Fix error on sqlite 3.7

Create the url_cache index on local_media_repository as a background update, so
that we can detect whether we are on sqlite or not and create a partial or
complete index accordingly.

To avoid running the cleanup job before we have built the index, add a bailout
which will defer the cleanup if the bg updates are still running.

Fixes https://github.com/matrix-org/synapse/issues/2572.

synapse/storage/media_repository.py

@@ -12,13 +12,23 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
-from ._base import SQLBaseStore
+from synapse.storage.background_updates import BackgroundUpdateStore
 
 
-class MediaRepositoryStore(SQLBaseStore):
+class MediaRepositoryStore(BackgroundUpdateStore):
     """Persistence for attachments and avatars"""
 
+    def __init__(self, db_conn, hs):
+        super(MediaRepositoryStore, self).__init__(db_conn, hs)
+
+        self.register_background_index_update(
+            update_name='local_media_repository_url_idx',
+            index_name='local_media_repository_url_idx',
+            table='local_media_repository',
+            columns=['created_ts'],
+            where_clause='url_cache IS NOT NULL',
+        )
+
     def get_default_thumbnails(self, top_level_type, sub_type):
         return []