Don't unnecessarily stack GCM message processing.

If we already have two active processing GCM messages, there's no
benefit to a third. In fact, enqueuing additional ones will likely only
end up showing the foreground notification unnecessariliy.

src/org/thoughtcrime/securesms/gcm/GcmBroadcastReceiver.java

@@ -19,10 +19,12 @@ import org.thoughtcrime.securesms.jobs.PushNotificationReceiveJob;
 import org.thoughtcrime.securesms.service.GenericForegroundService;
 import org.thoughtcrime.securesms.util.PowerManagerCompat;
 import org.thoughtcrime.securesms.util.TextSecurePreferences;
+import org.thoughtcrime.securesms.util.concurrent.SignalExecutors;
 import org.whispersystems.signalservice.api.SignalServiceMessageReceiver;
 import org.whispersystems.signalservice.internal.util.Util;
 
 import java.io.IOException;
+import java.util.concurrent.Executor;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 import javax.inject.Inject;
@@ -31,6 +33,10 @@ public class GcmBroadcastReceiver extends WakefulBroadcastReceiver implements In
 
   private static final String TAG = GcmBroadcastReceiver.class.getSimpleName();
 
+  private static final Executor MESSAGE_EXECUTOR = SignalExecutors.newCachedSingleThreadExecutor("GcmMessageProcessing");
+
+  private static int activeCount = 0;
+
   @Inject SignalServiceMessageReceiver messageReceiver;
 
   @Override
@@ -62,6 +68,11 @@ public class GcmBroadcastReceiver extends WakefulBroadcastReceiver implements In
   }
 
   private void handleReceivedNotification(Context context) {
+    if (!incrementActiveGcmCount()) {
+      Log.i(TAG, "Skipping GCM processing -- there's already one enqueued.");
+      return;
+    }
+
     TextSecurePreferences.setNeedsMessagePull(context, true);
 
     long          startTime    = System.currentTimeMillis();
@@ -81,9 +92,7 @@ public class GcmBroadcastReceiver extends WakefulBroadcastReceiver implements In
       callback.finish();
     }
 
-    new Thread("GcmMessageProcessing") {
-      @Override
-      public void run() {
+    MESSAGE_EXECUTOR.execute(() -> {
       try {
         new PushNotificationReceiveJob(context).pullAndProcessMessages(messageReceiver, TAG, startTime);
       } catch (IOException e) {
@@ -100,10 +109,11 @@ public class GcmBroadcastReceiver extends WakefulBroadcastReceiver implements In
           }
           taskCompleted.set(true);
         }
+
+        decrementActiveGcmCount();
         Log.i(TAG, "Processing complete.");
       }
-      }
-    }.start();
+    });
 
     if (!foregroundRunning.get()) {
       new Thread("GcmForegroundServiceTimer") {
@@ -121,4 +131,16 @@ public class GcmBroadcastReceiver extends WakefulBroadcastReceiver implements In
       }.start();
     }
   }
+
+  private static synchronized boolean incrementActiveGcmCount() {
+    if (activeCount < 2) {
+      activeCount++;
+      return true;
+    }
+    return false;
+  }
+
+  private static synchronized void decrementActiveGcmCount() {
+    activeCount--;
+  }
 }

src/org/thoughtcrime/securesms/util/concurrent/SignalExecutors.java

@@ -4,7 +4,10 @@ import android.support.annotation.NonNull;
 
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
 
 public class SignalExecutors {
@@ -16,4 +19,11 @@ public class SignalExecutors {
       return new Thread(r, "signal-io-" + counter.getAndIncrement());
     }
   });
+
+
+  public static ExecutorService newCachedSingleThreadExecutor(final String name) {
+    ThreadPoolExecutor executor = new ThreadPoolExecutor(1, 1, 15, TimeUnit.SECONDS, new LinkedBlockingQueue<>(), r -> new Thread(r, name));
+    executor.allowCoreThreadTimeOut(true);
+    return executor;
+  }
 }