Use Delta for subscription calendars instead of deleting and reinserting all events

[ChristophWurst]

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

#42345 / #43261 shows a long running transaction for the webcal background sync:

Exception: Long running transaction
#9 /var/www/nextcloud/lib/private/DB/Connection.php(680): OC\DB\Connection::commit
#8 /var/www/nextcloud/lib/private/DB/ConnectionAdapter.php(154): OC\DB\ConnectionAdapter::commit
#7 /var/www/nextcloud/lib/public/AppFramework/Db/TTransactional.php(64): OCA\DAV\CalDAV\CalDavBackend::atomic
#6 /dav/lib/CalDAV/CalDavBackend.php(3075): OCA\DAV\CalDAV\CalDavBackend::purgeAllCachedEventsForSubscription
#5 /dav/lib/CalDAV/WebcalCaching/RefreshWebcalService.php(101): OCA\DAV\CalDAV\WebcalCaching\RefreshWebcalService::refreshSubscription
#4 /dav/lib/BackgroundJob/RefreshWebcalJob.php(115): OCA\DAV\BackgroundJob\RefreshWebcalJob::run
#3 /var/www/nextcloud/lib/public/BackgroundJob/Job.php(81): OCP\BackgroundJob\Job::start
#2 /var/www/nextcloud/lib/public/BackgroundJob/Job.php(67): OCP\BackgroundJob\Job::execute
#1 /dav/lib/BackgroundJob/RefreshWebcalJob.php(108): OCA\DAV\BackgroundJob\RefreshWebcalJob::execute
#0 /var/www/nextcloud/cron.php(152): null

Steps to reproduce

1. Set up Nextcloud 28.0.2
2. Install Calendar
3. Add a webcal subscription

Expected behavior

Fast transaction

Installation method

None

Nextcloud Server version

28

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

[ChristophWurst]

Stepped through the code quickly. Looks like

SELECT `synctoken` FROM `oc_calendarsubscriptions` WHERE `id` = :dcValue1;
INSERT INTO `oc_calendarchanges` (`uri`, `synctoken`, `calendarid`, `operation`, `calendartype`) VALUES(:dcValue1, :dcValue2, :dcValue3, :dcValue4, :dcValue5);
UPDATE `oc_calendarsubscriptions` SET `synctoken` = :dcValue1 WHERE `id` = :dcValue2;

is called for each item in the subscription. This subscription has >2k entries. That makes 6k queries just to purge old data.