Proxying a web service with Play

Application.scala

package controllers

import play.api.libs.concurrent.Execution.Implicits.defaultContext
import play.api.libs.iteratee.{ Concurrent, Iteratee }
import play.api.libs.ws.{ ResponseHeaders, WS }
import play.api.mvc.{ Action, Controller, SimpleResult }
import scala.concurrent.Promise

object Application extends Controller {

  def proxy(url: String) = Action.async { request =>
    val resultP = Promise[SimpleResult]()

    WS.url(url).get { headers =>
      val iterateeP = Promise[Iteratee[Array[Byte], Unit]]

      // I wonder if we can propagate error or disconnect of the client
      // to the WS library.
      val enumerator = Concurrent.unicast[Array[Byte]](
        onStart = channel => {
          val iteratee = Iteratee.foreach[Array[Byte]](channel.push).map(_ => channel.eofAndEnd())
          iterateeP.success(iteratee)
        })
      resultP.success(Status(headers.status).chunked(enumerator).withHeaders(seqHeaders(headers): _*))
      Iteratee.flatten(iterateeP.future)
    }.map(_.run)

    resultP.future
  }

  /**
   * Transform headers from Map[String, Seq[String]] to Seq[(String, String)]
   */
  def seqHeaders(responseHeaders: ResponseHeaders): Seq[(String, String)] =
    for {
      (name, values) <- responseHeaders.headers.toSeq
      value <- values
    } yield name -> value

}