This project contains two applications for iOS and macOS. That should demonstrate strange behavior of NSURLSession.
After streamTaskWithHostName:port: call invalidateAndCancel stop working. URLSession:didBecomeInvalidWithError: will not be called. And delegate will not be released.
Start iOS app and run steps from 1 to 6. You will see the log:
StreamTask-ios[9314:315321] create
StreamTask-ios[9314:315321] run
StreamTask-ios[9314:315321] Capturing streams...
StreamTask-ios[9314:315321] Streams are captured
StreamTask-ios[9314:315321] Cancelling stream task...
StreamTask-ios[9314:315321] URL Session invalidate...
StreamTask-ios[9314:316823] URLSession become invalid with error:
(null)
StreamTask-ios[9314:315321] URL Session delegate wait for release...
StreamTask-ios[9314:315321] URL Session released delegate
Then run same steps for the macOS app:
StreamTask-mac[9331:318999] create
StreamTask-mac[9331:318999] run
StreamTask-mac[9331:318999] Capturing streams...
StreamTask-mac[9331:318999] Cancelling stream task...
StreamTask-mac[9331:318999] URL Session invalidate...
StreamTask-mac[9331:318999] URL Session delegate wait for release...
StreamTask-mac[9331:318999] Streams are captured
2. NSURLSession doesn't release delegate if NSURLSessionStreamTask created but streams is not captured
After streamTaskWithHostName:port: call invalidateAndCancel works only if captureStreams method called.
Start iOS app and run steps 1 - 6 except step 3 (Capture Streams)
StreamTask-ios[9450:337676] create
StreamTask-ios[9450:337676] run
StreamTask-ios[9450:337676] Cancelling stream task...
StreamTask-ios[9450:337676] URL Session invalidate...
StreamTask-ios[9450:337676] URL Session delegate wait for release...
After cancel method called URLSession:task:didCompleteWithError: will not be called.
Run iOS or macOS app create session (1), run task (2), cancel stream task (4) and nothing happens.
StreamTask-ios[9463:340258] create
StreamTask-ios[9463:340258] run
StreamTask-ios[9463:340258] Cancelling stream task...