Add a new SWHTask class to be used as base class for swh celery tasks

It is meant to be used to declare swh tasks via the task decorator
instead of subclassing the (now deprecated) Task class.

It is typically used like:

  from swh.scheduler.celery_backend.config import app
  from swh.scheduler.tasks import SWHTask

  @app.task(base=SWHTask)
  def ping():
      return 'pong'

swh/scheduler/task.py

@@ -7,9 +7,35 @@ import celery.app.task
 from celery.utils.log import get_task_logger
 
 
-class Task(celery.app.task.Task):
+class SWHTask(celery.app.task.Task):
     """a schedulable task (abstract class)
 
+    Current implementation is based on Celery. See
+    http://docs.celeryproject.org/en/latest/reference/celery.app.task.html for
+    how to use tasks once instantiated
+
+    """
+
+    _log = None
+
+    def on_failure(self, exc, task_id, args, kwargs, einfo):
+        self.send_event('task-result-exception')
+
+    def on_success(self, retval, task_id, args, kwargs):
+        self.send_event('task-result', result=retval)
+
+    @property
+    def log(self):
+        if self._log is None:
+            self._log = get_task_logger(self.name)
+        return self._log
+
+
+class Task(SWHTask):
+    """a schedulable task (abstract class)
+
+    DEPRECATED! Please use SWHTask as base for decorated functions instead.
+
     Sub-classes must implement the run_task() method.
 
     Current implementation is based on Celery. See
@@ -25,14 +51,7 @@ class Task(celery.app.task.Task):
 
         Should not be overridden as we need our special events to be sent for
         the reccurrent scheduler. Override run_task instead."""
-        try:
-            result = self.run_task(*args, **kwargs)
-        except Exception as e:
-            self.send_event('task-result-exception')
-            raise e from None
-        else:
-            self.send_event('task-result', result=result)
-            return result
+        return self.run_task(*args, **kwargs)
 
     def run_task(self, *args, **kwargs):
         """Perform the task.
@@ -41,10 +60,3 @@ class Task(celery.app.task.Task):
         scheduler using a celery event.
         """
         raise NotImplementedError('tasks must implement the run_task() method')
-
-    @property
-    def log(self):
-        if not hasattr(self, '__log'):
-            self.__log = get_task_logger('%s.%s' %
-                                         (__name__, self.__class__.__name__))
-        return self.__log