This is in UPGRADING, and see #1029.

Rack spec (from what I remember after reading a lot of it :) just says you're supposed to return a Rack::Response or a "file-like object". With the change you describe above we are able to return a file stream. I couldn't find a single example where returning a Rack::Response itself would be an issue.

Are you seeing something broken or are you just worried about the convention?

Thanks for the answer ;)
I didn't realize this was for streaming (and I didn't notice the UPGRADING untill now)

This broke a custom rack middleware in my app, because in some scenarios it will be just before the Formatter middleware.

If I need to make changes to the response before returning from my middleware, I must now first check if the response is an Array or a Rack::Response (The middleware is used in many different places).

If this is expected behavior, issue can be closed ;)

Yes, since UPGRADING documents this pretty thoroughly I think we're good. It's also easier to work with a Rack::Response in another middleware, you might want to consider adjusting all your code to that.

@dblock sorry to reopen an old issue here. From what I can tell, the Rack API docs do imply that you should not return the Rack::Response itself from the app.call:

Your application's call should end returning Response#finish.

via http://www.rubydoc.info/github/rack/rack/Rack/Response

Also:

A Rack application is a Ruby object (not a class) that responds to call. It takes exactly one argument, the environment and returns an Array of exactly three values: The status, the headers, and the body.

http://www.rubydoc.info/github/rack/rack/master/file/SPEC

I just don't read it the same way I suppose. If we undo this the whole streaming thing doesn't work at all, no?

@dblock, I haven't read the code, but wouldn't you do streaming by having the third value of the array be a special object that responds to each? http://www.rubydoc.info/github/rack/rack/file/SPEC#The_Body

From what I understand that doesn't work because of how Rack treats this, but I could be totally wrong. Maybe @dan-corneanu can help us here?

@dblock yeah, I'm just reading the spec, might be things I'm missing. It looks like Rails is just doing something with Rack::Chunked::Body: https://github.com/rails/rails/blob/b6760d8f14af6b00250395f2c830b89e43621860/actionpack/lib/action_controller/metal/streaming.rb

@dblock ran across this again and I still believe that Grape's behavior here is incorrect.

I'm virtually certain that the correct approach for streaming is to return a body (the third array element in the response) that handles the streaming. The response from call should always be a simple array.

I am not sure what to do with this, I think you should raise a PR and we can look at the change?

@dblock With some recent changes to Rack it seems like this may not work correctly anymore as implicit destructuring is no longer supported: rack/rack@72959eb#diff-10b933d2c1fdc82ceecade456c64e1c2