Add support for more HTTP cache controls

From <https://tools.ietf.org/html/rfc5861>:

> The stale-if-error HTTP Cache-Control extension allows a cache to
> return a stale response when an error -- e.g., a 500 Internal Server
> Error, a network segment, or DNS failure -- is encountered, rather
> than returning a "hard" error.  This improves availability.
>
> The stale-while-revalidate HTTP Cache-Control extension allows a
> cache to immediately return a stale response while it revalidates it
> in the background, thereby hiding latency (both in the network and on
> the server) from clients.

These are useful, fully standardized parts of the HTTP protocol with
widespread support among CDN vendors. Supporting them will make it
easier to utilize reverse proxies and CDNs from Rails.

actionpack/lib/action_controller/metal/conditional_get.rb

@@ -235,7 +235,9 @@ def expires_in(seconds, options = {})
       response.cache_control.merge!(
         max_age: seconds,
         public: options.delete(:public),
-        must_revalidate: options.delete(:must_revalidate)
+        must_revalidate: options.delete(:must_revalidate),
+        stale_while_revalidate: options.delete(:stale_while_revalidate),
+        stale_if_error: options.delete(:stale_if_error),
       )
       options.delete(:private)
 

actionpack/lib/action_dispatch/http/cache.rb

@@ -204,11 +204,15 @@ def merge_and_normalize_cache_control!(cache_control)
           else
             extras = control[:extras]
             max_age = control[:max_age]
+            stale_while_revalidate = control[:stale_while_revalidate]
+            stale_if_error = control[:stale_if_error]
 
             options = []
             options << "max-age=#{max_age.to_i}" if max_age
             options << (control[:public] ? PUBLIC : PRIVATE)
             options << MUST_REVALIDATE if control[:must_revalidate]
+            options << "stale-while-revalidate=#{stale_while_revalidate.to_i}" if stale_while_revalidate
+            options << "stale-if-error=#{stale_if_error.to_i}" if stale_if_error
             options.concat(extras) if extras
 
             self._cache_control = options.join(", ")

actionpack/test/controller/render_test.rb

@@ -141,6 +141,16 @@ def conditional_hello_with_expires_in_with_public_and_must_revalidate
     render action: "hello_world"
   end
 
+  def conditional_hello_with_expires_in_with_stale_while_revalidate
+    expires_in 1.minute, public: true, stale_while_revalidate: 5.minutes
+    render action: "hello_world"
+  end
+
+  def conditional_hello_with_expires_in_with_stale_if_error
+    expires_in 1.minute, public: true, stale_if_error: 5.minutes
+    render action: "hello_world"
+  end
+
   def conditional_hello_with_expires_in_with_public_with_more_keys
     expires_in 1.minute, :public => true, "s-maxage" => 5.hours
     render action: "hello_world"
@@ -358,6 +368,16 @@ def test_expires_in_header_with_public_and_must_revalidate
     assert_equal "max-age=60, public, must-revalidate", @response.headers["Cache-Control"]
   end
 
+  def test_expires_in_header_with_stale_while_revalidate
+    get :conditional_hello_with_expires_in_with_stale_while_revalidate
+    assert_equal "max-age=60, public, stale-while-revalidate=300", @response.headers["Cache-Control"]
+  end
+
+  def test_expires_in_header_with_stale_if_error
+    get :conditional_hello_with_expires_in_with_stale_if_error
+    assert_equal "max-age=60, public, stale-if-error=300", @response.headers["Cache-Control"]
+  end
+
   def test_expires_in_header_with_additional_headers
     get :conditional_hello_with_expires_in_with_public_with_more_keys
     assert_equal "max-age=60, public, s-maxage=18000", @response.headers["Cache-Control"]