Include the content of the flash in the auto-generated etag (#26250)

Include the content of the flash in the auto-generated etag

actionpack/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Include the content of the flash in the auto-generated etag. This solves the following problem:
+
+      1. POST /messages
+      2. redirect_to messages_url, notice: 'Message was created'
+      3. GET /messages/1
+      4. GET /messages
+      
+      Step 4 would before still include the flash message, even though it's no longer relevant,
+      because the etag cache was recorded with the flash in place and didn't change when it was gone.
+
+    *DHH*
+
 *   SSL: Changes redirect behavior for all non-GET and non-HEAD requests
     (like POST/PUT/PATCH etc) to `http://` resources to redirect to `https://`
     with a [307 status code](http://tools.ietf.org/html/rfc7231#section-6.4.7) instead of [301 status code](http://tools.ietf.org/html/rfc7231#section-6.4.2).

actionpack/lib/action_controller.rb

@@ -23,6 +23,7 @@ module ActionController
     autoload :Cookies
     autoload :DataStreaming
     autoload :EtagWithTemplateDigest
+    autoload :EtagWithFlash
     autoload :Flash
     autoload :ForceSSL
     autoload :Head

actionpack/lib/action_controller/base.rb

@@ -213,6 +213,7 @@ module ActionController
       Renderers::All,
       ConditionalGet,
       EtagWithTemplateDigest,
+      EtagWithFlash,
       Caching,
       MimeResponds,
       ImplicitRender,

actionpack/lib/action_controller/metal/etag_with_flash.rb

@@ -0,0 +1,16 @@
+module ActionController
+  # When you're using the flash, it's generally used as a conditional on the view.
+  # This means the content of the view depends on the flash. Which in turn means
+  # that the etag for a response should be computed with the content of the flash
+  # in mind. This does that by including the content of the flash as a component
+  # in the etag that's generated for a response.
+  module EtagWithFlash
+    extend ActiveSupport::Concern
+
+    include ActionController::ConditionalGet
+
+    included do
+      etag { flash unless flash.empty? }
+    end
+  end
+end

actionpack/test/controller/flash_test.rb

@@ -263,6 +263,13 @@ class FlashIntegrationTest < ActionDispatch::IntegrationTest
       flash[:bar] = "for great justice"
       head :ok
     end
+
+    def set_flash_optionally
+      flash.now.notice = params[:flash]
+      if stale? etag: "abe"
+        render inline: "maybe flash"
+      end
+    end
   end
 
   def test_flash
@@ -310,6 +317,28 @@ class FlashIntegrationTest < ActionDispatch::IntegrationTest
     end
   end
 
+  def test_flash_factored_into_etag
+    with_test_route_set do
+      get "/set_flash_optionally"
+      no_flash_etag = response.etag
+
+      get "/set_flash_optionally", params: { flash: "hello!" }
+      hello_flash_etag = response.etag
+
+      assert_not_equal no_flash_etag, hello_flash_etag
+
+      get "/set_flash_optionally", params: { flash: "hello!" }
+      another_hello_flash_etag = response.etag
+
+      assert_equal another_hello_flash_etag, hello_flash_etag
+
+      get "/set_flash_optionally", params: { flash: "goodbye!" }
+      goodbye_flash_etag = response.etag
+
+      assert_not_equal another_hello_flash_etag, goodbye_flash_etag
+    end
+  end
+
   private
 
     # Overwrite get to send SessionSecret in env hash