3 files changed, 28 insertions, 12 deletions
diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index b57408ede3..43880f2412 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,4 +1,11 @@
 ## Rails 4.0.0 (unreleased) ##
+
+*   Invert precedence in `ActionDispatch::Static` so that dynamic content is preferred.
+    This prevents precompiled assets inadvertently being included twice when running
+    in development mode. Fixes #6421
+
+    *Andrew White*
+
 *   Add :if / :unless conditions to fragment cache:
 
         <%= cache @model, if: some_condition(@model) do %>
diff --git a/actionpack/lib/action_dispatch/middleware/static.rb b/actionpack/lib/action_dispatch/middleware/static.rb
index e3b15b43b9..65670b0ff1 100644
--- a/actionpack/lib/action_dispatch/middleware/static.rb
+++ b/actionpack/lib/action_dispatch/middleware/static.rb
@@ -51,16 +51,20 @@ module ActionDispatch
     end
 
     def call(env)
-      case env['REQUEST_METHOD']
-      when 'GET', 'HEAD'
-        path = env['PATH_INFO'].chomp('/')
-        if match = @file_handler.match?(path)
-          env["PATH_INFO"] = match
-          return @file_handler.call(env)
+      path = env['PATH_INFO'].chomp('/')
+      response = @app.call(env)
+
+      if response[1]['X-Cascade'] == 'pass'
+        case env['REQUEST_METHOD']
+        when 'GET', 'HEAD'
+          if match = @file_handler.match?(path)
+            env["PATH_INFO"] = match
+            return @file_handler.call(env)
+          end
         end
       end
 
-      @app.call(env)
+      response
     end
   end
 end
diff --git a/actionpack/test/dispatch/static_test.rb b/actionpack/test/dispatch/static_test.rb
index 112f470786..f90be450e2 100644
--- a/actionpack/test/dispatch/static_test.rb
+++ b/actionpack/test/dispatch/static_test.rb
@@ -4,11 +4,19 @@ require 'rbconfig'
 
 module StaticTests
   def test_serves_dynamic_content
+    dummy_app = lambda { |env| [200, {"Content-Type" => "text/plain"}, ["Hello, World!"]] }
+    @app = ActionDispatch::Static.new(dummy_app, "#{FIXTURE_LOAD_PATH}/public", "public, max-age=60")
     assert_equal "Hello, World!", get("/nofile").body
   end
 
+  def test_dynamic_content_has_precedence_over_static_files
+    dummy_app = lambda { |env| [200, {"Content-Type" => "text/html"}, ["/foo/baz.html"]] }
+    @app = ActionDispatch::Static.new(dummy_app, "#{FIXTURE_LOAD_PATH}/public", "public, max-age=60")
+    assert_html "/foo/baz.html", get("/foo/bar.html")
+  end
+
   def test_handles_urls_with_bad_encoding
-    assert_equal "Hello, World!", get("/doorkeeper%E3E4").body
+    assert_equal "", get("/doorkeeper%E3E4").body
   end
 
   def test_sets_cache_control
@@ -40,7 +48,6 @@ module StaticTests
     assert_html "means hello in Japanese\n", get("/foo/#{Rack::Utils.escape("こんにちは.html")}")
   end
 
-
   def test_serves_static_file_with_exclamation_mark_in_filename
     with_static_file "/foo/foo!bar.html" do |file|
       assert_html file, get("/foo/foo%21bar.html")
@@ -142,9 +149,7 @@ module StaticTests
 end
 
 class StaticTest < ActiveSupport::TestCase
-  DummyApp = lambda { |env|
-    [200, {"Content-Type" => "text/plain"}, ["Hello, World!"]]
-  }
+  DummyApp = lambda { |env| [404, {"X-Cascade" => "pass"}, []] }
   App = ActionDispatch::Static.new(DummyApp, "#{FIXTURE_LOAD_PATH}/public", "public, max-age=60")
 
   def setup