Overriding Content-Length header with Rack/Sinatra

Question

In Sinatra, how can I override the Content-Length header in the response to set my own value?

The last line in my method returns the following:

[200, {'Content-Type' => component.content_type,
'Content-Length' => component.content_length.to_s}, component.content.data]

This way I was hoping to override the content value, but it results in an exception:

Unexpected error while processing request: Content-Length header was 2, but should be 0

I would like to return a different value for the content length. Is there a way to do this?

Why do you want to override content-length – that can surely only cause problems. The behaviour depends on which server you’re using (I can’t reproduce the error you see with any of the default servers Sinatra uses). What are you trying to do? — matt
In this case the user agent is an SAP system that is querying the data and is expecting the length of a document but not the document itself, i.e. it is interpreting the data in its own way. I am using Thin, if that makes any difference, although I am thinking that Rack is handling the adding of content length, as I understand there is a middleware for it, something I haven't quite grasped. — mydoghasworms

matt matt · Accepted Answer · 2014-06-05T19:57:47

This error is being raised by the Rack::Lint middleware, so the quick fix would be to not use that piece of middleware. Depending on how you are starting your application that may be tricky though – Rack adds it in certain cases if you use rackup.

A better solution would be to change your client to use a HTTP HEAD request rather than a GET. In Sinatra defining a GET route automatically defines a matching HEAD route. Using HEAD will cause the server to send the headers but not the body, and you should be able to set the Content-Length to whatever you want. It will also avoid the Rack::Lint error.

Here is a gist explaining how to disable Rack::Lint:

module Rack
  class Lint
    def call(env = nil)
      @app.call(env)
    end
  end
end

(Taken from https://gist.github.com/shtirlic/2146256).

Overriding Content-Length header with Rack/Sinatra

1 Answers