Lock down new `ImplicitRender` behavior for 5.0 RC

1. Conceptually revert #20276

   The feature was implemented for the `responders` gem. In the end,
   they did not need that feature, and have found a better fix (see
   plataformatec/responders#131).

   `ImplicitRender` is the place where Rails specifies our default
   policies for the case where the user did not explicitly tell us
   what to render, essentially describing a set of heuristics. If
   the gem (or the user) knows exactly what they want, they could
   just perform the correct `render` to avoid falling through to
   here, as `responders` did (the user called `respond_with`).

   Reverting the patch allows us to avoid exploding the complexity
   and defining “the fallback for a fallback” policies.

2. `respond_to` and templates are considered exhaustive enumerations

   If the user specified a list of formats/variants in a `respond_to`
   block, anything that is not explicitly included should result
   in an `UnknownFormat` error (which is then caught upstream to
   mean “406 Not Acceptable” by default). This is already how it
   works before this commit.

   Same goes for templates – if the user defined a set of templates
   (usually in the file system), that set is now considered exhaustive,
   which means that “missing” templates are considered `UnknownFormat`
   errors (406).

3. To keep API endpoints simple, the implicit render behavior for
   actions with no templates defined at all (regardless of formats,
   locales, variants, etc) are defaulted to “204 No Content”. This
   is a strictly narrower version of the feature landed in #19036 and
   #19377.

4. To avoid confusion when interacting in the browser, these actions
   will raise an `UnknownFormat` error for “interactive” requests
   instead. (The precise definition of “interactive” requests might
   change – the spirit here is to give helpful messages and avoid
   confusions.)

Closes #20666, #23062, #23077, #23564

[Godfrey Chan, Jon Moss, Kasper Timm Hansen, Mike Clark, Matthew Draper]

6 files changed, 86 insertions, 35 deletions

diff --git a/actionpack/test/controller/mime/respond_to_test.rb b/actionpack/test/controller/mime/respond_to_test.rb
index 76e2d3ff43..d0c7b2e06a 100644
--- a/actionpack/test/controller/mime/respond_to_test.rb
+++ b/actionpack/test/controller/mime/respond_to_test.rb
@@ -160,7 +160,14 @@ class RespondToController < ActionController::Base
     end
   end
 
-  def variant_with_implicit_rendering
+  def variant_with_implicit_template_rendering
+    # This has exactly one variant template defined in the file system (+mobile.html.erb),
+    # which raises the regular MissingTemplate error for other variants.
+  end
+
+  def variant_without_implicit_template_rendering
+    # This differs from the above in that it does not have any templates defined in the file
+    # system, which triggers the ImplicitRender (204 No Content) behavior.
   end
 
   def variant_with_format_and_custom_render
@@ -272,6 +279,8 @@ class RespondToController < ActionController::Base
 end
 
 class RespondToControllerTest < ActionController::TestCase
+  NO_CONTENT_WARNING = "No template found for RespondToController#variant_without_implicit_template_rendering, rendering head :no_content"
+
   def setup
     super
     @request.host = "www.example.com"
@@ -616,30 +625,69 @@ class RespondToControllerTest < ActionController::TestCase
   end
 
   def test_invalid_variant
+    assert_raises(ActionController::UnknownFormat) do
+      get :variant_with_implicit_template_rendering, params: { v: :invalid }
+    end
+  end
+
+  def test_variant_not_set_regular_unknown_format
+    assert_raises(ActionController::UnknownFormat) do
+      get :variant_with_implicit_template_rendering
+    end
+  end
+
+  def test_variant_with_implicit_template_rendering
+    get :variant_with_implicit_template_rendering, params: { v: :mobile }
+    assert_equal "text/html", @response.content_type
+    assert_equal "mobile", @response.body
+  end
+
+  def test_variant_without_implicit_rendering_from_browser
+    assert_raises(ActionController::UnknownFormat) do
+      get :variant_without_implicit_template_rendering, params: { v: :does_not_matter }
+    end
+  end
+
+  def test_variant_variant_not_set_and_without_implicit_rendering_from_browser
+    assert_raises(ActionController::UnknownFormat) do
+      get :variant_without_implicit_template_rendering
+    end
+  end
+
+  def test_variant_without_implicit_rendering_from_xhr
     logger = ActiveSupport::LogSubscriber::TestHelper::MockLogger.new
     old_logger, ActionController::Base.logger = ActionController::Base.logger, logger
 
-    get :variant_with_implicit_rendering, params: { v: :invalid }
+    get :variant_without_implicit_template_rendering, xhr: true, params: { v: :does_not_matter }
     assert_response :no_content
-    assert_equal 1, logger.logged(:info).select{ |s| s =~ /No template found/ }.size, "Implicit head :no_content not logged"
+
+    assert_equal 1, logger.logged(:info).select{ |s| s == NO_CONTENT_WARNING }.size, "Implicit head :no_content not logged"
   ensure
     ActionController::Base.logger = old_logger
   end
 
-  def test_variant_not_set_regular_template_missing
-    get :variant_with_implicit_rendering
+  def test_variant_without_implicit_rendering_from_api
+    logger = ActiveSupport::LogSubscriber::TestHelper::MockLogger.new
+    old_logger, ActionController::Base.logger = ActionController::Base.logger, logger
+
+    get :variant_without_implicit_template_rendering, format: 'json', params: { v: :does_not_matter }
     assert_response :no_content
+
+    assert_equal 1, logger.logged(:info).select{ |s| s == NO_CONTENT_WARNING }.size, "Implicit head :no_content not logged"
+  ensure
+    ActionController::Base.logger = old_logger
   end
 
-  def test_variant_with_implicit_rendering
-    get :variant_with_implicit_rendering, params: { v: :implicit }
+  def test_variant_variant_not_set_and_without_implicit_rendering_from_xhr
+    logger = ActiveSupport::LogSubscriber::TestHelper::MockLogger.new
+    old_logger, ActionController::Base.logger = ActionController::Base.logger, logger
+
+    get :variant_without_implicit_template_rendering, xhr: true
     assert_response :no_content
-  end
 
-  def test_variant_with_implicit_template_rendering
-    get :variant_with_implicit_rendering, params: { v: :mobile }
-    assert_equal "text/html", @response.content_type
-    assert_equal "mobile", @response.body
+    assert_equal 1, logger.logged(:info).select { |s| s == NO_CONTENT_WARNING }.size, "Implicit head :no_content not logged"
+  ensure
+    ActionController::Base.logger = old_logger
   end
 
   def test_variant_with_format_and_custom_render
@@ -778,24 +826,3 @@ class RespondToControllerTest < ActionController::TestCase
     assert_equal "phone", @response.body
   end
 end
-
-class RespondToWithBlockOnDefaultRenderController < ActionController::Base
-  def show
-    default_render do
-      render body: 'default_render yielded'
-    end
-  end
-end
-
-class RespondToWithBlockOnDefaultRenderControllerTest < ActionController::TestCase
-  def setup
-    super
-    @request.host = "www.example.com"
-  end
-
-  def test_default_render_uses_block_when_no_template_exists
-    get :show
-    assert_equal "default_render yielded", @response.body
-    assert_equal "text/plain", @response.content_type
-  end
-end
diff --git a/actionpack/test/controller/render_test.rb b/actionpack/test/controller/render_test.rb
index 60c6518c62..83d7405e4d 100644
--- a/actionpack/test/controller/render_test.rb
+++ b/actionpack/test/controller/render_test.rb
@@ -26,6 +26,9 @@ end
 class ImplicitRenderTestController < ActionController::Base
   def empty_action
   end
+
+  def empty_action_with_template
+  end
 end
 
 class TestController < ActionController::Base
@@ -537,10 +540,28 @@ end
 class ImplicitRenderTest < ActionController::TestCase
   tests ImplicitRenderTestController
 
-  def test_implicit_no_content_response
-    get :empty_action
+  def test_implicit_no_content_response_as_browser
+    assert_raises(ActionController::UnknownFormat) do
+      get :empty_action
+    end
+  end
+
+  def test_implicit_no_content_response_as_xhr
+    get :empty_action, xhr: true
     assert_response :no_content
   end
+
+  def test_implicit_success_response_with_right_format
+    get :empty_action_with_template
+    assert_equal "<h1>Empty action rendered this implicitly.</h1>\n", @response.body
+    assert_response :success
+  end
+
+  def test_implicit_unknown_format_response
+    assert_raises(ActionController::UnknownFormat) do
+      get :empty_action_with_template, format: 'json'
+    end
+  end
 end
 
 class HeadRenderTest < ActionController::TestCase
diff --git a/actionpack/test/fixtures/implicit_render_test/empty_action_with_mobile_variant.html+mobile.erb b/actionpack/test/fixtures/implicit_render_test/empty_action_with_mobile_variant.html+mobile.erb
new file mode 100644
index 0000000000..ded99ba52d
--- /dev/null
+++ b/actionpack/test/fixtures/implicit_render_test/empty_action_with_mobile_variant.html+mobile.erb
@@ -0,0 +1 @@
+mobile
diff --git a/actionpack/test/fixtures/implicit_render_test/empty_action_with_template.html.erb b/actionpack/test/fixtures/implicit_render_test/empty_action_with_template.html.erb
new file mode 100644
index 0000000000..dd294f8cf6
--- /dev/null
+++ b/actionpack/test/fixtures/implicit_render_test/empty_action_with_template.html.erb
@@ -0,0 +1 @@
+<h1>Empty action rendered this implicitly.</h1>
diff --git a/actionpack/test/fixtures/implicit_render_test/some_action_2.html+zomg.erb b/actionpack/test/fixtures/implicit_render_test/some_action_2.html+zomg.erb
new file mode 100644
index 0000000000..7d8187321c
--- /dev/null
+++ b/actionpack/test/fixtures/implicit_render_test/some_action_2.html+zomg.erb
@@ -0,0 +1 @@
+zomg
diff --git a/actionpack/test/fixtures/respond_to/variant_with_implicit_rendering.html+mobile.erb b/actionpack/test/fixtures/respond_to/variant_with_implicit_template_rendering.html+mobile.erb
index 317801ad30..317801ad30 100644
--- a/actionpack/test/fixtures/respond_to/variant_with_implicit_rendering.html+mobile.erb
+++ b/actionpack/test/fixtures/respond_to/variant_with_implicit_template_rendering.html+mobile.erb