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[SR-14815] Regression (Linux): NotificationCenter silently failing since 5.4.1 #3218

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mickeyl opened this issue Jun 23, 2021 · 6 comments
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@mickeyl
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@mickeyl mickeyl commented Jun 23, 2021

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
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@weissi weissi commented Jun 23, 2021

@swift-ci create

@weissi
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@weissi weissi commented Jun 23, 2021

@swift-ci
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@swift-ci swift-ci commented Jun 23, 2021

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
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@mickeyl mickeyl commented Jun 23, 2021

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
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@swift-ci swift-ci commented Jun 23, 2021

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
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@mickeyl mickeyl commented Nov 9, 2021

Same behavior in 5.5.1.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
@shahmishal shahmishal transferred this issue from apple/swift May 5, 2022
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