Fix web socket normalClosure on iOS 13

SignalServiceKit/src/Network/WebSocketPromise.swift

@@ -4,6 +4,7 @@
 //
 
 import Foundation
+import Network
 
 extension WebSocketFactory {
     func webSocketPromise(request: URLRequest, queue: DispatchQueue) -> WebSocketPromise? {
@@ -149,6 +150,14 @@ final class WebSocketPromise: SSKWebSocketDelegate {
         case .waitingForAllResponses(let future):
             switch state.socketError {
             case .some(WebSocketError.closeError(statusCode: WebSocketError.normalClosure, closeReason: _)):
+                // On iOS 13, we expect exactly one message. If we notice "!= 1" results on
+                // well-behaved platforms, surface it as an error since it's probably
+                // leading to incorrect behavior on iOS 13.
+                owsAssertDebug(state.receivedMessages.count == 1, "Must be exactly one message.")
+                let receivedMessages = state.receivedMessages
+                state.receivedMessages = []
+                return { future.resolve(receivedMessages) }
+            case .some(let socketError) where isNormalOS13Closure(state, error: socketError):
                 let receivedMessages = state.receivedMessages
                 state.receivedMessages = []
                 return { future.resolve(receivedMessages) }
@@ -162,6 +171,36 @@ final class WebSocketPromise: SSKWebSocketDelegate {
         }
     }
 
+    /// Checks if this is a "normalClosure" on iOS 13.
+    ///
+    /// On iOS 13, we get back a "Socket is not connected" error instead of a
+    /// callback to `urlSession(_:webSocketTask:didCloseWith:reason:)` when the
+    /// web socket is closed normally (ie, with a closeCode of 1000). However,
+    /// if the web socket is closed *abnormally*, the delegate method is called.
+    /// As a result, we interpret "Socket is not connected" as a normal teardown
+    /// on iOS 13 in some scenarios.
+    ///
+    /// Importantly, this heuristic behaves correctly since we expect exactly
+    /// one response when calling waitForAllResponses(). If we have a response,
+    /// it's successful. If we don't, it's a failure. In the future, if we
+    /// expect an arbitrary number of responses, we'll need a different
+    /// heuristic (it becomes impossible to know if we're still waiting for
+    /// additional responses or if we've received them all -- that EOF behavior
+    /// is exactly what the closeCode provides in the general case).
+    private func isNormalOS13Closure(_ state: State, error: Error) -> Bool {
+        guard #unavailable(iOS 14) else {
+            return false
+        }
+        let nsError = error as NSError
+        guard nsError.domain == kNWErrorDomainPOSIX as String && nsError.code == ENOTCONN else {
+            return false
+        }
+        guard state.receivedMessages.count == 1 else {
+            return false
+        }
+        return true
+    }
+
     // MARK: - SSKWebSocketDelegate
 
     func websocketDidConnect(socket: SSKWebSocket) {