CalDAV sync-collection REPORT with empty sync-token returns no items

[silbox-ch]

Description

When a CalDAV client performs an initial sync-collection REPORT (RFC 6578) with an empty sync-token (as required for initial synchronization), Stalwart returns only the collection itself instead of listing all calendar items. This breaks the initial sync for clients that prefer sync-collection over calendar-query, such as eM Client.

Steps to Reproduce

1. Have a calendar with events at /dav/cal/user%40domain/default/
2. Send a sync-collection REPORT with an empty sync-token:

<?xml version="1.0" encoding="UTF-8"?>
<d:sync-collection xmlns:d="DAV:">
  <d:sync-token/>
  <d:prop>
    <d:getetag/>
    <d:getcontenttype/>
  </d:prop>
</d:sync-collection>

3. Observe the response

Expected Behavior

Per RFC 6578 §3.8:

If the DAV:sync-token element is empty, the server MUST return all member URLs.

The response should list all .ics items in the calendar with their etags.

Actual Behavior

Stalwart returns only the collection itself (1 <D:response>) with the current sync-token, but no individual calendar items:

<?xml version="1.0" encoding="UTF-8"?>
<D:multistatus xmlns:D="DAV:" xmlns:A="urn:ietf:params:xml:ns:caldav">
  <D:response>
    <D:href>/dav/cal/user%40domain/default/</D:href>
    <D:propstat>
      <D:prop><D:getetag>"2242719613"</D:getetag></D:prop>
      <D:status>HTTP/1.1 200 OK</D:status>
    </D:propstat>
  </D:response>
  <D:sync-token>urn:stalwart:davsync:f0c6</D:sync-token>
</D:multistatus>

The same calendar has 828 VEVENT items that are correctly returned via a calendar-query REPORT.

Impact

• eM Client uses sync-collection as its primary sync mechanism. Since the initial sync returns 0 items, users see an empty calendar despite having hundreds of events. Contacts (CardDAV) work fine.
• Any CalDAV client that prefers sync-collection over calendar-query will be affected.
• Delta syncs (with a valid sync-token) seem to work correctly — only the initial sync with an empty token is broken.

Environment

• Stalwart version: 0.15.4
• Storage: RocksDB
• Affected client: eM Client (tested), likely any client using sync-collection for initial sync
• Working clients: Thunderbird (uses calendar-query instead)

Related

• eM Client forum: https://forum.emclient.com/t/caldav-calendar-not-syncing-on-desktop-and-android-app/110018
• eM Client developer confirmed: "the server's behavior goes against the specification"
• Previously opened as CalDAV sync-collection REPORT with empty sync-token returns no items #2783, closed because the process wasn't followed. Resubmitting here as a discussion per repository guidelines.

[mdecimus]

Based on the sample request you provided, emClient is not providing sync-level and Stalwart is defaulting to 1. According to the XML DTD, including sync-level is required:

<!ELEMENT sync-collection (sync-token, sync-level, limit?, prop)>