Commit flash changes when using a redirect route.

In ca324a0 the flash middleware was effectively removed by its
constructor returning the app it was passed and the `commit_flash`
call was moved to the `ActionController::Metal#dispatch` method.
This broke any redirect routes that modified the flash because the
redirect happens before `dispatch` gets called.

To fix it, this commit adds a `commit_flash` call in the `serve`
method of `ActionDispatch::Routing::Redirect`.

Fixes #27992.

1 files changed, 49 insertions, 0 deletions

diff --git a/actionpack/test/dispatch/routing_test.rb b/actionpack/test/dispatch/routing_test.rb
index 53758a4fbc..d563df91df 100644
--- a/actionpack/test/dispatch/routing_test.rb
+++ b/actionpack/test/dispatch/routing_test.rb
@@ -4913,3 +4913,52 @@ class TestInternalRoutingParams < ActionDispatch::IntegrationTest
     )
   end
 end
+
+class FlashRedirectTest < ActionDispatch::IntegrationTest
+  SessionKey = "_myapp_session"
+  Generator  = ActiveSupport::LegacyKeyGenerator.new("b3c631c314c0bbca50c1b2843150fe33")
+
+  class KeyGeneratorMiddleware
+    def initialize(app)
+      @app = app
+    end
+
+    def call(env)
+      env["action_dispatch.key_generator"] ||= Generator
+      @app.call(env)
+    end
+  end
+
+  class FooController < ActionController::Base
+    def bar
+      render plain: (flash[:foo] || "foo")
+    end
+  end
+
+  Routes = ActionDispatch::Routing::RouteSet.new
+  Routes.draw do
+    get "/foo", to: redirect { |params, req| req.flash[:foo] = "bar"; "/bar" }
+    get "/bar", to: "flash_redirect_test/foo#bar"
+  end
+
+  APP = build_app Routes do |middleware|
+    middleware.use KeyGeneratorMiddleware
+    middleware.use ActionDispatch::Session::CookieStore, key: SessionKey
+    middleware.use ActionDispatch::Flash
+    middleware.delete ActionDispatch::ShowExceptions
+  end
+
+  def app
+    APP
+  end
+
+  include Routes.url_helpers
+
+  def test_block_redirect_commits_flash
+    get "/foo", env: { "action_dispatch.key_generator" => Generator }
+    assert_response :redirect
+
+    follow_redirect!
+    assert_equal "bar", response.body
+  end
+end