From 2bcf34421a11f891c08c13922ae6b5386f0c9fa2 Mon Sep 17 00:00:00 2001
From: Bryan Davis <bd808@wikimedia.org>
Date: Mon, 22 Feb 2016 18:45:20 -0700
Subject: [PATCH] CachedBagOStuff: cache backend misses

Cache misses from the backend cache the same as hits.

Bug: T127772
Change-Id: If2fe1920411b24862acea888c627db13717da8bd
---
 includes/libs/objectcache/CachedBagOStuff.php  |  6 ++----
 includes/libs/objectcache/HashBagOStuff.php    | 13 ++++++++++++-
 .../libs/objectcache/CachedBagOStuffTest.php   | 18 ++++++++++++++++++
 3 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/includes/libs/objectcache/CachedBagOStuff.php b/includes/libs/objectcache/CachedBagOStuff.php
index 798357d596..3d5d383e42 100644
--- a/includes/libs/objectcache/CachedBagOStuff.php
+++ b/includes/libs/objectcache/CachedBagOStuff.php
@@ -50,11 +50,9 @@ class CachedBagOStuff extends HashBagOStuff {
 
 	protected function doGet( $key, $flags = 0 ) {
 		$ret = parent::doGet( $key, $flags );
-		if ( $ret === false ) {
+		if ( $ret === false && !$this->hasKey( $key ) ) {
 			$ret = $this->backend->doGet( $key, $flags );
-			if ( $ret !== false ) {
-				$this->set( $key, $ret, 0, self::WRITE_CACHE_ONLY );
-			}
+			$this->set( $key, $ret, 0, self::WRITE_CACHE_ONLY );
 		}
 		return $ret;
 	}
diff --git a/includes/libs/objectcache/HashBagOStuff.php b/includes/libs/objectcache/HashBagOStuff.php
index 6e7fb0cae0..e03cec6a2f 100644
--- a/includes/libs/objectcache/HashBagOStuff.php
+++ b/includes/libs/objectcache/HashBagOStuff.php
@@ -60,8 +60,19 @@ class HashBagOStuff extends BagOStuff {
 		return true;
 	}
 
+	/**
+	 * Does this bag have a non-null value for the given key?
+	 *
+	 * @param string $key
+	 * @return bool
+	 * @since 1.27
+	 */
+	protected function hasKey( $key ) {
+		return isset( $this->bag[$key] );
+	}
+
 	protected function doGet( $key, $flags = 0 ) {
-		if ( !isset( $this->bag[$key] ) ) {
+		if ( !$this->hasKey( $key ) ) {
 			return false;
 		}
 
diff --git a/tests/phpunit/includes/libs/objectcache/CachedBagOStuffTest.php b/tests/phpunit/includes/libs/objectcache/CachedBagOStuffTest.php
index 3b19c9a88e..7fe8055444 100644
--- a/tests/phpunit/includes/libs/objectcache/CachedBagOStuffTest.php
+++ b/tests/phpunit/includes/libs/objectcache/CachedBagOStuffTest.php
@@ -49,4 +49,22 @@ class CachedBagOStuffTest extends PHPUnit_Framework_TestCase {
 		$cache->delete( 'foo', CachedBagOStuff::WRITE_CACHE_ONLY );
 		$this->assertEquals( 'old', $cache->get( 'foo' ) ); // Reloaded from backend
 	}
+
+	public function testCacheBackendMisses() {
+		$backend = new HashBagOStuff;
+		$cache = new CachedBagOStuff( $backend );
+
+		// First hit primes the cache with miss from the backend
+		$this->assertEquals( false, $cache->get( 'foo' ) );
+
+		// Change the value in the backend
+		$backend->set( 'foo', true );
+
+		// Second hit returns the cached miss
+		$this->assertEquals( false, $cache->get( 'foo' ) );
+
+		// But a fresh value is read from the backend
+		$backend->set( 'bar', true );
+		$this->assertEquals( true, $cache->get( 'bar' ) );
+	}
 }
-- 
2.20.1