Android: use high priority for notification-inducing message types

Android normal notifications can induce a long delay before the device
gets woken up, and so we really want to use high notifications when we
can.

This gets complicated, though, because Google has some required
threshold of high priority notifications to actual device notifications,
but we cannot know in advance whether a message might induce a
notification (i.e. new DM) or not (typing indicator, or message in a
conversation with notifications disabled).

And so, for now, we disable high priority for messages that we know
cannot illicit a notification, and try high priority for those that
might, and just hope Android's limits aren't too severe.

Author: jagerman

spns/notifiers/firebase.py

@@ -69,14 +69,23 @@ def push_notification(msg: Message):
 
     enc_payload = encrypt_notify_payload(data, max_msg_size=MAX_MSG_SIZE)
 
+    # Send messages that are likely to illicit a device notification as high priority: specifically,
+    # has a body, and is in namespace 0 (DMs) or 11 (group "v2" messages).
+    # This doesn't catch them all, e.g. typing notifications, but definitely excludes things like
+    # data-too-big messages and config updates which definitely won't notify.
+    priority = "high" if b"~" in data and data[b"n"] in (0, 11) else "normal"
+
     device_token = data[b"&"].decode()  # unique service id, as we returned from validate
 
     msg = {
-        'fcm_token': device_token,
-        'data_payload': {
+        "fcm_token": device_token,
+        "data_payload": {
             "enc_payload": oxenc.to_base64(enc_payload),
             "spns": f"{SPNS_FIREBASE_VERSION}"
-        }
+        },
+        "android_config": {
+            "priority": priority,
+        },
     }
 
     global notify_queue, queue_lock