Create a multipart/form-data body parser of type Source[ByteString, _]

[Juan Miguel Cejuela]

I'm struggling with the following, I want to create a body parser that accepts multipart/form-data and outputs `Source[ByteString, _]`, that is, I want to create a body parser with this signature:

BodyParser[MultipartFormData[Source[ByteString, _]]

The reason is that I want to stream each file (source) later, pretty much as done in https://www.playframework.com/documentation/2.5.x/ScalaBodyParsers#Directing-the-body-elsewhere 

Using Play 2.5.18, I've achieved streaming the whole body as given in the example but I'm struggling with streaming file parts. 

I'm trying something like this:

def handleFilePartAsFile: FilePartHandler[Source[ByteString, _]] = {
    case FileInfo(partName, filename, contentType) =>
      Accumulator
        .source[ByteString]
        .map { source =>
          println("Hey, I'm here")
          FilePart(partName, filename, contentType, source)
        }
}


def experimental = Action(parse.multipartFormData(handleFilePartAsFile)) { implicit req =>
    val retString = req.body.files
      .map { filePart =>
        //stream source encapsulated in filePart...
        filePart.toString
      }
      .mkString(" -- ")


    Ok(retString)
}

However, the call never ends. I can see the `println`, but then, somehow the Source or Accumulator's Future never ends.

What am I doing wrong? 

[Juan Miguel Cejuela]

Update:

when I consume the Source right in the `FilePartHandler` (for example with a WSClient), then handler ends, and my actual goal of streaming the source is achieved.

Either way, I cannot seem to return back to the `Action` controller a `MultipartFormData` of `[Source[ByteString, _]]` -- I did achieve return a simpler `[Source[ByteString, _]]` (without prior consuming) and then process it however I want in the action controller.

[Greg Methvin]

In your original example, you're not consuming the source at all. Per the documentation for Accumulator.source:

Extreme care must be taken when using this accumulator - the source must always be materialized and consumed. If it isn't, this could lead to resource leaks and deadlocks upstream.

Did you try consuming the source or connecting it to a sink inside your action? What exactly are you trying to do with the source?

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Greg Methvin

Tech Lead - Play Framework

Lightbend, Inc.

[Juan Miguel Cejuela]

Thanks Greg,

Yes, in the second example I do consume the source and works. Concretely, I want to stream files elsewhere upon files uploads.

However, more generally, ideally I would be able to obtain the Source[ByteString, _] of each FilePart as a library-like utility (in a BodyParser), so that later different controllers’ Actions can do different things with those sources. What I would like is a general BodyParser that returns a Source[ByteString, _] and another parser that returns a MultipartFormData of type Source[ByteString, _]

I tested making a BodyParser that returns the whole Source[ByteString, _] of the body, and works (pretty much as in in this doc, but without making a WSClient call, just returning the Source.

I also saw that Play 2.6.x implemented a new BodyParser that returns a ByteString.

Makes sense?

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Greg Methvin

Tech Lead - Play Framework

Lightbend, Inc.

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