Make PuSHing the Pubhubsubbub server an async task (#436, #585)

Notifying the PuSH servers had 3 problems. 

1) it was done immediately after sending of the processing task to celery. So if celery was run in a separate
process we would notify the PuSH servers before the new media was processed/
visible. (#436)

2) Notification code was called in submit/views.py, so submitting via the
   API never resulted in notifications. (#585)

3) If Notifying the PuSH server failed, we would never retry.

The solution was to make the PuSH notification an asynchronous subtask. This
way: 1) it will only be called once async processing has finished, 2) it
is in the main processing code path, so even API calls will result in
notifications, and 3) We retry 3 times in case of failure before giving up.
If the server is in a separate process, we will wait 3x 2 minutes before
retrying the notification.

The only downside is that the celery server needs to have access to the internet
to ping the PuSH server. If that is a problem, we need to make the task belong
to a special group of celery servers that has access to the internet.

As a side effect, I believe I removed the limitation that prevented us from
upgrading celery.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

mediagoblin/processing/task.py

@@ -15,8 +15,10 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import logging
+import urllib
+import urllib2
 
-from celery.task import Task
+from celery import registry, task
 
 from mediagoblin import mg_globals as mgg
 from mediagoblin.db.models import MediaEntry
@@ -28,18 +30,51 @@ logging.basicConfig()
 _log.setLevel(logging.DEBUG)
 
 
+@task.task(default_retry_delay=2 * 60)
+def handle_push_urls(feed_url):
+    """Subtask, notifying the PuSH servers of new content
+
+    Retry 3 times every 2 minutes if run in separate process before failing."""
+    if not mgg.app_config["push_urls"]:
+        return # Nothing to do
+    _log.debug('Notifying Push servers for feed {0}'.format(feed_url))
+    hubparameters = {
+        'hub.mode': 'publish',
+        'hub.url': feed_url}
+    hubdata = urllib.urlencode(hubparameters)
+    hubheaders = {
+        "Content-type": "application/x-www-form-urlencoded",
+        "Connection": "close"}
+    for huburl in mgg.app_config["push_urls"]:
+        hubrequest = urllib2.Request(huburl, hubdata, hubheaders)
+        try:
+            hubresponse = urllib2.urlopen(hubrequest)
+        except (urllib2.HTTPError, urllib2.URLError) as exc:
+            # We retry by default 3 times before failing
+            _log.info("PuSH url %r gave error %r", huburl, exc)
+            try:
+                return handle_push_urls.retry(exc=exc, throw=False)
+            except Exception as e:
+                # All retries failed, Failure is no tragedy here, probably.
+                _log.warn('Failed to notify PuSH server for feed {0}. '
+                          'Giving up.'.format(feed_url))
+                return False
+
 ################################
 # Media processing initial steps
 ################################
 
-class ProcessMedia(Task):
+class ProcessMedia(task.Task):
     """
     Pass this entry off for processing.
     """
-    def run(self, media_id):
+    def run(self, media_id, feed_url):
         """
         Pass the media entry off to the appropriate processing function
         (for now just process_image...)
+
+        :param feed_url: The feed URL that the PuSH server needs to be
+            updated for.
         """
         entry = MediaEntry.query.get(media_id)
 
@@ -58,6 +93,9 @@ class ProcessMedia(Task):
             entry.state = u'processed'
             entry.save()
 
+            # Notify the PuSH servers as async task
+            handle_push_urls.subtask().delay(feed_url)
+
             json_processing_callback(entry)
         except BaseProcessingFail as exc:
             mark_entry_failed(entry.id, exc)
@@ -97,3 +135,7 @@ class ProcessMedia(Task):
 
         entry = mgg.database.MediaEntry.query.filter_by(id=entry_id).first()
         json_processing_callback(entry)
+
+# Register the task
+process_media = registry.tasks[ProcessMedia.name]
+