Property Executor: Add private dispatch queue

* Reproduced crashes by creating multiple async threads that added and removed tags concurrently.
* Added a private dispatch queue to synchronize access to the delta queue and request queue.
* Crashes no longer happened after this change.
* It is possible for the executor to be flushing while a client response is received and modify the request queue.
* Additionally, there are some code paths that enqueue and update request but does not go through the operation repo, such as updating session count at the start of a new session.

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalUser/Source/Executors/OSPropertyOperationExecutor.swift

@@ -33,6 +33,9 @@ class OSPropertyOperationExecutor: OSOperationExecutor {
     var deltaQueue: [OSDelta] = []
     var updateRequestQueue: [OSRequestUpdateProperties] = []
 
+    // The property executor dispatch queue, serial. This synchronizes access to `deltaQueue` and `updateRequestQueue`.
+    private let dispatchQueue = DispatchQueue(label: "OneSignal.OSPropertyOperationExecutor", target: .global())
+
     init() {
         // Read unfinished deltas from cache, if any...
         // Note that we should only have deltas for the current user as old ones are flushed..
@@ -78,42 +81,49 @@ class OSPropertyOperationExecutor: OSOperationExecutor {
     }
 
     func enqueueDelta(_ delta: OSDelta) {
-        OneSignalLog.onesignalLog(.LL_VERBOSE, message: "OSPropertyOperationExecutor enqueue delta\(delta)")
-        deltaQueue.append(delta)
+        self.dispatchQueue.async {
+            OneSignalLog.onesignalLog(.LL_VERBOSE, message: "OSPropertyOperationExecutor enqueue delta\(delta)")
+            self.deltaQueue.append(delta)
+        }
     }
 
     func cacheDeltaQueue() {
-        OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_DELTA_QUEUE_KEY, withValue: self.deltaQueue)
+        self.dispatchQueue.async {
+            OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_DELTA_QUEUE_KEY, withValue: self.deltaQueue)
+        }
     }
 
     func processDeltaQueue(inBackground: Bool) {
-        if !deltaQueue.isEmpty {
-            OneSignalLog.onesignalLog(.LL_VERBOSE, message: "OSPropertyOperationExecutor processDeltaQueue with queue: \(deltaQueue)")
-        }
-        for delta in deltaQueue {
-            guard let model = delta.model as? OSPropertiesModel else {
-                // Log error
-                continue
+        self.dispatchQueue.async {
+            if !self.deltaQueue.isEmpty {
+                OneSignalLog.onesignalLog(.LL_VERBOSE, message: "OSPropertyOperationExecutor processDeltaQueue with queue: \(self.deltaQueue)")
             }
+            for delta in self.deltaQueue {
+                guard let model = delta.model as? OSPropertiesModel else {
+                    // Log error
+                    continue
+                }
 
-            let request = OSRequestUpdateProperties(
-                properties: [delta.property: delta.value],
-                deltas: nil,
-                refreshDeviceMetadata: false, // Sort this out.
-                modelToUpdate: model,
-                identityModel: OneSignalUserManagerImpl.sharedInstance.user.identityModel // TODO: Make sure this is ok
-            )
-            updateRequestQueue.append(request)
-        }
-        self.deltaQueue = [] // TODO: Check that we can simply clear all the deltas in the deltaQueue
+                let request = OSRequestUpdateProperties(
+                    properties: [delta.property: delta.value],
+                    deltas: nil,
+                    refreshDeviceMetadata: false, // Sort this out.
+                    modelToUpdate: model,
+                    identityModel: OneSignalUserManagerImpl.sharedInstance.user.identityModel // TODO: Make sure this is ok
+                )
+                self.updateRequestQueue.append(request)
+            }
+            self.deltaQueue = [] // TODO: Check that we can simply clear all the deltas in the deltaQueue
 
-        // persist executor's requests (including new request) to storage
-        OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_UPDATE_REQUEST_QUEUE_KEY, withValue: self.updateRequestQueue)
+            // persist executor's requests (including new request) to storage
+            OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_UPDATE_REQUEST_QUEUE_KEY, withValue: self.updateRequestQueue)
 
-        OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_DELTA_QUEUE_KEY, withValue: self.deltaQueue) // This should be empty, can remove instead?
-        processRequestQueue(inBackground: inBackground)
+            OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_DELTA_QUEUE_KEY, withValue: self.deltaQueue) // This should be empty, can remove instead?
+            self.processRequestQueue(inBackground: inBackground)
+        }
     }
 
+    // This method is called by `processDeltaQueue` only and does not need to be added to the dispatchQueue.
     func processRequestQueue(inBackground: Bool) {
         if updateRequestQueue.isEmpty {
             return
@@ -141,38 +151,42 @@ class OSPropertyOperationExecutor: OSOperationExecutor {
         OneSignalCore.sharedClient().execute(request) { _ in
             // On success, remove request from cache, and we do need to hydrate
             // TODO: We need to hydrate after all ? What why ?
-            self.updateRequestQueue.removeAll(where: { $0 == request})
-            OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_UPDATE_REQUEST_QUEUE_KEY, withValue: self.updateRequestQueue)
-            if inBackground {
-                OSBackgroundTaskManager.endBackgroundTask(backgroundTaskIdentifier)
+            self.dispatchQueue.async {
+                self.updateRequestQueue.removeAll(where: { $0 == request})
+                OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_UPDATE_REQUEST_QUEUE_KEY, withValue: self.updateRequestQueue)
+                if inBackground {
+                    OSBackgroundTaskManager.endBackgroundTask(backgroundTaskIdentifier)
+                }
             }
         } onFailure: { error in
             OneSignalLog.onesignalLog(.LL_ERROR, message: "OSPropertyOperationExecutor update properties request failed with error: \(error.debugDescription)")
-            if let nsError = error as? NSError {
-                let responseType = OSNetworkingUtils.getResponseStatusType(nsError.code)
-                if responseType == .missing {
-                    // remove from cache and queue
-                    self.updateRequestQueue.removeAll(where: { $0 == request})
-                    OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_UPDATE_REQUEST_QUEUE_KEY, withValue: self.updateRequestQueue)
-                    // Logout if the user in the SDK is the same
-                    guard OneSignalUserManagerImpl.sharedInstance.isCurrentUser(request.identityModel)
-                    else {
-                        if inBackground {
-                            OSBackgroundTaskManager.endBackgroundTask(backgroundTaskIdentifier)
+            self.dispatchQueue.async {
+                if let nsError = error as? NSError {
+                    let responseType = OSNetworkingUtils.getResponseStatusType(nsError.code)
+                    if responseType == .missing {
+                        // remove from cache and queue
+                        self.updateRequestQueue.removeAll(where: { $0 == request})
+                        OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_UPDATE_REQUEST_QUEUE_KEY, withValue: self.updateRequestQueue)
+                        // Logout if the user in the SDK is the same
+                        guard OneSignalUserManagerImpl.sharedInstance.isCurrentUser(request.identityModel)
+                        else {
+                            if inBackground {
+                                OSBackgroundTaskManager.endBackgroundTask(backgroundTaskIdentifier)
+                            }
+                            return
                         }
-                        return
+                        // The subscription has been deleted along with the user, so remove the subscription_id but keep the same push subscription model
+                        OneSignalUserManagerImpl.sharedInstance.pushSubscriptionModel?.subscriptionId = nil
+                        OneSignalUserManagerImpl.sharedInstance._logout()
+                    } else if responseType != .retryable {
+                        // Fail, no retry, remove from cache and queue
+                        self.updateRequestQueue.removeAll(where: { $0 == request})
+                        OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_UPDATE_REQUEST_QUEUE_KEY, withValue: self.updateRequestQueue)
                     }
-                    // The subscription has been deleted along with the user, so remove the subscription_id but keep the same push subscription model
-                    OneSignalUserManagerImpl.sharedInstance.pushSubscriptionModel?.subscriptionId = nil
-                    OneSignalUserManagerImpl.sharedInstance._logout()
-                } else if responseType != .retryable {
-                    // Fail, no retry, remove from cache and queue
-                    self.updateRequestQueue.removeAll(where: { $0 == request})
-                    OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_UPDATE_REQUEST_QUEUE_KEY, withValue: self.updateRequestQueue)
                 }
-            }
-            if inBackground {
-                OSBackgroundTaskManager.endBackgroundTask(backgroundTaskIdentifier)
+                if inBackground {
+                    OSBackgroundTaskManager.endBackgroundTask(backgroundTaskIdentifier)
+                }
             }
         }
     }
@@ -201,8 +215,10 @@ extension OSPropertyOperationExecutor {
                 }
             }
         } else {
-            updateRequestQueue.append(request)
-            OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_UPDATE_REQUEST_QUEUE_KEY, withValue: self.updateRequestQueue)
+            self.dispatchQueue.async {
+                self.updateRequestQueue.append(request)
+                OneSignalUserDefaults.initShared().saveCodeableData(forKey: OS_PROPERTIES_EXECUTOR_UPDATE_REQUEST_QUEUE_KEY, withValue: self.updateRequestQueue)
+            }
         }
     }
 }