Digital PCT265 story ONEAPP-7249 - Better once implementation to avoid mutex lockups.

VDS/Extensions/DispatchQueue+Once.swift

@@ -8,7 +8,7 @@
 import Foundation
 
 extension DispatchQueue {
-    private static var _onceTracker = [String]()
+    private static var _onceTracker = Set<String>()
     
     public class func once(
         file: String = #file,
@@ -31,12 +31,17 @@ extension DispatchQueue {
         token: String,
         block: () -> Void
     ) {
+        // Peek ahead to avoid the intersection.
+        guard !_onceTracker.contains(token) else { return }
+        
         objc_sync_enter(self)
         defer { objc_sync_exit(self) }
         
+        // Double check we are the first in the critical section.
         guard !_onceTracker.contains(token) else { return }
         
-        _onceTracker.append(token)
+        // Execute.
+        _onceTracker.insert(token)
         block()
     }
 }