Introduce `render :plain` for render plain text

This is as an option to render content with a content type of
`text/plain`. This is the preferred option if you are planning to render
a plain text content.

Please see #12374 for more detail.

3 files changed, 186 insertions, 2 deletions

diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index dcccc99d5b..3318a088a3 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   Introduce `render :plain` as an option to render content with a content type
+    of `text/plain`. This is the preferred option if you are planning to render
+    a plain text content.
+
+    Please see #12374 for more detail.
+
+    *Prem Sichanugrist*
+
 *   Introduce `render :body` as an option for sending a raw content back to
     browser. Note that this rendering option will unset the default content type
     and does not include "Content-Type" header back in the response.
diff --git a/actionpack/lib/action_controller/metal/rendering.rb b/actionpack/lib/action_controller/metal/rendering.rb
index c9ae1ab388..3133334369 100644
--- a/actionpack/lib/action_controller/metal/rendering.rb
+++ b/actionpack/lib/action_controller/metal/rendering.rb
@@ -27,7 +27,7 @@ module ActionController
     end
 
     def render_to_body(options = {})
-      super || options[:body].presence || options[:text].presence || ' '
+      super || options[:body].presence || options[:text].presence || options[:plain].presence || ' '
     end
 
     private
@@ -40,6 +40,10 @@ module ActionController
         self.content_type = "none"
         self.headers.delete "Content-Type"
       end
+
+      if options[:plain].present?
+        self.content_type = Mime::TEXT
+      end
     end
 
     # Normalize arguments by catching blocks and setting them on :update.
@@ -59,7 +63,11 @@ module ActionController
         options[:text] = options[:text].to_text
       end
 
-      if options.delete(:nothing) || (options.key?(:body) && options[:body].nil?) || (options.key?(:text) && options[:text].nil?)
+      if options.key?(:plain) && options[:plain].respond_to?(:to_text)
+        options[:plain] = options[:plain].to_text
+      end
+
+      if options.delete(:nothing) || (options.key?(:body) && options[:body].nil?) || (options.key?(:text) && options[:text].nil?) || (options.key?(:plain) && options[:plain].nil?)
         options[:body] = " "
       end
 
diff --git a/actionpack/test/controller/new_base/render_plain_test.rb b/actionpack/test/controller/new_base/render_plain_test.rb
new file mode 100644
index 0000000000..dba2e9f13e
--- /dev/null
+++ b/actionpack/test/controller/new_base/render_plain_test.rb
@@ -0,0 +1,168 @@
+require 'abstract_unit'
+
+module RenderPlain
+  class MinimalController < ActionController::Metal
+    include AbstractController::Rendering
+    include ActionController::Rendering
+
+    def index
+      render plain: "Hello World!"
+    end
+  end
+
+  class SimpleController < ActionController::Base
+    self.view_paths = [ActionView::FixtureResolver.new]
+
+    def index
+      render plain: "hello david"
+    end
+  end
+
+  class WithLayoutController < ::ApplicationController
+    self.view_paths = [ActionView::FixtureResolver.new(
+      "layouts/application.text.erb" => "<%= yield %>, I'm here!",
+      "layouts/greetings.text.erb"   => "<%= yield %>, I wish thee well.",
+      "layouts/ivar.text.erb"        => "<%= yield %>, <%= @ivar %>"
+    )]
+
+    def index
+      render plain: "hello david"
+    end
+
+    def custom_code
+      render plain: "hello world", status: 404
+    end
+
+    def with_custom_code_as_string
+      render plain: "hello world", status: "404 Not Found"
+    end
+
+    def with_nil
+      render plain: nil
+    end
+
+    def with_nil_and_status
+      render plain: nil, status: 403
+    end
+
+    def with_false
+      render plain: false
+    end
+
+    def with_layout_true
+      render plain: "hello world", layout: true
+    end
+
+    def with_layout_false
+      render plain: "hello world", layout: false
+    end
+
+    def with_layout_nil
+      render plain: "hello world", layout: nil
+    end
+
+    def with_custom_layout
+      render plain: "hello world", layout: "greetings"
+    end
+
+    def with_ivar_in_layout
+      @ivar = "hello world"
+      render plain: "hello world", layout: "ivar"
+    end
+  end
+
+  class RenderPlainTest < Rack::TestCase
+    test "rendering text from a minimal controller" do
+      get "/render_plain/minimal/index"
+      assert_body "Hello World!"
+      assert_status 200
+    end
+
+    test "rendering text from an action with default options renders the text with the layout" do
+      with_routing do |set|
+        set.draw { get ':controller', action: 'index' }
+
+        get "/render_plain/simple"
+        assert_body "hello david"
+        assert_status 200
+      end
+    end
+
+    test "rendering text from an action with default options renders the text without the layout" do
+      with_routing do |set|
+        set.draw { get ':controller', action: 'index' }
+
+        get "/render_plain/with_layout"
+
+        assert_body "hello david"
+        assert_status 200
+      end
+    end
+
+    test "rendering text, while also providing a custom status code" do
+      get "/render_plain/with_layout/custom_code"
+
+      assert_body "hello world"
+      assert_status 404
+    end
+
+    test "rendering text with nil returns an empty body padded for Safari" do
+      get "/render_plain/with_layout/with_nil"
+
+      assert_body " "
+      assert_status 200
+    end
+
+    test "Rendering text with nil and custom status code returns an empty body padded for Safari and the status" do
+      get "/render_plain/with_layout/with_nil_and_status"
+
+      assert_body " "
+      assert_status 403
+    end
+
+    test "rendering text with false returns the string 'false'" do
+      get "/render_plain/with_layout/with_false"
+
+      assert_body "false"
+      assert_status 200
+    end
+
+    test "rendering text with layout: true" do
+      get "/render_plain/with_layout/with_layout_true"
+
+      assert_body "hello world, I'm here!"
+      assert_status 200
+    end
+
+    test "rendering text with layout: 'greetings'" do
+      get "/render_plain/with_layout/with_custom_layout"
+
+      assert_body "hello world, I wish thee well."
+      assert_status 200
+    end
+
+    test "rendering text with layout: false" do
+      get "/render_plain/with_layout/with_layout_false"
+
+      assert_body "hello world"
+      assert_status 200
+    end
+
+    test "rendering text with layout: nil" do
+      get "/render_plain/with_layout/with_layout_nil"
+
+      assert_body "hello world"
+      assert_status 200
+    end
+
+    test "rendering from minimal controller returns response with text/plain content type" do
+      get "/render_plain/minimal/index"
+      assert_content_type "text/plain"
+    end
+
+    test "rendering from normal controller returns response with text/plain content type" do
+      get "/render_plain/simple/index"
+      assert_content_type "text/plain; charset=utf-8"
+    end
+  end
+end