Use synchronized MastodonCache to prevent race condition crashes

Tusker/MastodonCache.swift

@@ -12,10 +12,10 @@ import Pachyderm
 
 class MastodonCache {
     
-    private static var statuses = [String: Status]()
+    private static var statuses = CachedDictionary<Status>(name: "Statuses")
-    private static var accounts = [String: Account]()
+    private static var accounts = CachedDictionary<Account>(name: "Accounts")
-    private static var relationships = [String: Relationship]()
+    private static var relationships = CachedDictionary<Relationship>(name: "Relationships")
-    private static var notifications = [String: Pachyderm.Notification]()
+    private static var notifications = CachedDictionary<Pachyderm.Notification>(name: "Notifications")
     
     static let statusSubject = PassthroughSubject<Status, Never>()
     static let accountSubject = PassthroughSubject<Account, Never>()
@@ -134,3 +134,29 @@ class MastodonCache {
     }
     
 }
+
+class CachedDictionary<Value> {
+    private let name: String
+    private var dict = [String: Value]()
+    private let queue: DispatchQueue
+    
+    init(name: String) {
+        self.name = name
+        self.queue = DispatchQueue(label: "CachedDictionary (\(name)) Coordinator", attributes: .concurrent)
+    }
+    
+    subscript(key: String) -> Value? {
+        get {
+            var result: Value? = nil
+            queue.sync {
+                result = dict[key]
+            }
+            return result
+        }
+        set(value) {
+            queue.async(flags: .barrier) {
+                self.dict[key] = value
+            }
+        }
+    }
+}