objectcache: avoid duplicate set() calls with lockTSE when no value is in cache

Each thread will still run the callback, but only one will save the value back

Bug: T203786
Change-Id: Idc4738aa005cc44ec0f1adc6dcf2e3f87d0c9480

diff --git a/includes/libs/objectcache/WANObjectCache.php b/includes/libs/objectcache/WANObjectCache.php
index 6e8d266..88f87f8 100644 (file)
--- a/includes/libs/objectcache/WANObjectCache.php
+++ b/includes/libs/objectcache/WANObjectCache.php
@@ -1334,7 +1334,8 @@ class WANObjectCache implements IExpiringStore, LoggerAwareInterface {
                        $this->setInterimValue( $key, $wrapped, $tempTTL );
                }
 
-               if ( $valueIsCacheable ) {
+               // Save the value unless a mutex-winning thread is already expected to do that
+               if ( $valueIsCacheable && ( !$useMutex || $lockAcquired ) ) {
                        $setOpts['lockTSE'] = $lockTSE;
                        $setOpts['staleTTL'] = $staleTTL;
                        // Use best known "since" timestamp if not provided