sys.processで複数のProcessBuilderを接続した場合にブロッキングが発生する問題（2.12.0-M1で解決予定） ref: http://qiita.com/rubyu/items/1c28f0b21dbd4d06262b

file0.scala

import sys.process._

def input = new ByteArrayInputStream("input".getBytes)
// wcコマンドにinputという文字列を標準入力から渡す x 100回
for (i <- 0 until 100)
  "wc" #< input !!

file1.scala

  private[process] class PipedProcesses(a: ProcessBuilder, b: ProcessBuilder, defaultIO: ProcessIO, toError: Boolean) extends CompoundProcess {
    protected[this] override def runAndExitValue() = {
      val currentSource = new SyncVar[Option[InputStream]]
      val pipeOut       = new PipedOutputStream
      val source        = new PipeSource(currentSource, pipeOut, a.toString)
      source.start()

      val pipeIn      = new PipedInputStream(pipeOut)
      val currentSink = new SyncVar[Option[OutputStream]]
      val sink        = new PipeSink(pipeIn, currentSink, b.toString)
      sink.start()

      def handleOutOrError(fromOutput: InputStream) = currentSource put Some(fromOutput)

      val firstIO =
        if (toError)
          defaultIO.withError(handleOutOrError)
        else
          defaultIO.withOutput(handleOutOrError)
      // この処理より
      val secondIO = defaultIO.withInput(toInput => currentSink put Some(toInput))
      val second = b.run(secondIO)
      val first = a.run(firstIO)
      try {
        runInterruptible {
          val exit1 = first.exitValue()
          currentSource put None
          // こっちのが早いタイミングで実行されると、プロセスが正常に接続されない
          currentSink put None
          val exit2 = second.exitValue()
          // Since file redirection (e.g. #>) is implemented as a piped process,
          // we ignore its exit value so cmd #> file doesn't always return 0.
          if (b.hasExitValue) exit2 else exit1
        } {
          first.destroy()
          second.destroy()
        }
      }
      finally {
        BasicIO close pipeIn
        BasicIO close pipeOut
      }
    }
  }