Abort function for io.Copy

[kemokemo]

Hello @hajimehoshi !

I was moved by the method of playing music by combining go-mp3 and oto library with io.Copy. here
I would like to create a mechanism like jukebox using this technique and incorporate it into my game using ebiten.

So I searched for a way to interrupt music play by interrupting io.Copy and implemented.
Here is an example of an interruption sample written based on your sample.

Would you please give me comments or advice?

[hajimehoshi]

Hi Kemo-kemo-san!

As far as I know, there is no feasible way to suspend copying of io.Copy.

Instead, how about making a 'abortable' copy function? The (pseudo) code would be:

func AbortableCopy(w io.Writer, r io.Reader, abortCh <-chan struct{}) (int, error) {
  type readResult struct {
    buf []uint8
    err error
  }
  readCh := make(chan, readResult)
  abortReadCh := make(chan struct{})
  go func() {
    buf := make([]uint8, 4096)
  readLoop:
    for {
      n, err := r.Read(buf)
      r := ReadResult{buf[:n], nil}
      if err != nil && err != io.EOF {
        r.err = err
      }
      select {
      case readCh <- r:
      case <-abortReadCh:
        break readLoop:
      }
      if err != nil {
        break
      }
    }
    close(readCh)
  }()
  written := 0
loop:
  for {
    select {
    case buf, ok := <-readCh:
      if !ok {
        readCh = nil
        break loop
      }
      n, err := w.Write(buf)
      if err != nil {
        return err
      }
      written += n
    case <-abortCh:
      close(readAbortCh)
      return written, nil
    }
  }
  return written, nil
}

Note that I've not tested that code and there might be something wrong in that.

EDIT: Added readAbortCh to stop the reader goroutine.

[hajimehoshi]

Hmm, that code has a bug: writtenBytes is not accurate when aborting. I don't think it is not hard to fix though.

[hajimehoshi]

I would like to create a mechanism like jukebox using this technique and incorporate it into my game using ebiten.

Sorry I missed this comment. If you are using Ebiten, you can use Player's Pause. http://godoc.org/github.com/hajimehoshi/ebiten/audio#Player.Pause Doesn't this work?

[kemokemo]

Hi hajime-hoshi-san!

I understood that the Copy function should be able to Abort. Thank you so much for the code of AbortableCopy! And now, I am convinced that ebiten's Player is the best choice for me.
I'm sorry to have disturbed you....

I'll try to implement the jukebox in my ebiten game!

[hajimehoshi]

No problem. Thank you for using Oto and Ebiten!