==== Error ====
* service.version: 1.42.0-wmf.25
* trace.id: b346a0e8-4737-4458-bc27-d6b7390ed2a9
* [[ https://logstash.wikimedia.org/app/dashboards#/view/AXFV7JE83bOlOASGccsT?_g=(time:(from:'2024-04-03T10:52:06.087Z',to:'2024-04-04T11:12:11.019Z'))&_a=(query:(query_string:(query:'reqId:%22b346a0e8-4737-4458-bc27-d6b7390ed2a9%22'))) | Find trace.id in Logstash ]]
```name=labels.normalized_message,lines=10
[{reqId}] {exception_url} PHP Notice: Undefined offset: 10
```
```name=error.stack_trace,lines=10
from /srv/mediawiki/php-1.42.0-wmf.25/includes/libs/rdbms/loadbalancer/LoadBalancer.php(1849)
#0 /srv/mediawiki/php-1.42.0-wmf.25/includes/libs/rdbms/loadbalancer/LoadBalancer.php(1849): MWExceptionHandler::handleError(integer, string, string, integer, array)
#1 /srv/mediawiki/php-1.42.0-wmf.25/includes/api/ApiMain.php(1571): Wikimedia\Rdbms\LoadBalancer->getMaxLag()
#2 /srv/mediawiki/php-1.42.0-wmf.25/includes/api/ApiMain.php(1608): ApiMain->getMaxLag()
#3 /srv/mediawiki/php-1.42.0-wmf.25/includes/api/ApiMain.php(1934): ApiMain->checkMaxLag(Wikibase\Repo\Api\GetEntities, array)
#4 /srv/mediawiki/php-1.42.0-wmf.25/includes/api/ApiMain.php(922): ApiMain->executeAction()
#5 /srv/mediawiki/php-1.42.0-wmf.25/includes/api/ApiMain.php(893): ApiMain->executeActionWithErrorHandling()
#6 /srv/mediawiki/php-1.42.0-wmf.25/includes/api/ApiEntryPoint.php(158): ApiMain->execute()
#7 /srv/mediawiki/php-1.42.0-wmf.25/includes/MediaWikiEntryPoint.php(199): MediaWiki\Api\ApiEntryPoint->execute()
#8 /srv/mediawiki/php-1.42.0-wmf.25/api.php(44): MediaWiki\MediaWikiEntryPoint->run()
#9 /srv/mediawiki/w/api.php(3): require(string)
#10 {main}
```
==== Notes ====
Error started around 05:00 UTC on 2024-04-03 (Wednesday) with wmf.25 at group0 and affected both wmf.24 and wmf.25. Change https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1004792 modified function `getLagTimes` which returns the indices that are currently causing the out of range error. However, the change was merged on 2024-03-22 and had already gone out in the wmf.24 train ago. All of this makes me think some other factor started triggering the error on Wednesday.
The issue affects mainly wikidate and happens in bursts, which points at some kind of data dump/snapshot.