From 213ee62469c6518af8423f00fb902b7665d61204 Mon Sep 17 00:00:00 2001
From: Jacob Vosmaer <contact@jacobvosmaer.nl>
Date: Fri, 18 Mar 2016 15:31:53 +0100
Subject: [PATCH] Be careful when setting class instance vars

---
 lib/gitlab/redis.rb | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/lib/gitlab/redis.rb b/lib/gitlab/redis.rb
index d03e6e4cd92..8c3aea2627c 100644
--- a/lib/gitlab/redis.rb
+++ b/lib/gitlab/redis.rb
@@ -2,12 +2,23 @@ module Gitlab
   class Redis
     attr_reader :url
 
+    # To be thread-safe we must be careful when writing the class instance
+    # variables @url and @pool. Because @pool depends on @url we need two
+    # mutexes to prevent deadlock.
+    URL_MUTEX = Mutex.new
+    POOL_MUTEX = Mutex.new
+    private_constant :URL_MUTEX, :POOL_MUTEX
+
     def self.url
-      @url ||= new.url
+      @url || URL_MUTEX.synchronize { @url = new.url }
     end
 
     def self.with
-      @pool ||= ConnectionPool.new { ::Redis.new(url: url) }
+      if @pool.nil?
+        POOL_MUTEX.synchronize do
+          @pool = ConnectionPool.new { ::Redis.new(url: url) }
+        end
+      end
       @pool.with { |redis| yield redis }
     end