[Android] Implements OS upgrade check and rescheduling

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

Index: components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java
diff --git a/components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java b/components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java
index d7570fca637dd1365ba0b07f4f7fea13f1c0f01c..a8b76b74702e6f2f3e3d0a417b64800603c67800 100644
--- a/components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java
+++ b/components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java
@@ -92,6 +92,40 @@ public class BackgroundTaskScheduler {
         mSchedulerDelegate.cancel(context, taskId);
     }
 
+    /**
+     * Checks whether OS was upgraded and triggers rescheduling if it is necessary.
+     * Rescheduling is necessary if type of background task scheduler delegate is different for a
+     * new version of the OS.
+     *
+     * @param context the current context.
+     */
+    public void checkForOSUpgrade(Context context) {
+        int oldSdkInt = BackgroundTaskSchedulerPrefs.getLastSdkVersion();
+        int newSdkInt = Build.VERSION.SDK_INT;
+        // No OS upgrade detected.
+        if (oldSdkInt == newSdkInt) return;
+
+        // Save the current SDK version to preferences.
+        BackgroundTaskSchedulerPrefs.setLastSdkVersion(newSdkInt);
+
+        // OS upgrade detected within the same delegate type. No is upgrade necessary.
+        if ((oldSdkInt < Build.VERSION_CODES.M && newSdkInt < Build.VERSION_CODES.M)

[nyquist, 2017/04/18 22:41:08]

Do we want to extract the conditional out to something like:

if (!osUpgradeChangesDelegateType(oldSdkInt, newSdkInt)) return;

[fgorski, 2017/04/20 22:36:07]

Done.

+                || (oldSdkInt >= Build.VERSION_CODES.M && newSdkInt >= Build.VERSION_CODES.M)) {

[nyquist, 2017/04/18 22:41:08]

Maybe stupid question, but if oldSdkInt was somehow written badly to disk and is
a very high number, should we force an upgrade? I.e. it would look like a
downgrade, which  I don't think we support, but in case of ... I don't know...
Fire? Maybe just accept that as a bad state so force reschedule?

[fgorski, 2017/04/20 22:36:07]

I proposed some changes that work, but I am not very thrilled about these.

This
1) makes us support OS downgrade,
2) is a bit complex to test. (I can probably do it by either mocking
jobScheduler, or adding hooks in the factory)

Let me know what you think.

[nyquist, 2017/04/25 20:43:56]

Yeah, I'm not sure if that is necessary. It feels weird :-/
Maybe just remove downgrade-support totally?
I mean, I don't even think it's officially supported anywhere.

+            return;
+        }
+
+        // Explicitly invoke create old delegate type to cancel all scheduled tasks.

[nyquist, 2017/04/18 22:41:08]

Something seems to be off with this sentence: "invoke create old...".
Maybe choose either invoke or create or add a word to bind them together?
Maybe something like: "Explicity create and invoke old delegate type to ..."

[fgorski, 2017/04/20 22:36:07]

Done.

+        // All preference entries are kept until reschedule call, which removes then then.
+        BackgroundTaskSchedulerDelegate oldDelegate =
+                new BackgroundTaskSchedulerGcmNetworkManager();
+        Set<Integer> scheduledTaskIds = BackgroundTaskSchedulerPrefs.getScheduledTaskIds();
+        for (int taskId : scheduledTaskIds) {
+            oldDelegate.cancel(context, taskId);
+        }
+
+        reschedule(context);
+    }
+
     public void reschedule(Context context) {
         Set<String> scheduledTasksClassNames = BackgroundTaskSchedulerPrefs.getScheduledTasks();
         BackgroundTaskSchedulerPrefs.removeAllTasks();