From 84758e6f107f939f313563964b2b31f4a71efad0 Mon Sep 17 00:00:00 2001
From: Aaron Schulz <aschulz@wikimedia.org>
Date: Thu, 21 May 2015 22:46:25 -0700
Subject: [PATCH] Made WANObjectCache::getCheckKey() automatically init the key

* This still allows If-Modified-Since logic but does not
  need to broadcast initialization values just because a
  key fell out of cache. The value can differ between DCs
  anyway via skew, this just lets them drift more. Actual
  purge events are still broadcasted, which is what matters.
* The User class has now been simplified given this change.
* Added more general comments to getCheckKeyTime().

Change-Id: Ic1f4bbb1947e0d1dd47499c9e9dc86991c30580c
---
 includes/User.php                             | 13 +++++------
 includes/libs/objectcache/WANObjectCache.php  | 23 +++++++++++++++++--
 .../objectcache/WANObjectCacheTest.php        |  4 +++-
 3 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/includes/User.php b/includes/User.php
index 46d53b876e..63c0d37e45 100644
--- a/includes/User.php
+++ b/includes/User.php
@@ -2326,6 +2326,10 @@ class User implements IDBAccessObject {
 
 	/**
 	 * Get the user touched timestamp
+	 *
+	 * Use this value only to validate caches via inequalities
+	 * such as in the case of HTTP If-Modified-Since response logic
+	 *
 	 * @return string TS_MW Timestamp
 	 */
 	public function getTouched() {
@@ -2334,14 +2338,9 @@ class User implements IDBAccessObject {
 		if ( $this->mId ) {
 			if ( $this->mQuickTouched === null ) {
 				$key = wfMemcKey( 'user-quicktouched', 'id', $this->mId );
+				$cache = ObjectCache::getMainWANInstance();
 
-				$timestamp = ObjectCache::getMainWANInstance()->getCheckKeyTime( $key );
-				if ( $timestamp ) {
-					$this->mQuickTouched = wfTimestamp( TS_MW, (int)$timestamp );
-				} else {
-					# Set the timestamp to get HTTP 304 cache hits
-					$this->touch();
-				}
+				$this->mQuickTouched = wfTimestamp( TS_MW, $cache->getCheckKeyTime( $key ) );
 			}
 
 			return max( $this->mTouched, $this->mQuickTouched );
diff --git a/includes/libs/objectcache/WANObjectCache.php b/includes/libs/objectcache/WANObjectCache.php
index 5d9557a1d6..c32efb9776 100644
--- a/includes/libs/objectcache/WANObjectCache.php
+++ b/includes/libs/objectcache/WANObjectCache.php
@@ -279,13 +279,32 @@ class WANObjectCache {
 	/**
 	 * Fetch the value of a timestamp "check" key
 	 *
+	 * The key will be *initialized* to the current time if not set,
+	 * so only call this method if this behavior is actually desired
+	 *
+	 * The timestamp can be used to check whether a cached value is valid.
+	 * Callers should not assume that this returns the same timestamp in
+	 * all datacenters due to relay delays.
+	 *
+	 * The level of staleness can roughly be estimated from this key, but
+	 * if the key was evicted from cache, such calculations may show the
+	 * time since expiry as ~0 seconds.
+	 *
 	 * Note that "check" keys won't collide with other regular keys
 	 *
 	 * @param string $key
-	 * @return float|bool TS_UNIX timestamp of the key; false if not present
+	 * @return float UNIX timestamp of the key
 	 */
 	final public function getCheckKeyTime( $key ) {
-		return self::parsePurgeValue( $this->cache->get( self::TIME_KEY_PREFIX . $key ) );
+		$key = self::TIME_KEY_PREFIX . $key;
+
+		$time = self::parsePurgeValue( $this->cache->get( $key ) );
+		if ( $time === false ) {
+			$time = microtime( true );
+			$this->cache->add( $key, self::PURGE_VAL_PREFIX . $time, self::CHECK_KEY_TTL );
+		}
+
+		return $time;
 	}
 
 	/**
diff --git a/tests/phpunit/includes/objectcache/WANObjectCacheTest.php b/tests/phpunit/includes/objectcache/WANObjectCacheTest.php
index 47a83b3fe2..10f64be716 100644
--- a/tests/phpunit/includes/objectcache/WANObjectCacheTest.php
+++ b/tests/phpunit/includes/objectcache/WANObjectCacheTest.php
@@ -229,8 +229,10 @@ class WANObjectCacheTest extends MediaWikiTestCase {
 	public function testTouchKeys() {
 		$key = wfRandomString();
 
+		$priorTime = microtime( true );
+		usleep( 1 );
 		$t0 = $this->cache->getCheckKeyTime( $key );
-		$this->assertFalse( $t0, 'Check key time is false' );
+		$this->assertGreaterThanOrEqual( $priorTime, $t0, 'Check key auto-created' );
 
 		$priorTime = microtime( true );
 		usleep( 1 );
-- 
2.20.1