Use SCAN during 'rake cache:clear'

This allows 'rake cache:clear' to delete millions of keys without
choking. It requires Redis 2.8.0 or newer but we needed that already
anyway.

config/application.rb

@@ -6,6 +6,8 @@ I18n.config.enforce_available_locales = false
 Bundler.require(:default, Rails.env)
 
 module Gitlab
+  REDIS_CACHE_NAMESPACE = 'cache:gitlab'
+
   class Application < Rails::Application
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration should go into files in config/initializers
@@ -89,7 +91,7 @@ module Gitlab
       redis_config_hash[:path] = redis_uri.path
     end
 
-    redis_config_hash[:namespace] = 'cache:gitlab'
+    redis_config_hash[:namespace] = REDIS_CACHE_NAMESPACE
     redis_config_hash[:expires_in] = 2.weeks # Cache should not grow forever
     config.cache_store = :redis_store, redis_config_hash
 

lib/tasks/cache.rake

@@ -1,11 +1,22 @@
 namespace :cache do
+  CLEAR_BATCH_SIZE = 1000
+  REDIS_SCAN_START_STOP = '0' # Magic value, see http://redis.io/commands/scan
+
   desc "GitLab | Clear redis cache"
   task :clear => :environment do
-    # Hack into Rails.cache until https://github.com/redis-store/redis-store/pull/225
-    # is accepted (I hope) and we can update the redis-store gem.
     redis_store = Rails.cache.instance_variable_get(:@data)
-    redis_store.keys.each_slice(1000) do |key_slice|
-      redis_store.del(*key_slice)
+    cursor = [REDIS_SCAN_START_STOP, []]
+    loop do
+      cursor = redis_store.scan(
+        cursor.first,
+        match: "#{Gitlab::REDIS_CACHE_NAMESPACE}*", 
+        count: CLEAR_BATCH_SIZE
+      )
+
+      keys = cursor.last
+      redis_store.del(*keys) if keys.any?
+
+      break if cursor.first == REDIS_SCAN_START_STOP
     end
   end
 end