URLSessionDataDelegate not being called using http2 connection - swift

Question

UPDATE
I've found that if I run the server and then the macOS app and leave it for 40 seconds (so the server has sent 40 "a" characters, one each second) then eventually the didReceive response delegate is called, and the didReceive data delegate then starts getting called with every new bit of data. This leads to logging like this in the console of the macOS app:
URLAuthenticationChallenge
Got response: <NSHTTPURLResponse: 0x6080000385c0> { URL: https://localhost:10443/sub } { status code: 200, headers {
"Content-Type" = "text/plain; charset=utf-8";
Date = "Thu, 03 Nov 2016 16:51:28 GMT";
Vary = "Accept-Encoding";
} }
Received data: Optional("{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n{\"Data\":\"a\"}\n")
Received data: Optional("{\"Data\":\"a\"}\n")
Received data: Optional("{\"Data\":\"a\"}\n")
Received data: Optional("{\"Data\":\"a\"}\n")
Received data: Optional("{\"Data\":\"a\"}\n")
Received data: Optional("{\"Data\":\"a\"}\n")
Received data: Optional("{\"Data\":\"a\"}\n")
...
which suggests that there's some buffering going on somewhere.
I've been testing out how URLSession works with HTTP/2 connections and I've run into some issues.
I've got an incredibly simple macOS app here: https://github.com/hamchapman/http2-barebones-mac-app although the whole code for it is basically just this:
class ViewController: NSViewController, URLSessionDelegate, URLSessionDataDelegate {
override func viewDidLoad() {
var urlComponents = URLComponents()
urlComponents.scheme = "https"
urlComponents.host = "localhost"
urlComponents.port = 10443
guard let url = urlComponents.url else {
print("Bad URL, try again")
return
}
var request = URLRequest(url: url.appendingPathComponent("/sub"))
request.httpMethod = "SUB"
request.timeoutInterval = REALLY_LONG_TIME
let sessionConfiguration = URLSessionConfiguration.ephemeral
sessionConfiguration.timeoutIntervalForResource = REALLY_LONG_TIME
sessionConfiguration.timeoutIntervalForRequest = REALLY_LONG_TIME
let session = Foundation.URLSession(configuration: sessionConfiguration, delegate: self, delegateQueue: nil)
let task: URLSessionDataTask = session.dataTask(with: request)
task.resume()
}
public func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse, completionHandler: #escaping (URLSession.ResponseDisposition) -> Void) {
print("Got response: \(response)")
completionHandler(.allow)
}
public func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
let dataString = String(data: data, encoding: .utf8)
print("Received data: \(dataString)")
}
public func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
print("Error: \(error)")
}
// So it works with self-signed certs (we don't care about TLS etc in this example)
public func urlSession(_ session: URLSession, didReceive challenge: URLAuthenticationChallenge, completionHandler: #escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) {
guard challenge.previousFailureCount == 0 else {
challenge.sender?.cancel(challenge)
completionHandler(.cancelAuthenticationChallenge, nil)
return
}
let allowAllCredential = URLCredential(trust: challenge.protectionSpace.serverTrust!)
completionHandler(.useCredential, allowAllCredential)
}
}
You can see that the http method being used is SUB. This is designed to be a method that you use if you want to subscribe to a given resource, which in my simple example is at path /sub. This should in theory be able to make use of HTTP/2 streaming to send new data over to the macOS app's connection, when the server has new data to send.
Here is the very basic (Go) app that I've been using as the server: https://github.com/hamchapman/http2-barebones-server (the readme has instructions on how to run it).
It's basically setup to accept a SUB request at /sub and send back a 200 OK immediately, and then every second it sends "a" as a bit of data.
The problem I'm facing is that as far as the Go server is concerned, the connection is being made fine. However, the URLSessionDelegate gets called with the expected URLAuthenticationChallenge (the server only allows encrypted connections), but the URLSessionDataDelegate methods that get called when a response is received and when data is received are never called.
You can verify that the server is working as expected by running it and then using the following curl command:
curl --http2 -k -v -X SUB https://localhost:10443/sub
(you might need to download the latest version of curl - see here for info: https://simonecarletti.com/blog/2016/01/http2-curl-macosx/)
I've also verified that the data is actually being received by the connection made in the macOS app (using Wireshark), but the delegate never gets called.
Does anyone know why this might be happening? Is data getting buffered somewhere? Is HTTP/2 support not fully there in URLSession?

Answer 1

It's because the first 512 bytes are buffered: https://forums.developer.apple.com/thread/64875