Fix crash dereferencing unowned _publisher in KVO trampoline (#112)

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):

```swift
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.

---------

Co-authored-by: Jesse Squires <[email protected]>

Author: nolanw

CHANGELOG.md

@@ -7,6 +7,11 @@ NEXT
 
 - TBA
 
+5.1.2
+-----
+
+- Fixed possible crash when receiving a KVO notification and so attempting to send a value to `FoilDefaultStorage` or `FoilDefaultStorageOptional`'s publisher after the storage is deallocated. ([#112](https://github.com/jessesquires/Foil/issues/112), [@nolanw](https://github.com/nolanw))
+
 5.1.1
 -----
 

Sources/FoilDefaultStorage.swift

@@ -53,10 +53,11 @@ public struct FoilDefaultStorage<T: UserDefaultsSerializable> {
         // Publisher must be initialized after `registerDefault`,
         // because `fetch` assumes that `registerDefault` has been called before
         // and uses force unwrap
-        self._publisher = CurrentValueSubject<T, Never>(userDefaults.fetch(keyName))
+        let publisher = CurrentValueSubject<T, Never>(userDefaults.fetch(keyName))
+        self._publisher = publisher
 
-        self._observer = ObserverTrampoline(userDefaults: userDefaults, key: keyName) { [unowned _publisher] in
-            _publisher.send(userDefaults.fetch(keyName))
+        self._observer = ObserverTrampoline(userDefaults: userDefaults, key: keyName) {
+            publisher.send(userDefaults.fetch(keyName))
         }
     }
 }

Sources/FoilDefaultStorageOptional.swift

@@ -51,9 +51,10 @@ public struct FoilDefaultStorageOptional<T: UserDefaultsSerializable> {
     public init(key keyName: String, userDefaults: UserDefaults = .standard) {
         self.key = keyName
         self._userDefaults = userDefaults
-        self._publisher = CurrentValueSubject<T?, Never>(userDefaults.fetchOptional(keyName))
-        self._observer = ObserverTrampoline(userDefaults: userDefaults, key: keyName) { [unowned _publisher] in
-            _publisher.send(userDefaults.fetchOptional(keyName))
+        let publisher = CurrentValueSubject<T?, Never>(userDefaults.fetchOptional(keyName))
+        self._publisher = publisher
+        self._observer = ObserverTrampoline(userDefaults: userDefaults, key: keyName) {
+            publisher.send(userDefaults.fetchOptional(keyName))
         }
     }
 }