#798: Add support for auto-acknowledging pulled messages.

docs/pubsub-usage.rst

@@ -272,3 +272,29 @@ Fetch messages for a pull subscription without blocking (none pending):
    >>> messages = [recv[1] for recv in received]
    >>> [message.message_id for message in messages]
    []
+
+
+Fetch pending messages, acknowledging those whose processing doesn't raise an
+error:
+
+.. doctest::
+
+   >>> from gcloud import pubsub
+   >>> client = pubsub.Client()
+   >>> topic = client.topic('topic_name')
+   >>> subscription = topic.subscription('subscription_name')
+   >>> with topic.batch() as batch:
+   ...     batch.publish('this is the first message_payload')
+   ...     batch.publish('this is the second message_payload',
+   ...                   attr1='value1', attr2='value2')
+   >>> from gcloud.pubsub.subscription import AutoAck
+   >>> for ack_id, message in subscription.pull(max_messages=10):  # API request
+   ...     with AutoAck(subscription, ack_id, message):
+   ...         do_something_with(message)
+
+.. note::
+
+   One ``acknowledge`` API request occurs at the end of each ``with`` block,
+   passing only the ``ack_id`` of the message just processed.  If
+   ``do_something_with`` raises an exception, the ``acknowledge`` API
+   request is skipped.

gcloud/pubsub/subscription.py

@@ -263,3 +263,29 @@ def delete(self, client=None):
         """
         client = self._require_client(client)
         client.connection.api_request(method='DELETE', path=self.path)
+
+
+class AutoAck(object):
+    """Automatically acknowlege a single message if processed without error.
+
+    :type subscription: :class:`Subscription`
+    :param subscription: the subscription from which the message was pulled,
+                         and to which it must be acknowledged.
+
+    :type ack_id: string
+    :param ack_id: the ID for acknowledging the message
+
+    :type message: :class:`gcloud.pubsub.message.Message`
+    :param message: the message to be acknowleged
+    """
+    def __init__(self, subscription, ack_id, message):
+        self.subscription = subscription
+        self.ack_id = ack_id
+        self.message = message
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        if exc_type is None:
+            self.subscription.acknowledge([self.ack_id])

gcloud/pubsub/test_subscription.py

@@ -485,6 +485,48 @@ def test_delete_w_alternate_client(self):
         self.assertEqual(req['path'], '/%s' % SUB_PATH)
 
 
+class TestAutoAck(unittest2.TestCase):
+
+    def _getTargetClass(self):
+        from gcloud.pubsub.subscription import AutoAck
+        return AutoAck
+
+    def _makeOne(self, *args, **kw):
+        return self._getTargetClass()(*args, **kw)
+
+    def test_ctor(self):
+        ACK_ID = 'ACK_ID'
+        MESSAGE = object()
+        subscription = _FauxSubscription()
+        auto_ack = self._makeOne(subscription, ACK_ID, MESSAGE)
+        self.assertTrue(auto_ack.subscription is subscription)
+        self.assertEqual(auto_ack.ack_id, ACK_ID)
+        self.assertTrue(auto_ack.message is MESSAGE)
+
+    def test_as_context_mgr_no_error(self):
+        ACK_ID = 'ACK_ID'
+        MESSAGE = object()
+        subscription = _FauxSubscription()
+
+        with self._makeOne(subscription, ACK_ID, MESSAGE):
+            pass
+
+        self.assertEqual(list(subscription._acknowledged), [ACK_ID])
+        self.assertEqual(subscription._ack_client, None)
+
+    def test_as_context_mgr_w_error(self):
+        ACK_ID = 'ACK_ID'
+        MESSAGE = object()
+        subscription = _FauxSubscription()
+
+        with self.assertRaises(ValueError):
+            with self._makeOne(subscription, ACK_ID, MESSAGE):
+                raise ValueError()
+
+        self.assertEqual(list(subscription._acknowledged), [])
+        self.assertTrue(getattr(subscription, '_ack_client', self) is self)
+
+
 class _Connection(object):
 
     def __init__(self, *responses):
@@ -522,3 +564,14 @@ def __init__(self, project, connection=None):
     def topic(self, name, timestamp_messages=False):
         from gcloud.pubsub.topic import Topic
         return Topic(name, client=self, timestamp_messages=timestamp_messages)
+
+
+class _FauxSubscription(object):
+
+    def __init__(self):
+        self._acknowledged = set()
+
+    def acknowledge(self, ack_ids, client=None):
+        self._ack_client = client
+        for ack_id in ack_ids:
+            self._acknowledged.add(ack_id)