From 6b2f13b055c84d7672652f54deee096762c525cb Mon Sep 17 00:00:00 2001
From: Aaron Schulz <aschulz@wikimedia.org>
Date: Wed, 18 Oct 2017 21:03:26 -0700
Subject: [PATCH] objectcache: Always use interim values on WAN cache
 tombstones

This stores values for very short times while the main
value is a tombstone in case of particularly high traffic.

Also make mutex keys expire immediately on unlock.

Change-Id: I4ec5cf7f8b49239fdd2518e5d955534877a0f7ee
---
 includes/libs/objectcache/WANObjectCache.php              | 8 ++++++--
 .../includes/libs/objectcache/WANObjectCacheTest.php      | 6 ++++--
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/includes/libs/objectcache/WANObjectCache.php b/includes/libs/objectcache/WANObjectCache.php
index 15e5759ce8..120ae453c0 100644
--- a/includes/libs/objectcache/WANObjectCache.php
+++ b/includes/libs/objectcache/WANObjectCache.php
@@ -964,6 +964,10 @@ class WANObjectCache implements IExpiringStore, LoggerAwareInterface {
 
 		// A deleted key with a negative TTL left must be tombstoned
 		$isTombstone = ( $curTTL !== null && $value === false );
+		if ( $isTombstone && $lockTSE <= 0 ) {
+			// Use the INTERIM value for tombstoned keys to reduce regeneration load
+			$lockTSE = 1;
+		}
 		// Assume a key is hot if requested soon after invalidation
 		$isHot = ( $curTTL !== null && $curTTL <= 0 && abs( $curTTL ) <= $lockTSE );
 		// Use the mutex if there is no value and a busy fallback is given
@@ -1032,7 +1036,7 @@ class WANObjectCache implements IExpiringStore, LoggerAwareInterface {
 
 		if ( $lockAcquired ) {
 			// Avoid using delete() to avoid pointless mcrouter broadcasting
-			$this->cache->changeTTL( self::MUTEX_KEY_PREFIX . $key, 1 );
+			$this->cache->changeTTL( self::MUTEX_KEY_PREFIX . $key, (int)$preCallbackTime - 60 );
 		}
 
 		return $value;
@@ -1751,7 +1755,7 @@ class WANObjectCache implements IExpiringStore, LoggerAwareInterface {
 		return array_diff( $keys, $keysFound );
 	}
 
-		/**
+	/**
 	 * @param array $keys
 	 * @param array $checkKeys
 	 * @return array Map of (cache key => mixed)
diff --git a/tests/phpunit/includes/libs/objectcache/WANObjectCacheTest.php b/tests/phpunit/includes/libs/objectcache/WANObjectCacheTest.php
index c5a1759ff9..e23f3183df 100644
--- a/tests/phpunit/includes/libs/objectcache/WANObjectCacheTest.php
+++ b/tests/phpunit/includes/libs/objectcache/WANObjectCacheTest.php
@@ -159,8 +159,9 @@ class WANObjectCacheTest extends PHPUnit_Framework_TestCase {
 		$this->assertEquals( 9, $hit, "Values evicted" );
 
 		$key = reset( $keys );
-		// Get into cache
+		// Get into cache (default process cache group)
 		$this->cache->getWithSetCallback( $key, 100, $callback, [ 'pcTTL' => 5 ] );
+		$this->assertEquals( 10, $hit, "Value calculated" );
 		$this->cache->getWithSetCallback( $key, 100, $callback, [ 'pcTTL' => 5 ] );
 		$this->assertEquals( 10, $hit, "Value cached" );
 		$outerCallback = function () use ( &$callback, $key ) {
@@ -168,7 +169,8 @@ class WANObjectCacheTest extends PHPUnit_Framework_TestCase {
 
 			return 43 + $v;
 		};
-		$this->cache->getWithSetCallback( $key, 100, $outerCallback );
+		// Outer key misses and refuses inner key process cache value
+		$this->cache->getWithSetCallback( "$key-miss-outer", 100, $outerCallback );
 		$this->assertEquals( 11, $hit, "Nested callback value process cache skipped" );
 	}
 
-- 
2.20.1