callback(item, method_name):
     method.assert_called_once_with([item])
 
 
-def test_dispatch_callback_inactive():
+@pytest.mark.parametrize(
+    "item,method_name",
+    [
+        (requests.AckRequest("0", 0, 0, ""), "ack"),
+        (requests.DropRequest("0", 0, ""), "drop"),
+        (requests.LeaseRequest("0", 0, ""), "lease"),
+        (requests.ModAckRequest("0", 0), "modify_ack_deadline"),
+        (requests.NackRequest("0", 0, ""), "nack"),
+    ],
+)
+def test_dispatch_callback_inactive_manager(item, method_name):
     manager = mock.create_autospec(
         streaming_pull_manager.StreamingPullManager, instance=True
     )
     manager.is_active = False
     dispatcher_ = dispatcher.Dispatcher(manager, mock.sentinel.queue)
 
-    dispatcher_.dispatch_callback([requests.AckRequest(0, 0, 0, "")])
+    items = [item]
 
-    manager.send.assert_not_called()
+    with mock.patch.object(dispatcher_, method_name) as method:
+        dispatcher_.dispatch_callback(items)
+
+    method.assert_called_once_with([item])
 
 
 def test_ack():
diff --git a/tests/unit/pubsub_v1/subscriber/test_futures_subscriber.py b/tests/unit/pubsub_v1/subscriber/test_futures_subscriber.py
index 909337c..62a3ea1 100644
--- a/tests/unit/pubsub_v1/subscriber/test_futures_subscriber.py
+++ b/tests/unit/pubsub_v1/subscriber/test_futures_subscriber.py
@@ -69,10 +69,18 @@ class TestStreamingPullFuture(object):
         result = future.result()
         assert result == "foo"  # on close callback was a no-op
 
-    def test_cancel(self):
+    def test_cancel_default_nonblocking_manager_shutdown(self):
         future = self.make_future()
 
         future.cancel()
 
-        future._manager.close.assert_called_once()
+        future._manager.close.assert_called_once_with(await_msg_callbacks=False)
+        assert future.cancelled()
+
+    def test_cancel_blocking_manager_shutdown(self):
+        future = self.make_future()
+
+        future.cancel(await_msg_callbacks=True)
+
+        future._manager.close.assert_called_once_with(await_msg_callbacks=True)
         assert future.cancelled()
diff --git a/tests/unit/pubsub_v1/subscriber/test_heartbeater.py b/tests/unit/pubsub_v1/subscriber/test_heartbeater.py
index 8f50496..1a52af2 100644
--- a/tests/unit/pubsub_v1/subscriber/test_heartbeater.py
+++ b/tests/unit/pubsub_v1/subscriber/test_heartbeater.py
@@ -22,22 +22,44 @@ import mock
 import pytest
 
 
-def test_heartbeat_inactive(caplog):
-    caplog.set_level(logging.INFO)
+def test_heartbeat_inactive_manager_active_rpc(caplog):
+    caplog.set_level(logging.DEBUG)
+
+    manager = mock.create_autospec(
+        streaming_pull_manager.StreamingPullManager, instance=True
+    )
+    manager.is_active = False
+    manager.heartbeat.return_value = True  # because of active rpc
+
+    heartbeater_ = heartbeater.Heartbeater(manager)
+    make_sleep_mark_event_as_done(heartbeater_)
+
+    heartbeater_.heartbeat()
+
+    assert "Sent heartbeat" in caplog.text
+    assert "exiting" in caplog.text
+
+
+def test_heartbeat_inactive_manager_inactive_rpc(caplog):
+    caplog.set_level(logging.DEBUG)
+
     manager = mock.create_autospec(
         streaming_pull_manager.StreamingPullManager, instance=True
     )
     manager.is_active = False
+    manager.heartbeat.return_value = False  # because of inactive rpc
 
     heartbeater_ = heartbeater.Heartbeater(manager)
+    make_sleep_mark_event_as_done(heartbeater_)
 
     heartbeater_.heartbeat()
 
+    assert "Sent heartbeat" not in caplog.text
     assert "exiting" in caplog.text
 
 
 def test_heartbeat_stopped(caplog):
-    caplog.set_level(logging.INFO)
+    caplog.set_level(logging.DEBUG)
     manager = mock.create_autospec(
         streaming_pull_manager.StreamingPullManager, instance=True
     )
@@ -47,17 +69,18 @@ def test_heartbeat_stopped(caplog):
 
     heartbeater_.heartbeat()
 
+    assert "Sent heartbeat" not in caplog.text
     assert "exiting" in caplog.text
 
 
-def make_sleep_mark_manager_as_inactive(heartbeater):
-    # Make sleep mark the manager as inactive so that heartbeat()
+def make_sleep_mark_event_as_done(heartbeater):
+    # Make sleep actually trigger the done event so that heartbeat()
     # exits at the end of the first run.
-    def trigger_inactive(timeout):
+    def trigger_done(timeout):
         assert timeout
-        heartbeater._manager.is_active = False
+        heartbeater._stop_event.set()
 
-    heartbeater._stop_event.wait = trigger_inactive
+    heartbeater._stop_event.wait = trigger_done
 
 
 def test_heartbeat_once():
@@ -65,7 +88,7 @@ def test_heartbeat_once():
         streaming_pull_manager.StreamingPullManager, instance=True
     )
     heartbeater_ = heartbeater.Heartbeater(manager)
-    make_sleep_mark_manager_as_inactive(heartbeater_)
+    make_sleep_mark_event_as_done(heartbeater_)
 
     heartbeater_.heartbeat()
 
diff --git a/tests/unit/pubsub_v1/subscriber/test_leaser.py b/tests/unit/pubsub_v1/subscriber/test_leaser.py
index 17409cb..2ecc0b9 100644
--- a/tests/unit/pubsub_v1/subscriber/test_leaser.py
+++ b/tests/unit/pubsub_v1/subscriber/test_leaser.py
@@ -88,15 +88,21 @@ def create_manager(flow_control=types.FlowControl()):
     return manager
 
 
-def test_maintain_leases_inactive(caplog):
+def test_maintain_leases_inactive_manager(caplog):
     caplog.set_level(logging.INFO)
     manager = create_manager()
     manager.is_active = False
 
     leaser_ = leaser.Leaser(manager)
+    make_sleep_mark_event_as_done(leaser_)
+    leaser_.add(
+        [requests.LeaseRequest(ack_id="my_ack_ID", byte_size=42, ordering_key="")]
+    )
 
     leaser_.maintain_leases()
 
+    # Leases should still be maintained even if the manager is inactive.
+    manager.dispatcher.modify_ack_deadline.assert_called()
     assert "exiting" in caplog.text
 
 
@@ -112,20 +118,20 @@ def test_maintain_leases_stopped(caplog):
     assert "exiting" in caplog.text
 
 
-def make_sleep_mark_manager_as_inactive(leaser):
-    # Make sleep mark the manager as inactive so that maintain_leases
+def make_sleep_mark_event_as_done(leaser):
+    # Make sleep actually trigger the done event so that heartbeat()
     # exits at the end of the first run.
-    def trigger_inactive(timeout):
+    def trigger_done(timeout):
         assert 0 < timeout < 10
-        leaser._manager.is_active = False
+        leaser._stop_event.set()
 
-    leaser._stop_event.wait = trigger_inactive
+    leaser._stop_event.wait = trigger_done
 
 
 def test_maintain_leases_ack_ids():
     manager = create_manager()
     leaser_ = leaser.Leaser(manager)
-    make_sleep_mark_manager_as_inactive(leaser_)
+    make_sleep_mark_event_as_done(leaser_)
     leaser_.add(
         [requests.LeaseRequest(ack_id="my ack id", byte_size=50, ordering_key="")]
     )
@@ -140,7 +146,7 @@ def test_maintain_leases_ack_ids():
 def test_maintain_leases_no_ack_ids():
     manager = create_manager()
     leaser_ = leaser.Leaser(manager)
-    make_sleep_mark_manager_as_inactive(leaser_)
+    make_sleep_mark_event_as_done(leaser_)
 
     leaser_.maintain_leases()
 
@@ -151,7 +157,7 @@ def test_maintain_leases_no_ack_ids():
 def test_maintain_leases_outdated_items(time):
     manager = create_manager()
     leaser_ = leaser.Leaser(manager)
-    make_sleep_mark_manager_as_inactive(leaser_)
+    make_sleep_mark_event_as_done(leaser_)
 
     # Add and start expiry timer at the beginning of the timeline.
     time.return_value = 0
diff --git a/tests/unit/pubsub_v1/subscriber/test_scheduler.py b/tests/unit/pubsub_v1/subscriber/test_scheduler.py
index 774d0d6..ede7c6b 100644
--- a/tests/unit/pubsub_v1/subscriber/test_scheduler.py
+++ b/tests/unit/pubsub_v1/subscriber/test_scheduler.py
@@ -14,6 +14,7 @@
 
 import concurrent.futures
 import threading
+import time
 
 import mock
 from six.moves import queue
@@ -38,19 +39,89 @@ def test_constructor_options():
     assert scheduler_._executor == mock.sentinel.executor
 
 
-def test_schedule():
+def test_schedule_executes_submitted_items():
     called_with = []
-    called = threading.Event()
+    callback_done_twice = threading.Barrier(3)  # 3 == 2x callback + 1x main thread
 
     def callback(*args, **kwargs):
-        called_with.append((args, kwargs))
-        called.set()
+        called_with.append((args, kwargs))  # appends are thread-safe
+        callback_done_twice.wait()
 
     scheduler_ = scheduler.ThreadScheduler()
 
     scheduler_.schedule(callback, "arg1", kwarg1="meep")
+    scheduler_.schedule(callback, "arg2", kwarg2="boop")
 
-    called.wait()
-    scheduler_.shutdown()
+    callback_done_twice.wait(timeout=3.0)
+    result = scheduler_.shutdown()
 
-    assert called_with == [(("arg1",), {"kwarg1": "meep"})]
+    assert result == []  # no scheduled items dropped
+
+    expected_calls = [(("arg1",), {"kwarg1": "meep"}), (("arg2",), {"kwarg2": "boop"})]
+    assert sorted(called_with) == expected_calls
+
+
+def test_shutdown_nonblocking_by_default():
+    called_with = []
+    at_least_one_called = threading.Event()
+    at_least_one_completed = threading.Event()
+
+    def callback(message):
+        called_with.append(message)  # appends are thread-safe
+        at_least_one_called.set()
+        time.sleep(1.0)
+        at_least_one_completed.set()
+
+    executor = concurrent.futures.ThreadPoolExecutor(max_workers=1)
+    scheduler_ = scheduler.ThreadScheduler(executor=executor)
+
+    scheduler_.schedule(callback, "message_1")
+    scheduler_.schedule(callback, "message_2")
+
+    at_least_one_called.wait()
+    dropped = scheduler_.shutdown()
+
+    assert len(called_with) == 1
+    assert called_with[0] in {"message_1", "message_2"}
+
+    assert len(dropped) == 1
+    assert dropped[0] in {"message_1", "message_2"}
+    assert dropped[0] != called_with[0]  # the dropped message was not the processed one
+
+    err_msg = (
+        "Shutdown should not have waited "
+        "for the already running callbacks to complete."
+    )
+    assert not at_least_one_completed.is_set(), err_msg
+
+
+def test_shutdown_blocking_awaits_running_callbacks():
+    called_with = []
+    at_least_one_called = threading.Event()
+    at_least_one_completed = threading.Event()
+
+    def callback(message):
+        called_with.append(message)  # appends are thread-safe
+        at_least_one_called.set()
+        time.sleep(1.0)
+        at_least_one_completed.set()
+
+    executor = concurrent.futures.ThreadPoolExecutor(max_workers=1)
+    scheduler_ = scheduler.ThreadScheduler(executor=executor)
+
+    scheduler_.schedule(callback, "message_1")
+    scheduler_.schedule(callback, "message_2")
+
+    at_least_one_called.wait()
+    dropped = scheduler_.shutdown(await_msg_callbacks=True)
+
+    assert len(called_with) == 1
+    assert called_with[0] in {"message_1", "message_2"}
+
+    # The work items that have not been started yet should still be dropped.
+    assert len(dropped) == 1
+    assert dropped[0] in {"message_1", "message_2"}
+    assert dropped[0] != called_with[0]  # the dropped message was not the processed one
+
+    err_msg = "Shutdown did not wait for the already running callbacks to complete."
+    assert at_least_one_completed.is_set(), err_msg
diff --git a/tests/unit/pubsub_v1/subscriber/test_streaming_pull_manager.py b/tests/unit/pubsub_v1/subscriber/test_streaming_pull_manager.py
index 242c080..a6454f8 100644
--- a/tests/unit/pubsub_v1/subscriber/test_streaming_pull_manager.py
+++ b/tests/unit/pubsub_v1/subscriber/test_streaming_pull_manager.py
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import functools
 import logging
 import threading
 import time
@@ -372,7 +373,6 @@ def test__maybe_release_messages_negative_on_hold_bytes_warning(caplog):
 
 def test_send_unary():
     manager = make_manager()
-    manager._UNARY_REQUESTS = True
 
     manager.send(
         gapic_types.StreamingPullRequest(
@@ -405,7 +405,6 @@ def test_send_unary():
 
 def test_send_unary_empty():
     manager = make_manager()
-    manager._UNARY_REQUESTS = True
 
     manager.send(gapic_types.StreamingPullRequest())
 
@@ -417,7 +416,6 @@ def test_send_unary_api_call_error(caplog):
     caplog.set_level(logging.DEBUG)
 
     manager = make_manager()
-    manager._UNARY_REQUESTS = True
 
     error = exceptions.GoogleAPICallError("The front fell off")
     manager._client.acknowledge.side_effect = error
@@ -431,7 +429,6 @@ def test_send_unary_retry_error(caplog):
     caplog.set_level(logging.DEBUG)
 
     manager, _, _, _, _, _ = make_running_manager()
-    manager._UNARY_REQUESTS = True
 
     error = exceptions.RetryError(
         "Too long a transient error", cause=Exception("Out of time!")
@@ -445,24 +442,15 @@ def test_send_unary_retry_error(caplog):
     assert "signaled streaming pull manager shutdown" in caplog.text
 
 
-def test_send_streaming():
-    manager = make_manager()
-    manager._UNARY_REQUESTS = False
-    manager._rpc = mock.create_autospec(bidi.BidiRpc, instance=True)
-
-    manager.send(mock.sentinel.request)
-
-    manager._rpc.send.assert_called_once_with(mock.sentinel.request)
-
-
 def test_heartbeat():
     manager = make_manager()
     manager._rpc = mock.create_autospec(bidi.BidiRpc, instance=True)
     manager._rpc.is_active = True
 
-    manager.heartbeat()
+    result = manager.heartbeat()
 
     manager._rpc.send.assert_called_once_with(gapic_types.StreamingPullRequest())
+    assert result
 
 
 def test_heartbeat_inactive():
@@ -472,7 +460,8 @@ def test_heartbeat_inactive():
 
     manager.heartbeat()
 
-    manager._rpc.send.assert_not_called()
+    result = manager._rpc.send.assert_not_called()
+    assert not result
 
 
 @mock.patch("google.api_core.bidi.ResumableBidiRpc", autospec=True)
@@ -632,14 +621,14 @@ class FakeDispatcher(object):
         while not self._stop:
             try:
                 self._manager.leaser.add([mock.Mock()])
-            except Exception as exc:
+            except Exception as exc:  # pragma: NO COVER
                 self._error_callback(exc)
             time.sleep(0.1)
 
         # also try to interact with the leaser after the stop flag has been set
         try:
             self._manager.leaser.remove([mock.Mock()])
-        except Exception as exc:
+        except Exception as exc:  # pragma: NO COVER
             self._error_callback(exc)
 
 
@@ -666,6 +655,27 @@ def test_close_callbacks():
     callback.assert_called_once_with(manager, "meep")
 
 
+def test_close_nacks_internally_queued_messages():
+    nacked_messages = []
+
+    def fake_nack(self):
+        nacked_messages.append(self.data)
+
+    MockMsg = functools.partial(mock.create_autospec, message.Message, instance=True)
+    messages = [MockMsg(data=b"msg1"), MockMsg(data=b"msg2"), MockMsg(data=b"msg3")]
+    for msg in messages:
+        msg.nack = stdlib_types.MethodType(fake_nack, msg)
+
+    manager, _, _, _, _, _ = make_running_manager()
+    dropped_by_scheduler = messages[:2]
+    manager._scheduler.shutdown.return_value = dropped_by_scheduler
+    manager._messages_on_hold._messages_on_hold.append(messages[2])
+
+    manager.close()
+
+    assert sorted(nacked_messages) == [b"msg1", b"msg2", b"msg3"]
+
+
 def test__get_initial_request():
     manager = make_manager()
     manager._leaser = mock.create_autospec(leaser.Leaser, instance=True)
@@ -979,3 +989,15 @@ def test_activate_ordering_keys():
     manager._messages_on_hold.activate_ordering_keys.assert_called_once_with(
         ["key1", "key2"], mock.ANY
     )
+
+
+def test_activate_ordering_keys_stopped_scheduler():
+    manager = make_manager()
+    manager._messages_on_hold = mock.create_autospec(
+        messages_on_hold.MessagesOnHold, instance=True
+    )
+    manager._scheduler = None
+
+    manager.activate_ordering_keys(["key1", "key2"])
+
+    manager._messages_on_hold.activate_ordering_keys.assert_not_called()
['google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py', 'tests/unit/pubsub_v1/subscriber/test_futures_subscriber.py', 'google/cloud/pubsub_v1/subscriber/_protocol/leaser.py', 'tests/system.py', 'google/cloud/pubsub_v1/subscriber/futures.py', 'google/cloud/pubsub_v1/subscriber/_protocol/dispatcher.py', 'google/cloud/pubsub_v1/subscriber/scheduler.py', 'google/cloud/pubsub_v1/subscriber/_protocol/heartbeater.py', 'tests/unit/pubsub_v1/subscriber/test_heartbeater.py', 'tests/unit/pubsub_v1/subscriber/test_leaser.py', 'tests/unit/pubsub_v1/subscriber/test_streaming_pull_manager.py', 'tests/unit/pubsub_v1/subscriber/test_scheduler.py', 'tests/unit/pubsub_v1/subscriber/test_dispatcher.py']