I tried the following code to create a framer that splits a stream of ASCII bytes into frames separated by the pipe ascii character: "|".
import Network
fileprivate let pipe = Character("|").asciiValue!
class PipeFramer: NWProtocolFramerImplementation {
static let label = "Pipe framer"
static let definition = NWProtocolFramer.Definition(implementation: PipeFramer.self)
var minLengthUntilNextMessage = 1 {
didSet { print("client: minLength set to", minLengthUntilNextMessage) }
}
required init(framer: NWProtocolFramer.Instance) {}
func start(framer: NWProtocolFramer.Instance) -> NWProtocolFramer.StartResult { .ready }
func handleInput(framer: NWProtocolFramer.Instance) -> Int {
while true {
var delimiterPosition: Int?
_ = framer.parseInput(minimumIncompleteLength: minLengthUntilNextMessage, maximumLength: 65535) { buffer, endOfMessage in
if let buffer = buffer {
print("client: parsing buffer: \"\(String(bytes: buffer, encoding: .utf8) ?? buffer.debugDescription)\"")
if let indexOfDelimiter = buffer.firstIndex(of: pipe) {
minLengthUntilNextMessage = 1
delimiterPosition = indexOfDelimiter
} else {
minLengthUntilNextMessage = buffer.count + 1
}
} else {
print("client: no buffer")
}
return 0
}
if let length = delimiterPosition {
guard framer.deliverInputNoCopy(length: length, message: .init(instance: framer), isComplete: true) else {
return 0
}
_ = framer.parseInput(minimumIncompleteLength: 1, maximumLength: 65535) { _,_ in 1 }
} else {
return minLengthUntilNextMessage
}
}
}
func handleOutput(framer: NWProtocolFramer.Instance, message: NWProtocolFramer.Message, messageLength: Int, isComplete: Bool) {
try! framer.writeOutputNoCopy(length: messageLength)
framer.writeOutput(data: [pipe])
}
func wakeup(framer: NWProtocolFramer.Instance) {}
func stop(framer: NWProtocolFramer.Instance) -> Bool { return true }
func cleanup(framer: NWProtocolFramer.Instance) { }
}
The problem is that from the moment I get a chunk that does not end with "|", the framer gets stuck on that chunk. So the other chunks that come after this incomplete chunk never fully arrive in the framer.parseInput(...) call. Because it always parses chunks of minimumIncompleteLength and hence never arrives to the point where the next "|" is.
Here is a simple reproduction of this problem:
- Create a TCP server
- Setup the server so that it sends chunks of messages when a client connects.
- Connect to the server (created in 1.) using the framer from above.
- Start receiving messages.
Swift Code:
import Network
let client = DispatchQueue(label: "Server")
let server = DispatchQueue(label: "Client")
let networkParameters = NWParameters.tcp
networkParameters.defaultProtocolStack.applicationProtocols.insert(NWProtocolFramer.Options(definition: PipeFramer.definition), at: 0)
let server = try! NWListener(using: .tcp)
server.newConnectionHandler = { connection in
print("server: new connection from", connection.endpoint)
print("server (client \(connection.endpoint)): state", connection.state)
connection.viabilityUpdateHandler = { viable in
print("server (client \(connection.endpoint)): state", connection.state)
if viable {
print("server: sending")
connection.send(content: "A|Be||Sea".data(using: .utf8)!, isComplete: false, completion: .idempotent)
serverQueue.asyncAfter(deadline: .now() + 5) {
print("server: sending second part")
connection.send(content: " is longer than expected|0|".data(using: .utf8)!, isComplete: true, completion: .idempotent)
}
serverQueue.asyncAfter(deadline: .now() + 8) {
print("server: sending last part")
connection.send(content: "Done|".data(using: .utf8)!, isComplete: true, completion: .idempotent)
}
}
}
connection.start(queue: serverQueue)
}
server.stateUpdateHandler = { state in
print("server:", state)
if state == .ready, let port = server.port {
print("server: listening on", port)
}
}
server.start(queue: serverQueue)
let client = NWConnection(to: .hostPort(host: "localhost", port: server.port!), using: networkParameters)
func receiveNext() {
client.receiveMessage { (data, context, complete, error) in
let content: String
if let data = data {
content = String(data: data, encoding: .utf8) ?? data.description
} else {
content = data?.debugDescription ?? "<no data>"
}
print("client: received \"\(content)\"", context.debugDescription, complete, error?.localizedDescription ?? "No error")
receiveNext()
}
}
client.stateUpdateHandler = { state in
print("client:", state)
if state == .ready {
print("client: receiving")
receiveNext()
}
}
client.start(queue: clientQueue)
Results in:
server: waiting(POSIXErrorCode: Network is down)
server: ready
server: listening on 54894
client: preparing
client: ready
client: receiving
server: new connection from ::1.53179
server (client ::1.53179): state setup
server (client ::1.53179): state ready
server: sending
client: parsing buffer: "A|Be||Sea"
client: minLength set to 1
client: parsing buffer: "Be||Sea"
client: minLength set to 1
client: parsing buffer: "|Sea"
client: minLength set to 1
client: parsing buffer: "Sea"
client: minLength set to 4
client: parsing buffer: ""
client: minLength set to 1
client: received "A" Optional(Network.NWConnection.ContentContext) true No error
client: received "Be" Optional(Network.NWConnection.ContentContext) true No error
client: received "<no data>" Optional(Network.NWConnection.ContentContext) true No error
client: parsing buffer: "Sea"
client: minLength set to 4
server: sending second part
client: parsing buffer: "Sea "
client: minLength set to 5
client: parsing buffer: "Sea i"
client: minLength set to 6
server: sending last part
client: parsing buffer: "Sea is"
client: minLength set to 7
client: parsing buffer: "Sea is "
client: minLength set to 8
Notice that the fourth and fifth message are never received by the client. How should I write the Framer so that it receives messages after an incoming incomplete chunk?