improve caching headers

lib/travis/api/app/extensions/scoping.rb

@@ -3,6 +3,16 @@ require 'travis/api/app'
 class Travis::Api::App
   module Extensions
     module Scoping
+      module Helpers
+        def scope
+          env['travis.scope'].to_sym
+        end
+
+        def public?
+          scope == :public
+        end
+      end
+
       def self.registered(app)
         app.set default_scope: :public, anonymous_scopes: [:public]
         app.helpers(Helpers)
@@ -16,8 +26,9 @@ class Travis::Api::App
           headers['X-Accepted-OAuth-Scopes'] = name.to_s
 
           if scopes.include? name
+            env['travis.scope'] = name
             headers['Vary'] = 'Accept'
-            headers['Vary'] << ', Authorization' if name == :public
+            headers['Vary'] << ', Authorization' unless public?
             true
           elsif env['travis.access_token']
             halt 403, "insufficient access"

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

@@ -14,12 +14,11 @@ module Travis::Api::App::Responders
     private
 
       def cache_control
-        if final?
-          endpoint.expires 31536000, :public # 1 year
-        elsif updated_at?
-          endpoint.cache_control :public, :must_revalidate
-          endpoint.last_modified resource.updated_at
-        end
+        mode = [endpoint.public? ? :public : :private]
+        mode << :must_revalidate unless final?
+        endpoint.expires(31536000, *mode) # 1 year
+        endpoint.etag resource.cache_key           if cache_key?
+        endpoint.last_modified resource.updated_at if updated_at?
       end
 
       def final?
@@ -30,6 +29,10 @@ module Travis::Api::App::Responders
         resource.respond_to?(:updated_at) && resource.updated_at
       end
 
+      def cache_key?
+        resource.respond_to?(:cache_key) && resource.cache_key
+      end
+
       # Services potentially return all sorts of things
       # If it's a string, true or false we'll wrap it into a hash.
       # If it's an active record or scope we just pass so it can be processed by the json responder.