Add a jobboard fetching context manager

Allow the jobboard backend to be fetched and automatically
connected and closed by providing a method that does the
fetching, connecting and closing as a context manager.

Change-Id: I3b2ea707009f8154f2c68652f101490c09f35c8c

taskflow/jobs/backends/__init__.py

@@ -14,6 +14,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import contextlib
 import logging
 
 import six
@@ -53,3 +54,14 @@ def fetch(name, conf, namespace=BACKEND_NAMESPACE, **kwargs):
         return mgr.driver
     except RuntimeError as e:
         raise exc.NotFound("Could not find jobboard %s" % (board), e)
+
+
+@contextlib.contextmanager
+def backend(name, conf, namespace=BACKEND_NAMESPACE, **kwargs):
+    """Fetches a jobboard backend, connects to it and allows it to be used in
+    a context manager statement with the jobboard being closed upon completion.
+    """
+    jb = fetch(name, conf, namespace=namespace, **kwargs)
+    jb.connect()
+    with contextlib.closing(jb):
+        yield jb

taskflow/jobs/jobboard.py

@@ -132,6 +132,18 @@ class JobBoard(object):
             this must be the same name that was used for claiming this job.
         """
 
+    @abc.abstractmethod
+    def connect(self):
+        """Opens the connection to any backend system."""
+
+    @abc.abstractmethod
+    def close(self):
+        """Close the connection to any backend system.
+
+        Once closed the jobboard can no longer be used (unless reconnection
+        occurs).
+        """
+
 
 # Jobboard events
 POSTED = 'POSTED'  # new job is/has been posted