Use os_unfair_lock for MultiThreadDictionary instead of DispatchQueue

Tusker/Caching/ImageCache.swift

@@ -24,7 +24,7 @@ class ImageCache {
     private let cache: ImageDataCache
     private let desiredPixelSize: CGSize?
     
-    private var groups = MultiThreadDictionary<URL, RequestGroup>(name: "ImageCache request groups")
+    private var groups = MultiThreadDictionary<URL, RequestGroup>()
     
     private var backgroundQueue = DispatchQueue(label: "ImageCache completion queue", qos: .default)
     
@@ -96,7 +96,7 @@ class ImageCache {
             if let data = data {
                 try? self.cache.set(url.absoluteString, data: data, image: image)
             }
-            self.groups.removeValueWithoutReturning(forKey: url)
+            _ = self.groups.removeValue(forKey: url)
         }
         groups[url] = group
         return group

Tusker/MultiThreadDictionary.swift

@@ -7,52 +7,79 @@
 //
 
 import Foundation
+import os
 
-class MultiThreadDictionary<Key: Hashable, Value> {
+// once we target iOS 16, replace uses of this with OSAllocatedUnfairLock<[Key: Value]>
-    private let name: String
+// to make the lock semantics more clear
-    private var dict = [Key: Value]()
+@available(iOS, obsoleted: 16.0)
-    private let queue: DispatchQueue
+class MultiThreadDictionary<Key: Hashable & Sendable, Value: Sendable> {
+    private let lock: any Lock<[Key: Value]>
     
-    init(name: String) {
+    init() {
-        self.name = name
+        if #available(iOS 16.0, *) {
-        self.queue = DispatchQueue(label: "MultiThreadDictionary (\(name)) Coordinator", attributes: .concurrent)
+            self.lock = OSAllocatedUnfairLock(initialState: [:])
+        } else {
+            self.lock = UnfairLock(initialState: [:])
+        }
     }
     
     subscript(key: Key) -> Value? {
         get {
-            var result: Value? = nil
+            return lock.withLock { dict in
-            queue.sync {
+                dict[key]
-                result = dict[key]
             }
-            return result
         }
         set(value) {
-            queue.async(flags: .barrier) {
+            lock.withLock { dict in
-                self.dict[key] = value
+                dict[key] = value
             }
         }
     }
     
-    func removeValueWithoutReturning(forKey key: Key) {
-        queue.async(flags: .barrier) {
-            self.dict.removeValue(forKey: key)
-        }
-    }
-    
     /// 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? {
-        var value: Value? = nil
+        return lock.withLock { dict in
-        queue.sync(flags: .barrier) {
+            dict.removeValue(forKey: key)
-            value = dict.removeValue(forKey: key)
         }
-        return value
     }
     
     func contains(key: Key) -> Bool {
-        var value: Bool!
+        return lock.withLock { dict in
-        queue.sync {
+            dict.keys.contains(key)
-            value = dict.keys.contains(key)
+        }
-        }
+    }
-        return value
+    
+    func withLock<R>(_ body: @Sendable (inout [Key: Value]) throws -> R) rethrows -> R where R: Sendable {
+        return try lock.withLock(body)
+    }
+}
+
+// TODO: replace this only with OSAllocatedUnfairLock
+@available(iOS, obsoleted: 16.0)
+fileprivate protocol Lock<State> {
+    associatedtype State
+    func withLock<R>(_ body: @Sendable (inout State) throws -> R) rethrows -> R where R: Sendable
+}
+
+@available(iOS 16.0, *)
+extension OSAllocatedUnfairLock: Lock {
+}
+
+// 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.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)
     }
 }

Tusker/Views/AccountDisplayNameLabel.swift

@@ -36,7 +36,7 @@ struct AccountDisplayNameLabel<Account: AccountProtocol>: View {
         let matches = emojiRegex.matches(in: account.displayName, options: [], range: fullRange)
         guard !matches.isEmpty else { return }
         
-        let emojiImages = MultiThreadDictionary<String, Image>(name: "AcccountDisplayNameLabel Emoji Images")
+        let emojiImages = MultiThreadDictionary<String, Image>()
         
         let group = DispatchGroup()
         

Tusker/Views/BaseEmojiLabel.swift

@@ -38,10 +38,7 @@ extension BaseEmojiLabel {
             return
         }
         
-        // not using a MultiThreadDictionary so that cached images can be added immediately
+        let emojiImages = MultiThreadDictionary<String, UIImage>()
-        // without jumping through various queues so that we can use them immediately
-        // in building either the final string or the string with placeholders
-        var emojiImages: [String: UIImage] = [:]
         var foundEmojis = false
         
         let group = DispatchGroup()
@@ -71,13 +68,9 @@ extension BaseEmojiLabel {
                               group.leave()
                               return
                     }
-                    // sync back to the main thread to add the dictionary
-                    // todo: using the main thread for this isn't great
-                    DispatchQueue.main.async {
                     emojiImages[emoji.shortcode] = transformedImage
                     group.leave()
                 }
-                }
                 if let request = request {
                     emojiRequests.append(request)
                 }
@@ -92,6 +85,11 @@ extension BaseEmojiLabel {
         func buildStringWithEmojisReplaced(usePlaceholders: Bool) -> NSAttributedString {
             let mutAttrString = NSMutableAttributedString(attributedString: attributedString)
             
+            // lock once for the entire loop, rather than lock/unlocking for each iteration to do the lookup
+            // OSAllocatedUnfairLock.withLock expects a @Sendable closure, so this warns about captures of non-sendable types (attribute dstrings, text checking results)
+            // even though the closures is invoked on the same thread that withLock is called, so it's unclear why it needs to be @Sendable (FB11494878)
+            // so, just ignore the warnings
+            emojiImages.withLock { emojiImages in
                 // replaces the emojis starting from the end of the string as to not alter the indices of preceeding emojis
                 for match in matches.reversed() {
                     let shortcode = (attributedString.string as NSString).substring(with: match.range(at: 1))
@@ -108,6 +106,7 @@ extension BaseEmojiLabel {
                     let attachmentStr = NSAttributedString(attachment: attachment)
                     mutAttrString.replaceCharacters(in: match.range, with: attachmentStr)
                 }
+            }
             
             return mutAttrString
         }