Hello,
Deployment of MediaWiki 1.34.0-wmf.22 on the Wikimedia cluster is currently blocked. It is blocked on a php7.2 bug that has yet to be tracked and determined.
Immediately after deploying to group1 (which has commons and wikidata), I noticed an infamous error in the logs:
PHP Notice: Undefined index:"
Which sounds similar to known errors with php7.2 (T229433, T231089 among others) but this time it hits LBFactoryMulti.php which is for the database load balancing. So that sounds scary and I thus decided to block the train.
The notice happens at:
$groupLoads[ILoadBalancer::GROUP_GENERIC]
The GROUP_GENERIC constant is an empty string. Brad "anomie" Jorsch quickly found out the code always set that key!
https://phabricator.wikimedia.org/T232613#5485181
So what is left is a edge case issue in php 7.2, maybe in the opcache or something even scarier.
Thanks to Tim Starling, we have a solution to issue a core dump of php when that issue happens. There is already available on the cluster, but it has to be investigated which is not that trivial.