fix(dav): Strip DTSTAMP before etag comparison in webcal refresh

[Olen]

Summary

• Exclude DTSTAMP from etag computation in RefreshWebcalService to prevent false change detection
• DTSTAMP is preserved in stored calendar data (required property per RFC 5545)

Problem

Many iCal providers (Google Calendar, Outlook 365, itslearning) set DTSTAMP to the current UTC time on every feed request per RFC 5545. Since Nextcloud computes the etag as md5(serialized_data) and DTSTAMP changes every time, every event appears modified on every refresh — even if nothing actually changed.

For a Google Calendar subscription with 2,621 events refreshed every ~15 minutes, this generates ~189K false change rows per day in oc_calendarchanges.

Reported at: #51120 (comment)

Approach

Instead of stripping DTSTAMP from the VObject (which would remove a required property from stored data), the fix strips DTSTAMP lines from a copy of the serialized string used only for etag computation:

$sObjectForEtag = preg_replace('/^DTSTAMP:.*\r?\n/m', '', $sObject);
$etag = md5($sObjectForEtag);

The stored calendar data ($sObject) is completely unchanged.

Test plan

• New test testDtstampChangeDoesNotTriggerUpdate verifies a DTSTAMP-only change does not trigger updateCalendarObject
• Updated identicalDataProvider etag computation to match the new DTSTAMP-stripped hashing
• Run RefreshWebcalServiceTest unit tests

Fixes: #51120

🤖 Generated with Claude Code

[tcitworld]

As the DTSTAMP is a required property, we shouldn't remove it from the saved payload, but just do the md5 hash without it.

[Olen]

Thanks for the review @tcitworld — you're absolutely right. DTSTAMP is a required property per RFC 5545 and shouldn't be removed from the stored data.

Apologies for being too quick with the initial approach. I've reworked the fix to be much more minimal: instead of stripping DTSTAMP from the VObject before serialization, it now only strips the DTSTAMP lines from a copy of the serialized string used for the etag/md5 computation. The stored calendar data is completely unchanged — DTSTAMP is preserved.

The diff is now just one line replaced in RefreshWebcalService.php:

-$etag = md5($sObject);
+$sObjectForEtag = preg_replace('/^DTSTAMP:.*\r?\n/m', '', $sObject);
+$etag = md5($sObjectForEtag);

[tcitworld]

There is a risk that the line might be over 75 characters even in this case (for instance if we have a TZID), so I would handle content folding as well in the regex (new line must not be followed by a white-space character).

https://icalendar.org/iCalendar-RFC-5545/3-1-content-lines.html