Fix crash dereferencing unowned _publisher in KVO trampoline

[nolanw]

A FoilDefaultStorage{,Optional} can deallocate while its ObserverTrampoline sticks around long enough to receive a KVO notification. (I'm not sure how.) When this happens, the attempt to dereference the captured _publisher crashes.

I don't know how to write a reliable test for the test suite, but I can reproduce the crash ~instantly with this code (e.g. pasted into a new test) in Xcode 16.0 beta 5 on an iPhone 8 running iOS 16.7.8 (as well as in iOS 15, 16, 17, and 18 simulators I have kicking around):

DispatchQueue.global().async {
    var storage: FoilDefaultStorageOptional<Int>?
    while true {
        storage = .init(key: "aloha")
    }
}
while true {
    UserDefaults.standard.set(Int.random(in: Int.min...Int.max), forKey: "aloha")
}

That's an admittedly silly test, but I'm seeing crashes in the wild attempting to dereference _publisher.

The obvious fix to me is to capture _publisher as either weak or strong, instead of unowned. A strong reference doesn't cause a cycle, so I chose it over the theoretically slower weak reference.

[jessesquires]

Ha, wow. What an edge case. Thanks for finding and fixing @nolanw! 💯 🙌🏼

• I left one comment/question about the capture.
• Also, could you add a changelog entry for this? Let's add a new 5.1.2 version.

After we merge this, I'll tag a new release. 😄

[jessesquires]

awesome. thanks @nolanw ! 💯