将套接字用于TCP上的自定义协议

Question

My protocol looks like this:
[size: UInt16][Channel: Uint16][Protobuff packet]
The problem is, it is my understanding that tcp does not guarantee that an the entire packet as sent by the user will arrive all in one. Therefore, there could technically be a situation, where I don't get the entire message at once, or even that a part of another message is sent with this message.

It was quite easy, when I was just assuming that I would be getting the entire message at once, because I just read the first 2 bytes from the stream, made a buffer with the proper size and called read(buffer) however if I start thinking about the possibility of cut-off, I am not sure how to handle it. I tried the following:

    val input = clientSocket.getInputStream()
    while (!isStopped) {
        val bufferedInputStream: BufferedInputStream = BufferedInputStream(input)
        val dataInputStream: DataInputStream = DataInputStream(bufferedInputStream)
        var messageSize: Int? = null
        var channel: Int? = null
        while (true) {
            if (messageSize == null) {
                messageSize = dataInputStream.readUnsignedShort()
            }
            if (channel == null) {
                channel = dataInputStream.readUnsignedShort()
            }
            var bytesRead = 0
            var didReachEnd = false
            val buffer = ByteArray(messageSize - 4)
            while (bytesRead != -1 && bytesRead != messageSize) {
                val temp = dataInputStream.read(buffer)
                if (temp == -1) {
                    didReachEnd = true
                } else {
                    bytesRead += temp
                }
                Logger.debug(this::class.java, "bytes read: $bytesRead out of $messageSize")
            }
            val packet = ChatChannel.Packet.parseFrom(buffer)
            print(packet.chatMessage.messageText)
        }
    }

And I attempt to imitate a message splitting by doing the following from my client socket:

        val chatPacket = ChatChannel.Packet.newBuilder().setChatMessage(chatChannel).build()
        val sendPacket = Packet(0,chatPacket.toByteArray())
        val sendPacketByteArray = sendPacket.toByteArray()


        val halfPoint = sendPacketByteArray.size/2
        val firstSlice = sendPacketByteArray.sliceArray(IntRange(0,halfPoint))
        val secondSlice = sendPacketByteArray.sliceArray(IntRange(halfPoint,halfPoint+(halfPoint%2)))

        s.getOutputStream().write(firstSlice)
        sleep(200)
        s.getOutputStream().write(secondSlice)

however my implementation just hangs in the following line in the second time its ran:

bytesRead = dataInputStream.read(buffer,0,buffer.size)

I have a feeling I am missing some idea of how Sockets work, however every implementation I have seen, assumes that when the user is done sending he will close the connection, but in my case it's a chat program, so the user will only close the connection when were done chatting.

What I would like to achieve:

• read size
• read channel
• read bytes until the buffer of size is full
• parse message
• notify
• back to step 0

Edit: After further research, I found that MessageLite library to work with protobuf in Java seems to have methods called writeDelimitedTo(outputStream) and parseDelimitedFrom(inputStream) so it seems most of the work has been done for me.

Answer 1

Two things:

while (bytesRead != -1 && bytesRead != messageSize) {
                bytesRead = dataInputStream.read(buffer,0,buffer.size)

Seems strange to me. You do no increment bytesRead. That is, if messageSize is 100 and you read 90 bytes in the first read, you will read 10 bytes in the second read. You cannot simply say 'bytesRead += ...' since it might return -1. So you need another variable. Check if the return value is non-negative and then do the increment to a byteCounter. This byteCounter needs to be compared to messageSize.

Also: If all you are doing is providing multiple channels for Google protobuf messages, don't do it like this! Create a protobuf message that contains the protobuf message and a channel variable. That should make life a lot easier.