From 1d526cae8112bdedbe03218ef0be6a4cd60d78d7 Mon Sep 17 00:00:00 2001
From: Ori Livneh <ori@wikimedia.org>
Date: Tue, 28 Jul 2015 16:22:57 -0700
Subject: [PATCH] RedisBagOStuff: if no alternatives, skip master link status
 check

If RedisBagOStuff::getConnection() is able to establish a connection, only
check the master link status if automatic failover is enabled and if there are
other viable servers left to consider. If there are no servers left to
consider, or if automatic failover is not configured, just return the
connection handle without subjecting it to further tests.

This will have the side-effect of making RedisBagOStuff compatible with
Nutcracker, which does not implement the INFO command. This is because when
MediaWiki is configured to use Nutcracker, the server pool will consist of a
single server (namely, Nutcracker itself), and thus there will be no other
server to consider, so INFO will never be executed.

Change-Id: I3812ec5a0b22df122bdf44350bc0496574c02ce8
---
 includes/objectcache/RedisBagOStuff.php | 46 ++++++++++++++++---------
 1 file changed, 29 insertions(+), 17 deletions(-)

diff --git a/includes/objectcache/RedisBagOStuff.php b/includes/objectcache/RedisBagOStuff.php
index b8a0dd5c2f..7e506f0d7a 100644
--- a/includes/objectcache/RedisBagOStuff.php
+++ b/includes/objectcache/RedisBagOStuff.php
@@ -372,31 +372,30 @@ class RedisBagOStuff extends BagOStuff {
 			}
 		}
 
-		foreach ( $candidates as $tag ) {
+		while ( ( $tag = array_shift( $candidates ) ) !== false ) {
 			$server = $this->serverTagMap[$tag];
-
 			$conn = $this->redisPool->getConnection( $server );
 			if ( !$conn ) {
 				continue;
 			}
 
-			try {
-				$info = $conn->info();
-				// Check if this server has an unreachable redis master
-				if ( $info['role'] === 'slave'
-					&& $info['master_link_status'] === 'down'
-					&& $this->automaticFailover
-				) {
-					// If the master cannot be reached, fail-over to the next server.
-					// If masters are in data-center A, and slaves in data-center B,
-					// this helps avoid the case were fail-over happens in A but not
-					// to the corresponding server in B (e.g. read/write mismatch).
+			// If automatic failover is enabled, check that the server's link
+			// to its master (if any) is up -- but only if there are other
+			// viable candidates left to consider.
+			if ( $this->automaticFailover && $candidates ) {
+				try {
+					if ( $this->getMasterLinkStatus( $conn ) === 'down' ) {
+						// If the master cannot be reached, fail-over to the next server.
+						// If masters are in data-center A, and slaves in data-center B,
+						// this helps avoid the case were fail-over happens in A but not
+						// to the corresponding server in B (e.g. read/write mismatch).
+						continue;
+					}
+				} catch ( RedisException $e ) {
+					// Server is not accepting commands
+					$this->handleException( $conn, $e );
 					continue;
 				}
-			} catch ( RedisException $e ) {
-				// Server is not accepting commands
-				$this->handleException( $conn, $e );
-				continue;
 			}
 
 			return array( $server, $conn );
@@ -407,6 +406,19 @@ class RedisBagOStuff extends BagOStuff {
 		return array( false, false );
 	}
 
+	/**
+	 * Check the master link status of a Redis server that is configured as a slave.
+	 * @param RedisConnRef $conn
+	 * @return string|null Master link status (either 'up' or 'down'), or null
+	 *  if the server is not a slave.
+	 */
+	protected function getMasterLinkStatus( RedisConnRef $conn ) {
+		$info = $conn->info();
+		return isset( $info['master_link_status'] )
+			? $info['master_link_status']
+			: null;
+	}
+
 	/**
 	 * Log a fatal error
 	 * @param string $msg
-- 
2.20.1