Crash when the actualDelegate property of URLSessionProxyDelegate is lost

[toupper]

Context

Our app crashes due to the exception
[PulseCore.URLSessionProxyDelegate URLSession:didBecomeInvalidWithError:]: unrecognized selector sent to instance
when the actualDelegate is deallocated and the reference lost. This happens in our app when we open and close an UIViewController instance without letting the URLSession finish, thus causing the invalidation of that URLSession.

Explanation

In URLSessionProxyDelegate

func responds(to aSelector: Selector!) -> Bool returns true because the actualDelegate reference (e.g Alamofire.SessionDelegate) implements URLSession:didBecomeInvalidWithError:.

func forwardingTarget(for selector: Selector!) -> Any? returns nil because the actualDelegate was deallocated right after calling func responds(to aSelector: Selector!) -> Bool and the weak reference lost. (e.g when closing the view controller that triggered that network request)

Because there is no forwardingTarget to receive the message and because we claimed that URLSessionProxyDelegate can respond to the selector, [PulseCore.URLSessionProxyDelegate URLSession:didBecomeInvalidWithError:]: is called, and since it does not implements this optional delegate method, crashes.

Note

A workaround is implemented in https://github.com/toupper/Pulse by adding an empty implementation of that optional delegate method in URLSessionProxyDelegate However that is not a final solution, since it could crash for any other delegate method call.

[kean]

However that is not a final solution, since it could crash for any other delegate method call.

I'm thinking about completely removing these options with clever method forwarding. They've been causing issues and I don't want to continue shipping something that I'm not sure is 100% reliable.

If you are using Alamofire, and I think most people do, there is EventMonitor. If you have a custom URLSession, you have access to the delegate and can add logging methods yourself.

[kean]

I think the root cause is that URLSessionProxyDelegate doesn't retain the real delegate the way it should (URLSession does).

[kean]

Should be fixed in Pulse 0.17.0. I didn't use the workaround, but instead made sure the real delegate is retained.

I also decided not to deprecate URLSessionProxyDelegate – it seems to be fine for the most part. But I no longer recommend it as a default option in the documentation.