Bug 1217544 is extracting some mozStorage-related functionality out of dom/cache so dom/backgroundsync could use it. (Namely mozStorageConnectionUtils.{h,cpp} and IncrementalVacuumConnect.{h,cpp}. Discussion starts at https://bugzilla.mozilla.org/show_bug.cgi?id=1217544#c129.) One conclusion was that IncrementalVacuumConnection (which exists only to provide automatic incremental vacuuming on close) really doesn't need to exist as a decorator and its logic can be integrated into our mozIStorageConnection implementation directly. Care should be taken to emphasize in comments and documentation that incremental vacuum is about minimizing disk usage of a file, not about reducing on-disk fragmentation or btree fragmentation. (mozStorageConnectionUtils' IncrementalVacuum method has some good comments to this effect about how aMaxFreePages is used to create hysteresis so that we avoid creating pathological on-disk fragmentation.)