From d7845da3f6829bd4a82a4fbd4a272252471896a7 Mon Sep 17 00:00:00 2001
From: Stephane Bisson <sbisson@wikimedia.org>
Date: Mon, 12 Sep 2016 12:05:33 -0400
Subject: [PATCH] resetNotificationCount() from replica with no lag

When notifications are being moderated, the unread count
for affected users has to be recalculated.

It was initially done using DB_SLAVE but it was leading
to inconsistent data since that database did not know
about recently deleted notifications.

It was changed to DB_MASTER but it potentially led
to too many queries on master.

This patch tries to wait for the replica to have
caught up with those changes and use it to update
the unread count for the affected users.

Bug: T93673
Change-Id: Ib4a845e82f686dd7ed807ab21a28490014b56604
---
 includes/controller/ModerationController.php | 22 +++++++++++++-------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/includes/controller/ModerationController.php b/includes/controller/ModerationController.php
index c849c6270..0c45357ed 100644
--- a/includes/controller/ModerationController.php
+++ b/includes/controller/ModerationController.php
@@ -1,4 +1,5 @@
 <?php
+use MediaWiki\MediaWikiServices;
 
 /**
  * This class represents the controller for moderating notifications
@@ -23,13 +24,18 @@ class EchoModerationController {
 		$affectedUserIds = $notificationMapper->fetchUsersWithNotificationsForEvents( $eventIds );
 		$eventMapper->toggleDeleted( $eventIds, $moderate );
 
-		/**
-		 * Recompute the notification count for the
-		 * users whose notifications have been moderated.
-		 */
-		foreach ( $affectedUserIds as $userId ) {
-			$user = User::newFromId( $userId );
-			MWEchoNotifUser::newFromUser( $user )->resetNotificationCount( DB_MASTER );
-		}
+		DeferredUpdates::addCallableUpdate( function () {
+			// This udate runs after than main transaction round commits.
+			// Wait for the events deletions to be propagated to replica DBs
+			$lbFactory = MediaWikiServices::getInstance()->getDBLoadBalancerFactory();
+			$lbFactory->waitForReplication( [ 'timeout' => 5 ] );
+			$lbFactory->flushReplicaSnapshots( __METHOD__ );
+			// Recompute the notification count for the
+			// users whose notifications have been moderated.
+			foreach ( $affectedUserIds as $userId ) {
+				$user = User::newFromId( $userId );
+				MWEchoNotifUser::newFromUser( $user )->resetNotificationCount( DB_REPLICA );
+			}
+		} );
 	}
 }