Raises exception when respond_to called multiple times in incompatible way

Nesting respond_to calls can lead to unexpected behavior, so it should be
avoided. Currently, the first respond_to format match sets the content-type
for the resulting response. But, if a nested respond_to occurs, it is possible
to match on a different format. For example:

    respond_to do |outer_type|
      outer_type.js do
        respond_to do |inner_type|
          inner_type.html { render body: "HTML" }
        end
      end
    end

Browsers will often include */* in their Accept headers. In the above example,
such a request would result in the outer_type.js match setting the content-
type of the response to text/javascript, while the inner_type.html match will
cause the actual response to return "HTML".

This change tries to minimize potential breakage by only raising an exception
if the nested respond_to calls are in conflict with each other. So, something
like the following example would not raise an exception:

    respond_to do |outer_type|
      outer_type.js do
        respond_to do |inner_type|
          inner_type.js { render body: "JS" }
        end
      end
    end

While the above is nested, it doesn't affect the content-type of the response.

2 files changed, 21 insertions, 0 deletions

diff --git a/actionpack/lib/action_controller/metal/exceptions.rb b/actionpack/lib/action_controller/metal/exceptions.rb
index ce9eb209fe..30034be018 100644
--- a/actionpack/lib/action_controller/metal/exceptions.rb
+++ b/actionpack/lib/action_controller/metal/exceptions.rb
@@ -51,6 +51,24 @@ module ActionController
   class UnknownFormat < ActionControllerError #:nodoc:
   end
 
+  # Raised when a nested respond_to is triggered and the content types of each
+  # are incompatible. For exampe:
+  #
+  #  respond_to do |outer_type|
+  #    outer_type.js do
+  #      respond_to do |inner_type|
+  #        inner_type.html { render body: "HTML" }
+  #      end
+  #    end
+  #  end
+  class RespondToMismatchError < ActionControllerError
+    DEFAULT_MESSAGE = "respond_to was called multiple times and matched with conflicting formats in this action. Please note that you may only call respond_to and match on a single format per action."
+
+    def initialize(message = nil)
+      super(message || DEFAULT_MESSAGE)
+    end
+  end
+
   class MissingExactTemplate < UnknownFormat #:nodoc:
   end
 end
diff --git a/actionpack/lib/action_controller/metal/mime_responds.rb b/actionpack/lib/action_controller/metal/mime_responds.rb
index 2233b93406..2b55b9347c 100644
--- a/actionpack/lib/action_controller/metal/mime_responds.rb
+++ b/actionpack/lib/action_controller/metal/mime_responds.rb
@@ -197,6 +197,9 @@ module ActionController #:nodoc:
       yield collector if block_given?
 
       if format = collector.negotiate_format(request)
+        if content_type && content_type != format
+          raise ActionController::RespondToMismatchError
+        end
         _process_format(format)
         _set_rendered_content_type format
         response = collector.response