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/rest/media/v1/preview_url_resource.py

@@ -348,11 +348,16 @@ class PreviewUrlResource(Resource):
     def _expire_url_cache_data(self):
         """Clean up expired url cache content, media and thumbnails.
         """
-
         # TODO: Delete from backup media store
 
         now = self.clock.time_msec()
 
+        logger.info("Running url preview cache expiry")
+
+        if not self.store.has_completed_background_updates():
+            logger.info("Still running DB updates; skipping expiry")
+            return
+
         # First we delete expired url cache entries
         media_ids = yield self.store.get_expired_url_cache(now)
 
@@ -426,8 +431,7 @@ class PreviewUrlResource(Resource):
 
         yield self.store.delete_url_cache_media(removed_media)
 
-        if removed_media:
-            logger.info("Deleted %d media from url cache", len(removed_media))
+        logger.info("Deleted %d media from url cache", len(removed_media))
 
 
 def decode_and_calc_og(body, media_uri, request_encoding=None):

synapse/storage/background_updates.py

@@ -85,6 +85,7 @@ class BackgroundUpdateStore(SQLBaseStore):
         self._background_update_performance = {}
         self._background_update_queue = []
         self._background_update_handlers = {}
+        self._all_done = False
 
     @defer.inlineCallbacks
     def start_doing_background_updates(self):
@@ -106,8 +107,17 @@ class BackgroundUpdateStore(SQLBaseStore):
                         "No more background updates to do."
                         " Unscheduling background update task."
                     )
+                    self._all_done = True
                     defer.returnValue(None)
 
+    def has_completed_background_updates(self):
+        """Check if all the background updates have completed
+
+        Returns:
+            bool: True if all background updates have completed
+        """
+        return self._all_done
+
     @defer.inlineCallbacks
     def do_next_background_update(self, desired_duration_ms):
         """Does some amount of work on the next queued background update
@@ -269,7 +279,7 @@ class BackgroundUpdateStore(SQLBaseStore):
             # Sqlite doesn't support concurrent creation of indexes.
             #
             # We don't use partial indices on SQLite as it wasn't introduced
-            # until 3.8, and wheezy has 3.7
+            # until 3.8, and wheezy and CentOS 7 have 3.7
             #
             # We assume that sqlite doesn't give us invalid indices; however
             # we may still end up with the index existing but the

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 []
 

synapse/storage/schema/delta/44/expire_url_cache.sql

@@ -13,7 +13,10 @@
  * limitations under the License.
  */
 
-CREATE INDEX local_media_repository_url_idx ON local_media_repository(created_ts) WHERE url_cache IS NOT NULL;
+-- this didn't work on SQLite 3.7 (because of lack of partial indexes), so was
+-- removed and replaced with 46/local_media_repository_url_idx.sql.
+--
+-- CREATE INDEX local_media_repository_url_idx ON local_media_repository(created_ts) WHERE url_cache IS NOT NULL;
 
 -- we need to change `expires` to `expires_ts` so that we can index on it. SQLite doesn't support
 -- indices on expressions until 3.9.

synapse/storage/schema/delta/46/local_media_repository_url_idx.sql

@@ -0,0 +1,24 @@
+/* Copyright 2017 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * 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.
+ */
+
+-- register a background update which will recreate the
+-- local_media_repository_url_idx index.
+--
+-- We do this as a bg update not because it is a particularly onerous
+-- operation, but because we'd like it to be a partial index if possible, and
+-- the background_index_update code will understand whether we are on
+-- postgres or sqlite and behave accordingly.
+INSERT INTO background_updates (update_name, progress_json) VALUES
+    ('local_media_repository_url_idx', '{}');