fix(background-task): ensure thread-safety on iOS (#779)

* fix(background-task): ensure thread-safety of the taskIds dictionary

* fix(background-task): ensure thread-safety of the taskIds dictionary

* Update .changeset/wet-clubs-begin.md

---------

Co-authored-by: anfuerer <admail@gmx.ch>
Co-authored-by: Robin Genz <mail@robingenz.dev>

Author: 3 people

.changeset/wet-clubs-begin.md

@@ -0,0 +1,5 @@
+---
+'@capawesome/capacitor-background-task': patch
+---
+
+fix(ios): ensure thread-safety

packages/background-task/ios/Plugin/BackgroundTask.swift

@@ -2,23 +2,30 @@ import Foundation
 import UIKit
 
 @objc public class BackgroundTask: NSObject {
-    var taskIds: [String: UIBackgroundTaskIdentifier] = [:]
+    private let isolationQueue = DispatchQueue(label: "io.capawesome.backgroundtask.isolation")
+    private var taskIds: [String: UIBackgroundTaskIdentifier] = [:]
 
     @objc public func beforeExit(_ callbackId: String) {
-        DispatchQueue.global().async {
+        // Use the isolation queue to synchronize access to the taskIds dictionary
+        isolationQueue.async { [weak self] in
+            guard let self = self else { return }
+            
             var taskId = UIBackgroundTaskIdentifier.invalid
             taskId = UIApplication.shared.beginBackgroundTask {
-                // Finish the task if time expires.
-                UIApplication.shared.endBackgroundTask(taskId)
-                self.taskIds.removeValue(forKey: callbackId)
+                // Finish the task if time expires. This expiration handler can run on any thread,
+                // so we MUST hop back to the isolation queue.
+                self.isolationQueue.async {
+                    UIApplication.shared.endBackgroundTask(taskId)
+                    self.taskIds.removeValue(forKey: callbackId)
+                }
             }
             self.taskIds[callbackId] = taskId
         }
     }
 
     @objc public func finish(_ callbackId: String) {
-        DispatchQueue.global().async {
-            guard let taskId = self.taskIds[callbackId] else {
+        isolationQueue.async { [weak self] in
+            guard let self = self, let taskId = self.taskIds[callbackId] else {
                 return
             }
             UIApplication.shared.endBackgroundTask(taskId)