[Android] BackgroundTaskScheduler reschedule implementation

components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.components.background_task_scheduler;
 
 import android.annotation.TargetApi;
 import android.content.Context;
 import android.os.Build;
 import android.support.annotation.Nullable;
 
 import org.chromium.base.Log;
 import org.chromium.base.ThreadUtils;
 
 import java.lang.reflect.Constructor;
+import java.util.Set;
 
 /**
  * A BackgroundTaskScheduler which is used to schedule jobs that run in the background.
  * It is backed by system APIs ({@link android.app.job.JobScheduler}) on newer platforms
  * and by GCM ({@link com.google.android.gms.gcm.GcmNetworkManager}) on older platforms.
  *
  * To get an instance of this class, use {@link BackgroundTaskSchedulerFactory#getScheduler()}.
  */
 @TargetApi(Build.VERSION_CODES.LOLLIPOP)
 public class BackgroundTaskScheduler {
@@ skipping 57 matching lines @@
      * Cancels the task specified by the task ID.
      *
      * @param context the current context.
      * @param taskId the ID of the task to cancel. See {@link TaskIds} for a list.
      */
     public void cancel(Context context, int taskId) {
         ThreadUtils.assertOnUiThread();
         BackgroundTaskSchedulerPrefs.removeScheduledTask(taskId);
         mSchedulerDelegate.cancel(context, taskId);
     }
+
+    public void reschedule(Context context) {
+        Set<String> scheduledTasksClassNames = BackgroundTaskSchedulerPrefs.getScheduledTasks();
+        BackgroundTaskSchedulerPrefs.removeAllTasks();

[nyquist, 2017/04/18 22:33:03]

So, conceptually we could do this before or after all the tasks, or before/after
each individual class.
You chose the before-all version, which is fine, but could you add a short
comment as to why? For example mention that if any of the tasks make us crash,
we won't crash repeatedly, etc., or whatever you thought process was behind
this.

[fgorski, 2017/04/20 19:09:58]

I did it to avoid updating the prefs entry from both directions (schedule and
here).
I can do one of the following:
* remove-all as here (upside: we start fresh, downside: if early task reschedule
crashes us, we lose all tasks)
* remove one task before: (upside: we don't lose tasks by crash of some other
task... downside: we update the prefs a lot...)
* remove one task after: (won't work as we will remove entry for rescheduled
task)
* don't remove tasks and make sure that next scheduling does not write if
already there (wont work if the task decides not to reschedule itself... it will
be pending there)

WDYT?

[nyquist, 2017/04/25 20:44:05]

I think what we have now is fine.

+        for (String className : scheduledTasksClassNames) {
+            BackgroundTask task = getBackgroundTaskFromClassName(className);
+            if (task == null) {
+                Log.w(TAG, "Cannot reschedule task for: " + className);
+                continue;
+            }
+
+            task.reschedule(context);
+        }
+    }
 }