A rebuilt sync engine now runs every sync

Over the past few weeks we moved every active sync onto a rebuilt sync engine. The last ones moved over the weekend of 26–27 September. You don't need to do anything.

What it does differently

• Every change is checked. After writing an event to a calendar, the engine reads it back to confirm it is the right event with the right content before counting it done. If something is unclear it re-reads the calendar before trying again, and after repeated failures it flags the sync for attention instead of reporting success.
• Protection against duplicates. Every copy carries a hidden marker pointing to its original. If the engine is not sure an earlier attempt worked, it looks for the existing copy rather than creating another, and it won't delete a copy it can't prove it made.
• More careful recurring events. Cancelled occurrences are applied when a series is first copied, and an occurrence is never removed just because a partial read didn't show it.
• Due syncs start sooner. A calendar that is due for its regular check is typically picked up within about 10 seconds, instead of waiting up to 30 minutes.
• A cleaner foundation. Each provider (Google, Outlook, CalDAV and iCal) has its own adapter, and every sync decision lives in one place. That makes future improvements and new providers safer to add.