channelRead not being called in Swift-NIO Datagram's ChannelInboundHandler

Question

I am trying to capture a UDP video stream within a (fresh) vapor application running in Xcode. The data is being streamed by ffmpeg and I can successfully view the stream on the target machine using VLC, which is also the one running the vapor application, using udp://0.0.0.0:5000. I have used various bits of Apple documentation to get to the code below. When I run it, I get these lines of output on the console log, but I wonder if they are not relevant:

2021-07-07 17:59:27.102681+0100 Run[10550:2494617] [si_destination_compare] send failed: Invalid argument
2021-07-07 17:59:27.104056+0100 Run[10550:2494617] [si_destination_compare] send failed: Undefined error: 0

In configure.swift:

try setupClient()

This is the client code:

final class FrameHandler : ChannelInboundHandler {
    typealias InboundIn = AddressedEnvelope<ByteBuffer>
    typealias OutboundOut = AddressedEnvelope<ByteBuffer>

    func channelRead(ctx: ChannelHandlerContext, data: NIOAny) {
        // minimal for question
    }
    
    func errorCaught(ctx: ChannelHandlerContext, error: Error) {
        // minimal for question
    }
}

func setupClient() throws {
    let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
    let bootstrap = DatagramBootstrap(group: group)
        .channelOption(ChannelOptions.socketOption(.so_reuseaddr), value: 1)
        .channelInitializer { channel in
            channel.pipeline.addHandler(FrameHandler())
    }
    defer {
        try! group.syncShutdownGracefully()
    }

    let channel = try bootstrap.bind(host: "0.0.0.0", port: 5000).wait()
    try channel.closeFuture.wait()
}

The problem is that although channelRegistered and channelActive are called, followed by a never-ending stream of readComplete, the important one channelRead never gets called - neither does errorCaught. If I comment out the call to setupClient then there is no network activity, however, if it runs then Xcode's network monitor shows activity consistent with the levels in ffmpeg. So, I believe the connection is being set up.

I wonder if the problem is in the way I am setting the handler up? All the examples use echo or reflecting chat examples, so the inbound handler is set up in the closure of the data-writing function using the context rather than adding it in the initialiser (although, the outbound handler is set up in this way).

Answer 1

I'm assuming you're using Vapor 4 which is based on SwiftNIO 2. And in NIO 2, the ChannelHandlerContext variable is called context and not ctx. So if you just rename all your ctx to context, I'd assume it'll work.