From 467fbef034601ed172cbc643a633c18e5f69a2b6 Mon Sep 17 00:00:00 2001
From: Stephane Bisson <sbisson@wikimedia.org>
Date: Wed, 30 Aug 2017 11:31:41 -0400
Subject: [PATCH] WLFilters: fix server-side tag filtering

Tag filtering was enabled in the UI but the
backend was not properly wired for it.

Change-Id: I1c50b67b0a4aaddb561e79fdacd9849ffe0ded8c
---
 .../specialpage/ChangesListSpecialPage.php    |  1 +
 includes/specials/SpecialRecentchanges.php    |  1 -
 includes/specials/SpecialWatchlist.php        | 24 +++++++++++++++----
 3 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/includes/specialpage/ChangesListSpecialPage.php b/includes/specialpage/ChangesListSpecialPage.php
index 4d27d35dd4..acd5d14fe6 100644
--- a/includes/specialpage/ChangesListSpecialPage.php
+++ b/includes/specialpage/ChangesListSpecialPage.php
@@ -904,6 +904,7 @@ abstract class ChangesListSpecialPage extends SpecialPage {
 		$opts->add( 'invert', false );
 		$opts->add( 'associated', false );
 		$opts->add( 'urlversion', 1 );
+		$opts->add( 'tagfilter', '' );
 
 		return $opts;
 	}
diff --git a/includes/specials/SpecialRecentchanges.php b/includes/specials/SpecialRecentchanges.php
index 6ee697ebc4..547a1b06c2 100644
--- a/includes/specials/SpecialRecentchanges.php
+++ b/includes/specials/SpecialRecentchanges.php
@@ -241,7 +241,6 @@ class SpecialRecentChanges extends ChangesListSpecialPage {
 
 		$opts->add( 'categories', '' );
 		$opts->add( 'categories_any', false );
-		$opts->add( 'tagfilter', '' );
 
 		return $opts;
 	}
diff --git a/includes/specials/SpecialWatchlist.php b/includes/specials/SpecialWatchlist.php
index 0dd66b023d..07889b7a9d 100644
--- a/includes/specials/SpecialWatchlist.php
+++ b/includes/specials/SpecialWatchlist.php
@@ -382,10 +382,6 @@ class SpecialWatchlist extends ChangesListSpecialPage {
 		$tables = array_merge( [ 'recentchanges', 'watchlist' ], $tables );
 		$fields = array_merge( RecentChange::selectFields(), $fields );
 
-		$query_options = array_merge( [
-			'ORDER BY' => 'rc_timestamp DESC',
-			'LIMIT' => $opts['limit']
-		], $query_options );
 		$join_conds = array_merge(
 			[
 				'watchlist' => [
@@ -422,13 +418,14 @@ class SpecialWatchlist extends ChangesListSpecialPage {
 			], LIST_OR );
 		}
 
+		$tagFilter = $opts['tagfilter'] ? explode( '|', $opts['tagfilter'] ) : [];
 		ChangeTags::modifyDisplayQuery(
 			$tables,
 			$fields,
 			$conds,
 			$join_conds,
 			$query_options,
-			''
+			$tagFilter
 		);
 
 		$this->runMainQueryHook( $tables, $fields, $conds, $query_options, $join_conds, $opts );
@@ -437,6 +434,23 @@ class SpecialWatchlist extends ChangesListSpecialPage {
 			return false;
 		}
 
+		$orderByAndLimit = [
+			'ORDER BY' => 'rc_timestamp DESC',
+			'LIMIT' => $opts['limit']
+		];
+		if ( in_array( 'DISTINCT', $query_options ) ) {
+			// ChangeTags::modifyDisplayQuery() adds DISTINCT when filtering on multiple tags.
+			// In order to prevent DISTINCT from causing query performance problems,
+			// we have to GROUP BY the primary key. This in turn requires us to add
+			// the primary key to the end of the ORDER BY, and the old ORDER BY to the
+			// start of the GROUP BY
+			$orderByAndLimit['ORDER BY'] = 'rc_timestamp DESC, rc_id DESC';
+			$orderByAndLimit['GROUP BY'] = 'rc_timestamp, rc_id';
+		}
+		// array_merge() is used intentionally here so that hooks can, should
+		// they so desire, override the ORDER BY / LIMIT condition(s)
+		$query_options = array_merge( $orderByAndLimit, $query_options );
+
 		return $dbr->select(
 			$tables,
 			$fields,
-- 
2.20.1