MSC4076: Let E2EE clients calculate app & encrypted-room badge counts themselves

proposals/4076-e2ee-aware-badges.md

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+# MSC4076: Let E2EE clients calculate app & encrypted-room badge counts themselves
+
+## Problem
+
+Some push notification implementations (e.g. APNS) let the server update the notification badge count shown on an app
+directly from the server, without the client running any code[^1].  In the early days of Matrix before we landed E2EE
+this was a desirable thing to do, because: 1) the server knew how many pushes it had sent to the client 2) the client
+displayed all pushes 3) the client typically didn't run code on receiving push, but just relied on the OS to display
+the push.
+
+However, since E2EE landed, this breaks down for E2EE rooms because only the client can reliably know whether a given
+message should trigger a push notification or not - given only the client can see the event type and event contents
+of E2EE messages, and so decide whether to notify based on the event type being visible - or based on mention keywords
+within the message contents.
+
+The workaround since we launched E2EE has been to assume that all events in E2EE rooms are visible, and so if an E2EE
+room is configured to notify for all messages, unrenderable events will still trigger a push notification and increment
+the `unread_notifications` count for that room and consider that room as unread when calculating the app badge count
+passed to the push gateway.  As a result, invisible events like VoIP signalling will trigger an app badge count.
+
+Meanwhile, clients now display notifications on both Android & iOS by running local code on receiving a push, rather
+than displaying it verbatim from the server.  This means the push rules can be executed locally and notifications should
+only be displayed if the event actually is visible and matched the push rules.  Moreover, the clients can calculate and
+track their own accurate `unread_notification` count for E2EE rooms (while still using the `unread_notification` count
+in the /sync response for non-E2EE rooms if they want), and from that calculate an accurate app badge count
+clientside.
+
+The problem is that the server still tries to override the app badge count whenever it sends a push to the client though,
+which at best will be wrong (either understeering due to missing E2EE mentions, or oversteering due to alerting about
+rooms with invisible events) - and at worst will race and fight[^2] with the app badge count maintained locally by the
+app, if any.
+
+Therefore we need a way for sophisticated E2EE clients to tell the server to stop overriding their app badge count.
+
+Also, the server currently tracks which events the user has read, and will send a 'null' push to the client to update
+the app badge count once a given pushed event has been read.  But given the server doesn't reliably know what events
+cause pushes in E2EE rooms, we will need a new mechanism to reduce the badge count as the user reads rooms (or parts
+them, or reconfigures their push settings).
+ 
+Finally, a separate but related problem is that historically the description of the `unread` count passed to push
+gateways in the Push API is currently wrong[^3]: it says it's "The number of unread messages a user has across all of
+the rooms they are a member of." but in practice it's "The number of rooms the user has with unread push
+notifications" (although synapse has a config option to change that, confusingly). So we should fix that too.
+
+## Solution
+
+When a pusher is registered for a given device via `POST /_matrix/client/v3/pushers/set`, we add a new boolean field
+to the request called `e2ee_badge_aware` which defaults to `false` if absent.
+
+If `e2ee_badge_aware` is set to true, then the pusher will not specify `unread` or `missed_calls` in the
+`POST /_matrix/push/v1/notify` request to the target push gateway, and so not override the client's app badge.
+
+We also add a new optional field to read receipts called `cleared-notifs` which defaults to `false` if absent.  This is
+set to `true` by e2ee-badge-aware clients to indicate that a given receipt means the user has read all push
+notifications for this room's main timeline.  The server should then send a 'null' push notification to all other
+clients to encourage them to sync and recalculate their app badge counts, ensuring that the app badge count decreases
+when the user catches up on a given room.
+
+## Potential issues
+
+1. It's not clear that the push extension will be able to reliably calculate badge counts, as they run with very
+constrained resources, and the client will need to have spidered all unread messages in all E2EE rooms to get a correct
+number.  So if the client has been offline for a while and comes back, we have a problem if the push service has given
+up pushing the client, and potential notifs get dropped.  Perhaps we can detect when this happens (e.g. seqnums on push
+notifs?) and warn the user to launch the app to catch up on any notifs they may have missed?
+
+2. Rather than sending a null push to clients to get them to update their app badge counts, should we instead send the
+`cleared-notifs` read receipt details through - to tell them which room cleared its notifs, and at which event, rather
+than requiring them to /sync and guess?  This seems likelier to succeed within resource constraints, but exposes more
+metadata to the push provider on what events have been read by the user.  This in turn could be mitigated by
+[MSC3013](https://github.com/matrix-org/matrix-spec-proposals/pull/3013).
+
+3. Just because one client thinks a room's notifs have been cleared doesn't mean a different client will agree, so if
+another client has more noisy per-device rules, it may end up with a stuck app badge count.  A workaround could be for
+e2ee_badge_aware clients to set `cleared-notifs: true` on every RR they send if they spot that the user has per-device
+rules.  Alternatively, they could set `cleared-notifs: true` whatever when the user reads the most recent message in
+the room as a guaranteed point where all clients will agree that there are no unread notifs.
+Given per-device rules aren't common currently in the wild, I suggest we punt this to a later MSC.
+
+4. Ideally we should also omit `unread_notifications` and `unread_thread_notifications` in /sync responses entirely for
+E2EE rooms for that device, given they will be subtly wrong and encourage the client not to correctly calculate them
+themselves.  However, this is hygiene rather than functionality and can be a different MSC for an `e2ee_badge_aware`
+room_filter or similar.
+
+## Alternatives
+
+We could track app badge count on the server, but let it be set your clients instead - e.g. by a dedicated API like
+`POST /_matrix/client/v3/pushers/set_badge { badge: 123 }`, which would in turn push it to all clients so they are in
+sync.  This would avoid each client individually trying to figure out when to reduce the badge count in its Push
+Extension - instead the client sending the read receipts would do it for them.  But on the other hand, it would not
+work with per-device push rules.  (Then again, nor does the proposed solution).
+
+Alternatively, we could push all devices every time the user sends a read receipt anywhere, just in case they need to
+recalculate their app badge count.  This would have an avoidable and large impact on battery life.
+
+In terms of configuring pushers as e2ee_badge_aware: we could also do this via a per-device underride push rule with
+a custom `action` of `e2ee_badge_aware`.  However, this isn't really a push action: it shouldn't be changeable by other
+push rules, for instance.  Instead it's config data at the point the pusher is created, hence putting it there.
+
+## Security considerations
+
+This assumes a user can trust their other apps to accurately say when they're sending a read receipt which will clear
+the badge count for a given room.  This doesn't seem unreasonable.
+
+## Unstable prefix
+
+Until this MSC is merged:
+ * `e2ee_badge_aware` should be prefixed as `org.matrix.msc4076.e2ee_badge_aware` when setting pushers
+ * `cleared-notifs` should be prefixed as `org.matrix.msc4076.cleared-notifs` when sending read receipts which clear
+   all the notifs in the room.
+
+## Dependencies
+
+None.
+
+## Footnotes
+
+[^1] https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/generating_a_remote_notification#2943362
+[^2] https://github.com/vector-im/element-x-ios/issues/2066
+[^3] https://github.com/matrix-org/matrix-spec/issues/644