Make `ActionDispatch::Response#content_type` behavior configurable

I changed return value of `ActionDispatch::Response#content_type` in #36034.
But this change seems to an obstacle to upgrading. https://github.com/rails/rails/pull/36034#issuecomment-498795893

Therefore, I restored the behavior of `ActionDispatch::Response#content_type`
to 5.2 and deprecated old behavior. Also, made it possible to control the
behavior with the config.

3 files changed, 43 insertions, 1 deletions

diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index 63d8f6b585..ea3692951f 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -86,6 +86,7 @@ module ActionDispatch # :nodoc:
 
     cattr_accessor :default_charset, default: "utf-8"
     cattr_accessor :default_headers
+    cattr_accessor :return_only_media_type_on_content_type, default: false
 
     include Rack::Response::Helpers
     # Aliasing these off because AD::Http::Cache::Response defines them.
@@ -243,7 +244,17 @@ module ActionDispatch # :nodoc:
 
     # Content type of response.
     def content_type
-      super.presence
+      if self.class.return_only_media_type_on_content_type
+        ActiveSupport::Deprecation.warn(
+          "Rails 6.1 will return Content-Type header without modification." \
+          " If you want just the MIME type, please use `#media_type` instead."
+        )
+
+        content_type = super
+        content_type ? content_type.split(/;\s*charset=/)[0].presence : content_type
+      else
+        super.presence
+      end
     end
 
     # Media type of response.
diff --git a/actionpack/lib/action_dispatch/railtie.rb b/actionpack/lib/action_dispatch/railtie.rb
index 5f711c7348..66f90980b9 100644
--- a/actionpack/lib/action_dispatch/railtie.rb
+++ b/actionpack/lib/action_dispatch/railtie.rb
@@ -23,6 +23,7 @@ module ActionDispatch
     config.action_dispatch.use_authenticated_cookie_encryption = false
     config.action_dispatch.use_cookies_with_metadata = false
     config.action_dispatch.perform_deep_munge = true
+    config.action_dispatch.return_only_media_type_on_content_type = true
 
     config.action_dispatch.default_headers = {
       "X-Frame-Options" => "SAMEORIGIN",
@@ -43,6 +44,7 @@ module ActionDispatch
       ActionDispatch::Request::Utils.perform_deep_munge = app.config.action_dispatch.perform_deep_munge
       ActionDispatch::Response.default_charset = app.config.action_dispatch.default_charset || app.config.encoding
       ActionDispatch::Response.default_headers = app.config.action_dispatch.default_headers
+      ActionDispatch::Response.return_only_media_type_on_content_type = app.config.action_dispatch.return_only_media_type_on_content_type
 
       ActionDispatch::ExceptionWrapper.rescue_responses.merge!(config.action_dispatch.rescue_responses)
       ActionDispatch::ExceptionWrapper.rescue_templates.merge!(config.action_dispatch.rescue_templates)
diff --git a/actionpack/test/dispatch/response_test.rb b/actionpack/test/dispatch/response_test.rb
index 33cf86a081..ed64d89902 100644
--- a/actionpack/test/dispatch/response_test.rb
+++ b/actionpack/test/dispatch/response_test.rb
@@ -593,4 +593,33 @@ class ResponseIntegrationTest < ActionDispatch::IntegrationTest
     assert_equal("text/csv", @response.media_type)
     assert_equal("utf-16", @response.charset)
   end
+
+  test "`content type` returns header that excludes `charset` when specified `return_only_media_type_on_content_type`" do
+    original = ActionDispatch::Response.return_only_media_type_on_content_type
+    ActionDispatch::Response.return_only_media_type_on_content_type = true
+
+    @app = lambda { |env|
+      if env["PATH_INFO"] == "/with_parameters"
+        [200, { "Content-Type" => "text/csv; header=present; charset=utf-16" }, [""]]
+      else
+        [200, { "Content-Type" => "text/csv; charset=utf-16" }, [""]]
+      end
+    }
+
+    get "/"
+    assert_response :success
+
+    assert_deprecated do
+      assert_equal("text/csv", @response.content_type)
+    end
+
+    get "/with_parameters"
+    assert_response :success
+
+    assert_deprecated do
+      assert_equal("text/csv; header=present", @response.content_type)
+    end
+  ensure
+    ActionDispatch::Response.return_only_media_type_on_content_type = original
+  end
 end