Add you may have messages notification.

app/src/main/java/org/thoughtcrime/securesms/gcm/FcmFetchManager.kt

@@ -1,13 +1,23 @@
 package org.thoughtcrime.securesms.gcm
 
+import android.app.Notification
+import android.app.PendingIntent
 import android.content.Context
 import android.content.Intent
 import android.os.Build
+import androidx.core.app.NotificationCompat
+import androidx.core.app.NotificationManagerCompat
+import org.signal.core.util.PendingIntentFlags.mutable
 import org.signal.core.util.concurrent.SignalExecutors
 import org.signal.core.util.logging.Log
+import org.thoughtcrime.securesms.MainActivity
+import org.thoughtcrime.securesms.R
 import org.thoughtcrime.securesms.dependencies.ApplicationDependencies
 import org.thoughtcrime.securesms.jobs.PushNotificationReceiveJob
 import org.thoughtcrime.securesms.messages.WebSocketStrategy
+import org.thoughtcrime.securesms.notifications.NotificationChannels
+import org.thoughtcrime.securesms.notifications.NotificationIds
+import org.thoughtcrime.securesms.util.FeatureFlags
 import org.thoughtcrime.securesms.util.SignalLocalMetrics
 import org.thoughtcrime.securesms.util.concurrent.SerialMonoLifoExecutor
 import kotlin.time.Duration.Companion.minutes
@@ -38,6 +48,9 @@ object FcmFetchManager {
   @Volatile
   private var activeCount = 0
 
+  @Volatile
+  private var highPriority = false
+
   /**
    * @return True if a service was successfully started, otherwise false.
    */
@@ -56,13 +69,41 @@ object FcmFetchManager {
     FcmFetchForegroundService.startServiceIfNecessary(context)
   }
 
+  private fun postMayHaveMessagesNotification(context: Context) {
+    if (FeatureFlags.fcmMayHaveMessagesNotificationKillSwitch()) {
+      Log.w(TAG, "May have messages notification kill switch")
+      return
+    }
+    val mayHaveMessagesNotification: Notification = NotificationCompat.Builder(context, NotificationChannels.getInstance().ADDITIONAL_MESSAGE_NOTIFICATIONS)
+      .setSmallIcon(R.drawable.ic_notification)
+      .setContentTitle(context.getString(R.string.FcmFetchManager__you_may_have_messages))
+      .setCategory(NotificationCompat.CATEGORY_MESSAGE)
+      .setContentIntent(PendingIntent.getActivity(context, 0, MainActivity.clearTop(context), mutable()))
+      .setVibrate(longArrayOf(0))
+      .setOnlyAlertOnce(true)
+      .build()
+
+    NotificationManagerCompat.from(context)
+      .notify(NotificationIds.MAY_HAVE_MESSAGES_NOTIFICATION_ID, mayHaveMessagesNotification)
+  }
+
+  private fun cancelMayHaveMessagesNotification(context: Context) {
+    NotificationManagerCompat.from(context).cancel(NotificationIds.MAY_HAVE_MESSAGES_NOTIFICATION_ID)
+  }
+
   private fun fetch(context: Context) {
+    val hasHighPriorityContext = highPriority
+
     val metricId = SignalLocalMetrics.PushWebsocketFetch.startFetch()
     val success = retrieveMessages(context)
-    if (!success) {
-      SignalLocalMetrics.PushWebsocketFetch.onTimedOut(metricId)
-    } else {
+    if (success) {
       SignalLocalMetrics.PushWebsocketFetch.onDrained(metricId)
+      cancelMayHaveMessagesNotification(context)
+    } else {
+      SignalLocalMetrics.PushWebsocketFetch.onTimedOut(metricId)
+      if (hasHighPriorityContext) {
+        postMayHaveMessagesNotification(context)
+      }
     }
 
     synchronized(this) {
@@ -72,13 +113,22 @@ object FcmFetchManager {
         Log.i(TAG, "No more active. Stopping.")
         context.stopService(Intent(context, FcmFetchBackgroundService::class.java))
         FcmFetchForegroundService.stopServiceIfNecessary(context)
+        highPriority = false
       }
     }
   }
 
   @JvmStatic
-  fun enqueueFetch(context: Context) {
+  fun onForeground(context: Context) {
+    cancelMayHaveMessagesNotification(context)
+  }
+
+  @JvmStatic
+  fun enqueueFetch(context: Context, highPriority: Boolean) {
     synchronized(this) {
+      if (highPriority) {
+        this.highPriority = true
+      }
       val performedReplace = EXECUTOR.enqueue { fetch(context) }
       if (performedReplace) {
         Log.i(TAG, "Already have one running and one enqueued. Ignoring.")

app/src/main/java/org/thoughtcrime/securesms/gcm/FcmReceiveService.java

@@ -74,9 +74,8 @@ public class FcmReceiveService extends FirebaseMessagingService {
   }
 
   private static void handleReceivedNotification(Context context, @Nullable RemoteMessage remoteMessage) {
+    boolean highPriority = remoteMessage != null && remoteMessage.getPriority() == RemoteMessage.PRIORITY_HIGH;
     try {
-      boolean highPriority = remoteMessage != null && remoteMessage.getPriority() == RemoteMessage.PRIORITY_HIGH;
-
       Log.d(TAG, String.format(Locale.US, "[handleReceivedNotification] API: %s, RemoteMessagePriority: %s", Build.VERSION.SDK_INT, remoteMessage != null ? remoteMessage.getPriority() : "n/a"));
 
       if (highPriority) {
@@ -88,7 +87,7 @@ public class FcmReceiveService extends FirebaseMessagingService {
       Log.w(TAG, "Failed to start service.", e);
     }
 
-    FcmFetchManager.enqueueFetch(context);
+    FcmFetchManager.enqueueFetch(context, highPriority);
   }
 
   private static void handleRegistrationPushChallenge(@NonNull String challenge) {