From 9a2ba8e21d820478f96adead39b544d92d1d6306 Mon Sep 17 00:00:00 2001
From: Aaron Schulz <aschulz@wikimedia.org>
Date: Mon, 11 Jun 2018 17:23:49 -0700
Subject: [PATCH] Reduce frequency of refreshCounts() calls in
 LinksDeletionUpdate

Bug: T195397
Change-Id: I0a39c735ec516b70c43c7a40583c43289550b687
---
 includes/Category.php                     |  2 +-
 includes/deferred/LinksDeletionUpdate.php | 47 ++++++++++++++++-------
 2 files changed, 35 insertions(+), 14 deletions(-)

diff --git a/includes/Category.php b/includes/Category.php
index 46b86d8cd8..fb5916bba2 100644
--- a/includes/Category.php
+++ b/includes/Category.php
@@ -48,7 +48,7 @@ class Category {
 
 	/**
 	 * Set up all member variables using a database query.
-	 * @param int $mode
+	 * @param int $mode One of (Category::LOAD_ONLY, Category::LAZY_INIT_ROW)
 	 * @throws MWException
 	 * @return bool True on success, false on failure.
 	 */
diff --git a/includes/deferred/LinksDeletionUpdate.php b/includes/deferred/LinksDeletionUpdate.php
index 3c86d11ac6..9f6257c810 100644
--- a/includes/deferred/LinksDeletionUpdate.php
+++ b/includes/deferred/LinksDeletionUpdate.php
@@ -90,6 +90,7 @@ class LinksDeletionUpdate extends DataUpdate implements EnqueueableDataUpdate {
 		foreach ( $catBatches as $catBatch ) {
 			$this->page->updateCategoryCounts( [], $catBatch, $id );
 			if ( count( $catBatches ) > 1 ) {
+				// Only sacrifice atomicity if necessary due to size
 				$lbFactory->commitAndWaitForReplication(
 					__METHOD__, $this->ticket, [ 'domain' => $dbw->getDomainID() ]
 				);
@@ -98,19 +99,10 @@ class LinksDeletionUpdate extends DataUpdate implements EnqueueableDataUpdate {
 
 		// Refresh counts on categories that should be empty now
 		if ( $title->getNamespace() === NS_CATEGORY ) {
-			$row = $dbw->selectRow(
-				'category',
-				[ 'cat_id', 'cat_title', 'cat_pages', 'cat_subcats', 'cat_files' ],
-				[ 'cat_title' => $title->getDBkey(), 'cat_pages <= 100' ],
-				__METHOD__
-			);
-			if ( $row ) {
-				$cat = Category::newFromRow( $row, $title );
-				// T166757: do the update after the main job DB commit
-				DeferredUpdates::addCallableUpdate( function () use ( $cat ) {
-					$cat->refreshCounts();
-				} );
-			}
+			// T166757: do the update after the main job DB commit
+			DeferredUpdates::addCallableUpdate( function () use ( $title ) {
+				$this->refreshCategoryIfEmpty( $title );
+			} );
 		}
 
 		$this->batchDeleteByPK(
@@ -195,6 +187,35 @@ class LinksDeletionUpdate extends DataUpdate implements EnqueueableDataUpdate {
 		ScopedCallback::consume( $scopedLock );
 	}
 
+	/**
+	 * @param Title $title
+	 */
+	private function refreshCategoryIfEmpty( Title $title ) {
+		$dbw = $this->getDB();
+
+		$row = $dbw->selectRow(
+			'category',
+			[ 'cat_id', 'cat_title', 'cat_pages', 'cat_subcats', 'cat_files' ],
+			[ 'cat_title' => $title->getDBkey(), 'cat_pages <= 100' ],
+			__METHOD__
+		);
+
+		if ( !$row ) {
+			return; // nothing to delete
+		}
+
+		$cat = Category::newFromRow( $row, $title );
+		$hasLink = $dbw->selectField(
+			'categorylinks',
+			'1',
+			[ 'cl_to' => $title->getDBkey() ],
+			__METHOD__
+		);
+		if ( !$hasLink ) {
+			$cat->refreshCounts(); // delete the category table entry
+		}
+	}
+
 	private function batchDeleteByPK( $table, array $conds, array $pk, $bSize ) {
 		$services = MediaWikiServices::getInstance();
 		$lbFactory = $services->getDBLoadBalancerFactory();
-- 
2.20.1