From 1a2f4b55aa665f65d3fe1db44e0d520e337211d7 Mon Sep 17 00:00:00 2001
From: Aaron Schulz <aschulz@wikimedia.org>
Date: Wed, 22 May 2013 16:19:16 -0700
Subject: [PATCH] [Database] Tweaked LoadMonitor::getLagTimes() to further
 reduce stampedes

Change-Id: I79e3232e3e87bb298ae1a3fe214fcd2506818884
---
 includes/db/LoadMonitor.php | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/includes/db/LoadMonitor.php b/includes/db/LoadMonitor.php
index 4b9eccea4b..519e2dfd0e 100644
--- a/includes/db/LoadMonitor.php
+++ b/includes/db/LoadMonitor.php
@@ -142,12 +142,12 @@ class LoadMonitor_MySQL implements LoadMonitor {
 		$masterName = $this->parent->getServerName( 0 );
 		$memcKey = wfMemcKey( 'lag_times', $masterName );
 		$times = $wgMemc->get( $memcKey );
-		if ( $times ) {
+		if ( is_array( $times ) ) {
 			# Randomly recache with probability rising over $expiry
 			$elapsed = time() - $times['timestamp'];
 			$chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 			if ( mt_rand( 0, $chance ) != 0 ) {
-				unset( $times['timestamp'] );
+				unset( $times['timestamp'] ); // hide from caller
 				wfProfileOut( __METHOD__ );
 				return $times;
 			}
@@ -157,6 +157,17 @@ class LoadMonitor_MySQL implements LoadMonitor {
 		}
 
 		# Cache key missing or expired
+		if ( $wgMemc->add( "$memcKey:lock", 1, 10 ) ) {
+			# Let this process alone update the cache value
+			$unlocker = new ScopedCallback( function() use ( $wgMemc, $memcKey ) {
+				$wgMemc->delete( $memcKey );
+			} );
+		} elseif ( is_array( $times ) ) {
+			# Could not acquire lock but an old cache exists, so use it
+			unset( $times['timestamp'] ); // hide from caller
+			wfProfileOut( __METHOD__ );
+			return $times;
+		}
 
 		$times = array();
 		foreach ( $serverIndexes as $i ) {
@@ -171,14 +182,11 @@ class LoadMonitor_MySQL implements LoadMonitor {
 
 		# Add a timestamp key so we know when it was cached
 		$times['timestamp'] = time();
-		$wgMemc->set( $memcKey, $times, $expiry );
-
-		# But don't give the timestamp to the caller
-		unset( $times['timestamp'] );
-		$lagTimes = $times;
+		$wgMemc->set( $memcKey, $times, $expiry + 10 );
+		unset( $times['timestamp'] ); // hide from caller
 
 		wfProfileOut( __METHOD__ );
-		return $lagTimes;
+		return $times;
 	}
 
 	/**
-- 
2.20.1