Middleware unable to write headers in before blocks

[neptuwunium]

Reproduction:

class API < Grape::API
  before do
    header "X-Test-A", "Success"
  end

  class HeaderTest < Grape::Middleware::Base
    def before
      endpoint = env['api.endpoint']
      endpoint.header "X-Test-B", "Success"
    end
  end

  use HeaderTest

  get '/' do
    present ""
  end
end

Expected result:

  HTTP/1.1 200 OK
  Content-Type: text/plain
  Content-Length: 0
  Cache-Control: max-age=0, private, must-revalidate
  Connection: keep-alive
  X-Test-A: Success
  X-Test-B: Success
  Server: thin

Actual result:

  HTTP/1.1 500 Internal Server Error
  Content-Type: text/plain
  Content-Length: 39
  Cache-Control: max-age=0, private, must-revalidate
  Connection: keep-alive
  Server: thin

  undefined method `[]=' for nil:NilClass

Cause: Headers are sent before any before calls in middleware Before is called before any response is created.

Workaround: Set headers in after (@app_response[1])

[neptuwunium]

N.B. trying to access response also yields undefined method []' for nil:NilClass`

[dblock]

I agree it should be possible, marking this as a feature request.

[gottfrois]

I also encounter the exact same issue. I was expecting to be able to add custom headers in the before method but the headers instance variable is defined after. Looking forward for this to be solved :)

[namusyaka]

I'd like to fix this after solving #1243

[namusyaka]

@dblock This can be closed by #1244.