From e264144aea7b7b474792d69802805281b4964dd4 Mon Sep 17 00:00:00 2001
From: Michael Hamann <michael@content-space.de>
Date: Mon, 11 Jan 2016 20:49:17 +0100
Subject: [PATCH] GCM: Wake with exact guarantees for heartbeat

This changes how the alarms for heartbeat pings are scheduled. Instead
of a repeating, inexact alarm that may be delayed (at least up to 15
minutes), either an exact alarm (below Android 4.4) or an interval is used
which ensures that the alarm fires between half the configured interval
and the configured interval. This interval allows the OS to optimize
alarms a bit. For Android 6.0 further adjustments are probably
necessary.
---
 .../java/org/microg/gms/gcm/McsService.java   | 23 ++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/play-services-core/src/main/java/org/microg/gms/gcm/McsService.java b/play-services-core/src/main/java/org/microg/gms/gcm/McsService.java
index 08192d0b..c8326b60 100644
--- a/play-services-core/src/main/java/org/microg/gms/gcm/McsService.java
+++ b/play-services-core/src/main/java/org/microg/gms/gcm/McsService.java
@@ -22,6 +22,7 @@ import android.app.Service;
 import android.content.Context;
 import android.content.Intent;
 import android.content.SharedPreferences;
+import android.os.Build;
 import android.os.Handler;
 import android.os.IBinder;
 import android.os.Looper;
@@ -145,10 +146,25 @@ public class McsService extends Service implements Handler.Callback {
         AlarmManager alarmManager = (AlarmManager) context.getSystemService(ALARM_SERVICE);
         long delay = getCurrentDelay();
         Log.d(TAG, "Scheduling reconnect in " + delay / 1000 + " seconds...");
-        alarmManager.set(AlarmManager.ELAPSED_REALTIME, SystemClock.elapsedRealtime() + delay,
+        alarmManager.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime() + delay,
                 PendingIntent.getBroadcast(context, 1, new Intent(ACTION_RECONNECT, null, context, TriggerReceiver.class), 0));
     }
 
+    public void scheduleHeartbeat(Context context) {
+        AlarmManager alarmManager = (AlarmManager) context.getSystemService(ALARM_SERVICE);
+        Log.d(TAG, "Scheduling heartbeat in " + heartbeatMs / 1000 + " seconds...");
+
+        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.KITKAT) {
+            alarmManager.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime() + heartbeatMs, heartbeatIntent);
+        } else {
+            // with KitKat, the alarms become inexact by default, but with the newly available setWindow we can get inexact alarms with guarantees.
+            // Schedule the alarm to fire within the interval [heartbeatMs/2, heartbeatMs]
+            alarmManager.setWindow(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime() + heartbeatMs / 2, heartbeatMs / 2,
+                    heartbeatIntent);
+        }
+
+    }
+
     public synchronized static long getCurrentDelay() {
         long delay = currentDelay == 0 ? 5000 : currentDelay;
         if (currentDelay < 60000) currentDelay += 10000;
@@ -181,7 +197,7 @@ public class McsService extends Service implements Handler.Callback {
                 WakefulBroadcastReceiver.completeWakefulIntent(intent);
             }
         }
-        return START_STICKY;
+        return START_REDELIVER_INTENT;
     }
 
     private synchronized void connect() {
@@ -196,7 +212,7 @@ public class McsService extends Service implements Handler.Callback {
             inputStream.start();
             outputStream.start();
 
-            alarmManager.setInexactRepeating(AlarmManager.ELAPSED_REALTIME, SystemClock.elapsedRealtime(), heartbeatMs, heartbeatIntent);
+            scheduleHeartbeat(this);
         } catch (Exception e) {
             Log.w(TAG, "Exception while connecting!", e);
             rootHandler.sendMessage(rootHandler.obtainMessage(MSG_TEARDOWN, e));
@@ -340,6 +356,7 @@ public class McsService extends Service implements Handler.Callback {
                         ping.last_stream_id_received(inputStream.getStreamId());
                     }
                     send(ping.build());
+                    scheduleHeartbeat(this);
                 } else {
                     Log.d(TAG, "Ignoring heartbeat, not connected!");
                     scheduleReconnect(this);