make taskQueueWaitingForInit thread safe

There are two calls to startPendingTasks that can happen on two threads,
which means sometimes taskQueueWaitingForInit.poll() can return null and
throws a NPE when it is given to submit. To handle this we are making
taskQueueWaitingForInit thread safe by calling synchronized on. Doing so
handles this case but also rare cases where something is being added as
we process the queue.

Author: jkasten2

OneSignalSDK/onesignal/src/main/java/com/onesignal/OSTaskController.java

@@ -49,25 +49,27 @@ void addTaskToQueue(Runnable runnable) {
     }
 
     private void addTaskToQueue(PendingTaskRunnable task) {
-        task.taskId = lastTaskId.incrementAndGet();
-
-        if (pendingTaskExecutor == null) {
-            logger.debug("Adding a task to the pending queue with ID: " + task.taskId);
-            // The tasks haven't been executed yet...add them to the waiting queue
-            taskQueueWaitingForInit.add(task);
-        } else if (!pendingTaskExecutor.isShutdown()) {
-            logger.debug("Executor is still running, add to the executor with ID: " + task.taskId);
-            try {
-                // If the executor isn't done with tasks, submit the task to the executor
-                pendingTaskExecutor.submit(task);
-            } catch (RejectedExecutionException e) {
-                logger.info("Executor is shutdown, running task manually with ID: " + task.taskId);
-                // Run task manually when RejectedExecutionException occurs due to the ThreadPoolExecutor.AbortPolicy
-                // The pendingTaskExecutor is already shutdown by the time it tries to run the task
-                // Issue #669
-                // https://github.com/OneSignal/OneSignal-Android-SDK/issues/669
-                task.run();
-                e.printStackTrace();
+        synchronized (taskQueueWaitingForInit) {
+            task.taskId = lastTaskId.incrementAndGet();
+
+            if (pendingTaskExecutor == null) {
+                logger.debug("Adding a task to the pending queue with ID: " + task.taskId);
+                // The tasks haven't been executed yet...add them to the waiting queue
+                taskQueueWaitingForInit.add(task);
+            } else if (!pendingTaskExecutor.isShutdown()) {
+                logger.debug("Executor is still running, add to the executor with ID: " + task.taskId);
+                try {
+                    // If the executor isn't done with tasks, submit the task to the executor
+                    pendingTaskExecutor.submit(task);
+                } catch (RejectedExecutionException e) {
+                    logger.info("Executor is shutdown, running task manually with ID: " + task.taskId);
+                    // Run task manually when RejectedExecutionException occurs due to the ThreadPoolExecutor.AbortPolicy
+                    // The pendingTaskExecutor is already shutdown by the time it tries to run the task
+                    // Issue #669
+                    // https://github.com/OneSignal/OneSignal-Android-SDK/issues/669
+                    task.run();
+                    e.printStackTrace();
+                }
             }
         }
     }
@@ -77,19 +79,21 @@ private void addTaskToQueue(PendingTaskRunnable task) {
      * Run available pending tasks on an Executor
      */
     void startPendingTasks() {
-        OneSignal.Log(OneSignal.LOG_LEVEL.DEBUG, "startPendingTasks with task queue quantity: " + taskQueueWaitingForInit.size());
-        if (!taskQueueWaitingForInit.isEmpty()) {
-            pendingTaskExecutor = Executors.newSingleThreadExecutor(new ThreadFactory() {
-                @Override
-                public Thread newThread(@NonNull Runnable runnable) {
-                    Thread newThread = new Thread(runnable);
-                    newThread.setName(OS_PENDING_EXECUTOR + newThread.getId());
-                    return newThread;
+        synchronized (taskQueueWaitingForInit) {
+            OneSignal.Log(OneSignal.LOG_LEVEL.DEBUG, "startPendingTasks with task queue quantity: " + taskQueueWaitingForInit.size());
+            if (!taskQueueWaitingForInit.isEmpty()) {
+                pendingTaskExecutor = Executors.newSingleThreadExecutor(new ThreadFactory() {
+                    @Override
+                    public Thread newThread(@NonNull Runnable runnable) {
+                        Thread newThread = new Thread(runnable);
+                        newThread.setName(OS_PENDING_EXECUTOR + newThread.getId());
+                        return newThread;
+                    }
+                });
+
+                while (!taskQueueWaitingForInit.isEmpty()) {
+                    pendingTaskExecutor.submit(taskQueueWaitingForInit.poll());
                 }
-            });
-
-            while (!taskQueueWaitingForInit.isEmpty()) {
-                pendingTaskExecutor.submit(taskQueueWaitingForInit.poll());
             }
         }
     }
@@ -102,7 +106,9 @@ private void onTaskRan(long taskId) {
     }
 
     ConcurrentLinkedQueue<Runnable> getTaskQueueWaitingForInit() {
-        return taskQueueWaitingForInit;
+        synchronized (taskQueueWaitingForInit) {
+            return taskQueueWaitingForInit;
+        }
     }
 
     void shutdownNow() {