Fix MultiThreadDictionary crash on iOS 15 due to using existential types

See #178

Tusker/MultiThreadDictionary.swift

@@ -13,24 +13,26 @@ import os
 // to make the lock semantics more clear
 @available(iOS, obsoleted: 16.0)
 class MultiThreadDictionary<Key: Hashable & Sendable, Value: Sendable> {
-    private let lock: any Lock<[Key: Value]>
+    private let lock: LockHolder<[AnyHashable: Any]>
     
     init() {
         if #available(iOS 16.0, *) {
-            self.lock = OSAllocatedUnfairLock(initialState: [:])
+            let lock = OSAllocatedUnfairLock(initialState: [:])
+            self.lock = LockHolder(withLock: lock.withLock(_:))
         } else {
-            self.lock = MutexLock(initialState: [:])
+            let lock = UnfairLock(initialState: [:])
+            self.lock = LockHolder(withLock: lock.withLock(_:))
         }
     }
     
     subscript(key: Key) -> Value? {
         get {
-            return lock.withLock { dict in
+            return try! lock.withLock { dict in
                 dict[key]
-            }
+            } as! Value?
         }
         set(value) {
-            lock.withLock { dict in
+            _ = try! lock.withLock { dict in
                 dict[key] = value
             }
         }
@@ -38,20 +40,32 @@ class MultiThreadDictionary<Key: Hashable & Sendable, Value: Sendable> {
     
     /// If the result of this function is unused, it is preferable to use `removeValueWithoutReturning` as it executes asynchronously and doesn't block the calling thread.
     func removeValue(forKey key: Key) -> Value? {
-        return lock.withLock { dict in
+        return try! lock.withLock { dict in
             dict.removeValue(forKey: key)
-        }
+        } as! Value?
     }
     
     func contains(key: Key) -> Bool {
-        return lock.withLock { dict in
+        return try! lock.withLock { dict in
             dict.keys.contains(key)
+        } as! Bool
+    }
+    
+    // TODO: this should really be throws/rethrows but the stupid type-erased lock makes that not possible
+    func withLock<R>(_ body: @Sendable (inout [Key: Value]) -> R) -> R where R: Sendable {
+        return try! lock.withLock { dict in
+            var downcasted = dict as! [Key: Value]
+            defer { dict = downcasted }
+            return body(&downcasted)
+        } as! R
     }
 }
 
-    func withLock<R>(_ body: @Sendable (inout [Key: Value]) throws -> R) rethrows -> R where R: Sendable {
-        return try lock.withLock(body)
-    }
+// this type erased struct is necessary due to a compiler bug with stored constrained existential types
+// see https://github.com/apple/swift/issues/61403
+// see #178
+fileprivate struct LockHolder<State> {
+    let withLock: (_ body: @Sendable (inout State) throws -> any Sendable) throws -> any Sendable
 }
 
 // TODO: replace this only with OSAllocatedUnfairLock
@@ -65,41 +79,22 @@ fileprivate protocol Lock<State> {
 extension OSAllocatedUnfairLock: Lock {
 }
 
-// something is wrong with the UnfairLock impl and it results in segv_accerrs 
-fileprivate class MutexLock<State>: Lock {
+// from http://www.russbishop.net/the-law
+fileprivate class UnfairLock<State>: Lock {
+    private var lock: UnsafeMutablePointer<os_unfair_lock>
     private var state: State
-    private var lock = NSLock()
-    
     init(initialState: State) {
         self.state = initialState
+        self.lock = .allocate(capacity: 1)
+        self.lock.initialize(to: os_unfair_lock())
     }
-    
-    func withLock<R>(_ body: @Sendable (inout State) throws -> R) rethrows -> R where R : Sendable {
-        if !lock.lock(before: Date(timeIntervalSinceNow: 1)) {
-            // if we can't acquire the lock after 1 second, something has gone catastrophically wrong
-            fatalError()
+    deinit {
+        self.lock.deinitialize(count: 1)
+        self.lock.deallocate()
     }
-        defer { lock.unlock() }
+    func withLock<R>(_ body: (inout State) throws -> R) rethrows -> R where R: Sendable {
+        os_unfair_lock_lock(lock)
+        defer { os_unfair_lock_unlock(lock) }
         return try body(&state)
     }
 }
-
-//// from http://www.russbishop.net/the-law
-//fileprivate class UnfairLock<State>: Lock {
-//    private var lock: UnsafeMutablePointer<os_unfair_lock>
-//    private var state: State
-//    init(initialState: State) {
-//        self.state = initialState
-//        self.lock = .allocate(capacity: 1)
-//        self.lock.initialize(to: os_unfair_lock())
-//    }
-//    deinit {
-//        self.lock.deinitialize(count: 1)
-//        self.lock.deallocate()
-//    }
-//    func withLock<R>(_ body: (inout State) throws -> R) rethrows -> R where R: Sendable {
-//        os_unfair_lock_lock(lock)
-//        defer { os_unfair_lock_unlock(lock) }
-//        return try body(&state)
-//    }
-//}