Why makes calling error or done in a BodyParser's Iteratee the request hang in Play Framework 2.0?

Question

I am trying to understand the reactive I/O concepts of Play 2.0 framework. In order to get a better understanding from the start I decided to skip the framework's helpers to construct iteratees of different kinds and to write a custom Iteratee from scratch to be used by a BodyParser to parse a request body.

Starting with the information available in Iteratees and ScalaBodyParser docs and two presentations about play reactive I/O this is what I came up with:

import play.api.mvc._
import play.api.mvc.Results._
import play.api.libs.iteratee.{Iteratee, Input}
import play.api.libs.concurrent.Promise
import play.api.libs.iteratee.Input.{El, EOF, Empty}

01 object Upload extends Controller {
02   def send = Action(BodyParser(rh => new SomeIteratee)) { request =>
03     Ok("Done")
04   }
05 }
06
07 case class SomeIteratee(state: Symbol = 'Cont, input: Input[Array[Byte]] = Empty, received: Int = 0) extends Iteratee[Array[Byte], Either[Result, Int]] {
08   println(state + " " + input + " " + received)
09
10   def fold[B](
11     done: (Either[Result, Int], Input[Array[Byte]]) => Promise[B],
12     cont: (Input[Array[Byte]] => Iteratee[Array[Byte], Either[Result, Int]]) => Promise[B],
13     error: (String, Input[Array[Byte]]) => Promise[B]
14   ): Promise[B] = state match {
15     case 'Done => { println("Done"); done(Right(received), Input.Empty) }
16     case 'Cont => cont(in => in match {
17       case in: El[Array[Byte]] => copy(input = in, received = received + in.e.length)
18       case Empty => copy(input = in)
19       case EOF => copy(state = 'Done, input = in)
20       case _ => copy(state = 'Error, input = in)
21     })
22     case _ => { println("Error"); error("Some error.", input) }
23   }
24 }

(Remark: All these things are new to me, so please forgive if something about this is total crap.) The Iteratee is pretty dumb, it just reads all chunks, sums up the number of received bytes and prints out some messages. Everything works as expected when I call the controller action with some data - I can observe all chunks are received by the Iteratee and when all data is read it switches to state done and the request ends.

Now I started to play around with the code because I wanted to see the behaviour for these two cases:

• Switching into state error before all input is read.
• Switching into state done before all input is read and returning a Result instead of the Int.

My understanding of the documentation mentioned above is that both should be possible but actually I am not able to understand the observed behaviour. To test the first case I changed line 17 of the above code to:

17       case in: El[Array[Byte]] => copy(state = if(received + in.e.length > 10000) 'Error else 'Cont, input = in, received = received + in.e.length)

So I just added a condition to switch into the error state if more than 10000 bytes were received. The output I get is this:

'Cont Empty 0
'Cont El([B@38ecece6) 8192
'Error El([B@4ab50d3c) 16384
Error
Error
Error
Error
Error
Error
Error
Error
Error
Error
Error

Then the request hangs forever and never ends. My expectation from the above mentioned docs was that when I call the error function inside fold of an Iteratee the processing should be stopped. What is happening here is that the Iteratee's fold method is called several times after error has been called - well and then the request hangs.

When I switch into done state before reading all input the behaviour is quite similar. Changing line 15 to:

15    case 'Done => { println("Done with " + input); done(if (input == EOF) Right(received) else Left(BadRequest), Input.Empty) }

and line 17 to:

17       case in: El[Array[Byte]] => copy(state = if(received + in.e.length > 10000) 'Done else 'Cont, input = in, received = received + in.e.length)

produces the following output:

'Cont Empty 0
'Cont El([B@16ce00a8) 8192
'Done El([B@2e8d214a) 16384
Done with El([B@2e8d214a)
Done with El([B@2e8d214a)
Done with El([B@2e8d214a)
Done with El([B@2e8d214a)

and again the request hangs forever.

My main question is why the request is hanging in the above mentioned cases. If anybody could shed light on this I would greatly appreciate it!

Answer 1

Your understanding is perfectly right and I have just push a fix to master:

https://github.com/playframework/Play20/commit/ef70e641d9114ff8225332bf18b4dd995bd39bcc

Fixed both cases plus exceptions in the Iteratees.

Nice use of copy in case class for doing an Iteratee BTW.