always use etag for caching when cache_key or updated_at are present, add the deploy_sha as a cache buster

lib/travis/api/app.rb

@@ -51,6 +51,10 @@ module Travis::Api
       super()
     end
 
+    def self.deploy_sha
+      @deploy_sha ||= File.exist?('.deploy_sha') ? File.read('.deploy-sha')[0..7] : 'deploy-sha'
+    end
+
     attr_accessor :app
 
     def initialize
@@ -64,11 +68,10 @@ module Travis::Api
 
         memcache_servers = ENV['MEMCACHE_SERVERS']
         if Travis::Features.feature_active?(:use_rack_cache) && memcache_server
-          namespace = File.read('.deploy-sha')[0..7]
           use Rack::Cache,
             verbose: true,
-            metastore:   "memcached://#{memcache_servers}/#{namespace}",
-            entitystore: "memcached://#{memcache_servers}/#{namespace}"
+            metastore:   "memcached://#{memcache_servers}/#{self.class.deploy_sha}",
+            entitystore: "memcached://#{memcache_servers}/#{self.class.deploy_sha}"
         end
 
         use Rack::Deflater

lib/travis/api/app/responders/service.rb

@@ -1,5 +1,11 @@
-module Travis::Api::App::Responders
+require 'digest/md5'
+
+module Travis::Api
+  class App
+    module Responders
       class Service < Base
+        include Helpers::Accept
+
         def apply?
           resource.respond_to?(:run)
         end
@@ -22,20 +28,26 @@ module Travis::Api::App::Responders
               endpoint.cache_control :no_cache
             end
 
-        endpoint.etag resource.cache_key           if cache_key?
-        endpoint.last_modified resource.updated_at if updated_at?
+            endpoint.etag cache_key if cache_key
           end
 
           def final?
             resource.respond_to?(:final?) && resource.final?
           end
 
-      def updated_at?
+          def cache_key
+            cache_key ||= begin
+              key = resource_cache_key || resource_updated_at
+              Digest::MD5.hexdigest([App.deploy_sha, key].join('-')) if key
+            end
+          end
+
+          def resource_cache_key
             resource.respond_to?(:updated_at) && resource.updated_at
           end
 
-      def cache_key?
-        resource.respond_to?(:cache_key) && resource.cache_key
+          def resource_updated_at
+            resource.respond_to?(:updated_at) && resource.updated_at.try(:strftime, '%FT%T%:z')
           end
 
           # Services potentially return all sorts of things
@@ -52,3 +64,5 @@ module Travis::Api::App::Responders
           end
       end
     end
+  end
+end