Akka.Net file IO - process only the first n bytes of a file

Question

I have this very simple stream to process a file - just a FileIO.FromFile source, a pass-through flow, and a Last sink:

    var source = FileIO.FromFile(new FileInfo(fileName));

    var flow = Flow.FromFunction<ByteString, ByteString>(x =>
    {
       Log.Info(x.Count.ToString());
       return x;
    });

    var sink = Sink.Last<ByteString>();

    var runnable = source.Via(flow).ToMaterialized(sink, Keep.Right);
    var result = runnable.Run(Context.Materializer()).Result;

The stream runs as expected: the logger spams out the bytestring sizes until the file source is completely exhausted.

Now, I change the sink to use First instead of Last

    var source = FileIO.FromFile(new FileInfo(fileName));

    var flow = Flow.FromFunction<ByteString, ByteString>(x =>
    {
       Log.Info(x.Count.ToString());
       return x;
    });

    var sink = Sink.First<ByteString>();

    var runnable = source.Via(flow).ToMaterialized(sink, Keep.Right);
    var result = runnable.Run(Context.Materializer()).Result;

The documentation for "First" states "cancels after receiving one element", which I assumed meant the sink signals a cancel up the stream, which would close the source. But when this stream is run, two things happen.

1) I get the following debug log message

[DEBUG][17/05/2018 13:55:16][Thread 0004][akka://Demo/user/DATReader/StreamSupervisor-0/Flow-0-1-fileSource] Unhandled message from akka://Demo/user/DATReader/StreamSupervisor-0/Flow-0-0-unknown-operation : Akka.Streams.Actors.Cancel

and

2) The file is locked, so any further attempt to read it fails with an access denied exception.

I also tried using Take(1) on the source, but the same effect is seen.

My question is: how do I read only the first n bytes from a file and shut down the stream gracefully so that any lock (obtained by FileIO.FromFile) is released?

Answer 1

In my case, there is no such error. Try using sharedKillSwitch to control completion. Documentation is here about Dynamic stream handling

            var sharedKillSwitch = KillSwitches.Shared("my-kill-switch");
            var source = FileIO.FromFile(new FileInfo(fileName), chunkSize: 1024)
                    .Via(sharedKillSwitch.Flow<ByteString>());

            var sink = Sink.First<ByteString>();

            var runnable = source.Select(x => {                 
                Log.Info(x.Count.ToString());
                sharedKillSwitch.Shutdown();
                return x;
            })
            .ToMaterialized(sink, Keep.Right);

            var result = runnable.Run(materializer).Result;