[SR-14815] Regression (Linux): NotificationCenter silently failing since 5.4.1

[mickeyl]

Previous ID	SR-14815
Radar	rdar://problem/79653034
Original Reporter	@mickeyl
Type	Bug

Environment

Swift 5.4, Ubuntu 20.x

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: cb6a9bc2b9d5e55753c5d7918a8daec1

Issue Description:

Please see the following example program:

import Foundation

class Adapter {

    enum State {
        case created
        case searching
        case notfound
        case present
        case gone
    }

    public static let DidUpdateState = Notification.Name("Adapter.DidUpdateState")

    public private(set) var state: State = .created {
        didSet {
            NotificationCenter.default.post(name: Self.DidUpdateState, object: self)
        }
    }

    func doSomething() {
        self.updateState(.searching)
    }
}

extension Adapter {

    func updateState(_ next: State) {
        guard self.state != next else {
            return
        }
        self.state = next
    }
}

class ConcreteAdapter: Adapter {

    override init() {
        super.init()
        NotificationCenter.default.addObserver(forName: Self.DidUpdateState, object: self, queue: nil) { n in
            print("Success: Notification did arrive!")
        }
        print("subscription done")
    }
}

let adapter = ConcreteAdapter()
adapter.doSomething()

print("exiting program...")

On Apple-platforms, this prints `Success: Notification did arrive!` – on Linux, it doesn't.

[weissi]

@swift-ci create

[weissi]

Cc @millenomi

[swift-ci]

Comment by Helge Heß (JIRA)

This is missing two things:

• `Adapter` needs to inherit from `NSObject` (otherwise casting crash at runtime)

• the subscription token returned by `addObserver` needs to be stored, otherwise the subscription is immediately discarded

With those fixes it runs.

[mickeyl]

Interestingly this seems to be a regression in Swift 5.4. Swift 5.3 warns you through a runtime crash that something is wrong, while Swift 5.4 just fails silently.

[swift-ci]

Comment by Helge Heß (JIRA)

Just to clarify:

The missing subscription storage is properly reported in both 5.3.1 and 5.4.1.

What is different is the `NSObject` fix, 5.3.1 complains about it at runtime:
```
root@a0e1f63ce52e:/tmp# ./test
Could not cast value of type 'test.ConcreteAdapter' (0x55936587d158) to 'Foundation.NSObject' (0x7fb617a5fbc8).
Aborted
```

But in 5.4.1 just nothing happens if the adapter does not inherit from NSObject. Actually you'd think the compiler would already complain in `addObserver` in both cases (i.e. why is the `object` on Linux `Any` if `NSObjectProtocol` is required).

[mickeyl]

Same behavior in 5.5.1.