Don't parent background managed object contexts to view context

Otherwise, certain operations require the background contexts to
interact with the view context, which can block the main thread from
accessing the view context (potentially causing hitches if the view
context access is in a critical path, like cell fetching).

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/CoreData/MastodonCachePersistentStore.swift

@@ -20,13 +20,15 @@ class MastodonCachePersistentStore: NSPersistentContainer {
     
     private(set) lazy var backgroundContext: NSManagedObjectContext = {
         let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
-        context.parent = self.viewContext
+        context.persistentStoreCoordinator = self.persistentStoreCoordinator
+        context.automaticallyMergesChangesFromParent = true
         return context
     }()
     
     private(set) lazy var prefetchBackgroundContext: NSManagedObjectContext = {
         let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
-        context.parent = self.viewContext
+        context.persistentStoreCoordinator = self.persistentStoreCoordinator
+        context.automaticallyMergesChangesFromParent = true
         return context
     }()
     
@@ -51,6 +53,8 @@ class MastodonCachePersistentStore: NSPersistentContainer {
             }
         }
         
+        viewContext.automaticallyMergesChangesFromParent = true
+        
         NotificationCenter.default.addObserver(self, selector: #selector(managedObjectsDidChange), name: .NSManagedObjectContextObjectsDidChange, object: viewContext)
     }
     

Tusker/MultiThreadDictionary.swift

@@ -7,52 +7,79 @@
 //
 
 import Foundation
+import os
 
-class MultiThreadDictionary<Key: Hashable, Value> {
-    private let name: String
-    private var dict = [Key: Value]()
-    private let queue: DispatchQueue
+// once we target iOS 16, replace uses of this with OSAllocatedUnfairLock<[Key: Value]>
+// 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]>
     
-    init(name: String) {
-        self.name = name
-        self.queue = DispatchQueue(label: "MultiThreadDictionary (\(name)) Coordinator", attributes: .concurrent)
+    init() {
+        if #available(iOS 16.0, *) {
+            self.lock = OSAllocatedUnfairLock(initialState: [:])
+        } else {
+            self.lock = UnfairLock(initialState: [:])
+        }
     }
     
     subscript(key: Key) -> Value? {
         get {
-            var result: Value? = nil
-            queue.sync {
-                result = dict[key]
+            return lock.withLock { dict in
+                dict[key]
             }
-            return result
         }
         set(value) {
-            queue.async(flags: .barrier) {
-                self.dict[key] = value
+            lock.withLock { dict in
+                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
-        queue.sync(flags: .barrier) {
-            value = dict.removeValue(forKey: key)
+        return lock.withLock { dict in
+            dict.removeValue(forKey: key)
         }
-        return value
     }
     
     func contains(key: Key) -> Bool {
-        var value: Bool!
-        queue.sync {
-            value = dict.keys.contains(key)
-        }
-        return value
+        return lock.withLock { dict in
+            dict.keys.contains(key)
+        }
+    }
+    
+    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/Screens/Explore/FeaturedProfileCollectionViewCell.swift

@@ -52,7 +52,7 @@ class FeaturedProfileCollectionViewCell: UICollectionViewCell {
         displayNameLabel.updateForAccountDisplayName(account: account)
         
         noteTextView.setTextFromHtml(account.note)
-        noteTextView.setEmojis(account.emojis)
+        noteTextView.setEmojis(account.emojis, identifier: account.id)
         
         avatarImageView.image = nil
         if let avatar = account.avatar {

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

@@ -24,6 +24,7 @@ extension BaseEmojiLabel {
         // blergh
         precondition(Thread.isMainThread)
         
+        emojiIdentifier = identifier
         emojiRequests.forEach { $0.cancel() }
         emojiRequests = []
         
@@ -38,10 +39,7 @@ extension BaseEmojiLabel {
             return
         }
         
-        // not using a MultiThreadDictionary so that cached images can be added immediately
-        // 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] = [:]
+        let emojiImages = MultiThreadDictionary<String, UIImage>()
         var foundEmojis = false
         
         let group = DispatchGroup()
@@ -71,13 +69,9 @@ 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 +86,11 @@ 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 +107,7 @@ extension BaseEmojiLabel {
                     let attachmentStr = NSAttributedString(attachment: attachment)
                     mutAttrString.replaceCharacters(in: match.range, with: attachmentStr)
                 }
+            }
             
             return mutAttrString
         }

Tusker/Views/ContentTextView.swift

@@ -57,8 +57,8 @@ class ContentTextView: LinkTextView, BaseEmojiLabel {
     }
     
     // MARK: - Emojis
-    func setEmojis(_ emojis: [Emoji]) {
-        replaceEmojis(in: attributedText!, emojis: emojis, identifier: emojiIdentifier) { attributedString, didReplaceEmojis in
+    func setEmojis(_ emojis: [Emoji], identifier: String?) {
+        replaceEmojis(in: attributedText!, emojis: emojis, identifier: identifier) { attributedString, didReplaceEmojis in
             guard didReplaceEmojis else {
                 return
             }

Tusker/Views/EmojiLabel.swift

@@ -21,13 +21,9 @@ class EmojiLabel: UILabel, BaseEmojiLabel {
     func setEmojis(_ emojis: [Emoji], identifier: String) {
         guard emojis.count > 0, let attributedText = attributedText else { return }
 
-        self.emojiIdentifier = identifier
-        emojiRequests.forEach { $0.cancel() }
-        emojiRequests = []
-        hasEmojis = true
-        
-        replaceEmojis(in: attributedText.string, emojis: emojis, identifier: identifier) { [weak self] (newAttributedText, _) in
+        replaceEmojis(in: attributedText.string, emojis: emojis, identifier: identifier) { [weak self] (newAttributedText, didReplaceEmojis) in
             guard let self = self, self.emojiIdentifier == identifier else { return }
+            self.hasEmojis = didReplaceEmojis
             self.attributedText = newAttributedText
             self.setNeedsLayout()
             self.setNeedsDisplay()

Tusker/Views/Profile Header/ProfileHeaderView.swift

@@ -110,7 +110,7 @@ class ProfileHeaderView: UIView {
         
         noteTextView.navigationDelegate = delegate
         noteTextView.setTextFromHtml(account.note)
-        noteTextView.setEmojis(account.emojis)
+        noteTextView.setEmojis(account.emojis, identifier: account.id)
         
         // don't show relationship label for the user's own account
         if accountID != mastodonController.account?.id {
@@ -148,7 +148,7 @@ class ProfileHeaderView: UIView {
             valueTextView.isSelectable = false
             valueTextView.font = .systemFont(ofSize: 17)
             valueTextView.setTextFromHtml(field.value)
-            valueTextView.setEmojis(account.emojis)
+            valueTextView.setEmojis(account.emojis, identifier: account.id)
             valueTextView.textAlignment = .left
             valueTextView.awakeFromNib()
             valueTextView.navigationDelegate = delegate

Tusker/Views/StatusContentTextView.swift

@@ -16,9 +16,8 @@ class StatusContentTextView: ContentTextView {
     
     func setTextFrom(status: StatusMO) {
         statusID = status.id
-        emojiIdentifier = status.id
         setTextFromHtml(status.content)
-        setEmojis(status.emojis)
+        setEmojis(status.emojis, identifier: status.id)
     }
     
     override func getMention(for url: URL, text: String) -> Mention? {

Tusker/en.lproj/Localizable.stringsdict

@@ -5,7 +5,7 @@
 	<key>poll votes count</key>
 	<dict>
 		<key>NSStringLocalizedFormatKey</key>
-		<string>%2$#@votes@</string>
+		<string>%#@votes@</string>
 		<key>votes</key>
 		<dict>
 			<key>NSStringFormatSpecTypeKey</key>