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ClearMessageBlobs uses TRUNCATE TABLE in a tight loop:

foreach ( $wgConf->getLocalDatabases() as $wiki ) {
	$lb = wfGetLB( $wiki );
	$db = $lb->getConnection( DB_MASTER, array(), $wiki );
	$db->query( "TRUNCATE TABLE " . $db->tableName( 'msg_resource' ), __METHOD__ );
	$db->query( "TRUNCATE TABLE " . $db->tableName( 'msg_resource_links' ), __METHOD__ );
	$lb->reuseConnection( $db );
}

This results in a lot of replication slave lag on WMF production shards.

It should use a batched delete if possible, instead of truncate.

It should definitely check slave lag in the loop.