Raise DoubleRenderError on head after rendering

Currently, calling `head` to set the response code silently removes a
previously set response body [0]. This can lead to unnecessary and
potentially expensive rendering, as shown in the following snippet:

```ruby
def index
  render json: User.all.as_json # expensive rendering
  head :ok # Overwrites `response_body` to ``
end
```

This behavior is inconsistent with calling `render` twice or calling
`redirect_to` after `render`: both of which raise a `DoubleRenderError`
[1, 2].

Since this change could break existing endpoints when upgrading Rails,
it might make sense to introduce a configuration option would allow
developers to choose whether this scenario raises an exception or
a warning.

[0] https://github.com/rails/rails/blob/b721c62de11c2c29fa2b8ef51b00d95dce163afc/actionpack/lib/action_controller/metal/head.rb#L50
[1] https://github.com/rails/rails/blob/b721c62de11c2c29fa2b8ef51b00d95dce163afc/actionpack/lib/action_controller/metal/redirecting.rb#L103-L105
[2] https://github.com/rails/rails/blob/b721c62de11c2c29fa2b8ef51b00d95dce163afc/actionpack/lib/action_controller/metal/rendering.rb#L171-L174

Author: viralpraxis

actionpack/CHANGELOG.md

@@ -1,3 +1,18 @@
+*   Raise `AbstractController::DoubleRenderError` if `head` is called after rendering.
+
+    After this change, invoking `head` will lead to an error if response body is already set:
+
+    ```ruby
+    class PostController < ApplicationController
+      def index
+        render locals: {}
+        head :ok
+      end
+    end
+    ```
+
+    *Iaroslav Kurbatov*
+
 *   The Cookie Serializer can now serialize an Active Support SafeBuffer when using message pack.
 
     Such code would previously produce an error if an application was using messagepack as its cookie serializer.

actionpack/lib/action_controller/metal/head.rb

@@ -25,6 +25,8 @@ def head(status, options = nil)
         raise ArgumentError, "#{status.inspect} is not a valid value for `status`."
       end
 
+      raise ::AbstractController::DoubleRenderError if response_body
+
       status ||= :ok
 
       if options

actionpack/test/controller/new_base/render_test.rb

@@ -49,6 +49,13 @@ def index
     end
   end
 
+  class DoubleRenderWithHeadController < ActionController::Base
+    def index
+      render plain: "hello"
+      head :bad_request
+    end
+  end
+
   class ChildRenderController < BlankRenderController
     append_view_path ActionView::FixtureResolver.new("render/child_render/overridden_with_own_view_paths_appended.html.erb" => "child content")
     prepend_view_path ActionView::FixtureResolver.new("render/child_render/overridden_with_own_view_paths_prepended.html.erb" => "child content")
@@ -83,6 +90,20 @@ class RenderTest < Rack::TestCase
         end
       end
     end
+
+    test "using head after rendering raises an exception" do
+      with_routing do |set|
+        set.draw do
+          ActionDispatch.deprecator.silence do
+            get ":controller", action: "index"
+          end
+        end
+
+        assert_raises(AbstractController::DoubleRenderError) do
+          get "/render/double_render_with_head", headers: { "action_dispatch.show_exceptions" => :none }
+        end
+      end
+    end
   end
 
   class TestOnlyRenderPublicActions < Rack::TestCase