Implement FCM Token Manager
epic-push-notification-delivery-engine-task-004 — Create the FCMTokenManager infrastructure service that retrieves the device FCM registration token via firebase_messaging, detects token refresh events, persists the current token to Supabase user_devices table with device_id and platform fields, and deletes the token on logout. Include retry logic for network failures and token invalidation handling.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 1 - 540 tasks
Can start after Tier 0 completes
Implementation Notes
Implement FCMTokenManager as a plain Dart class registered as a Riverpod Provider (not StateNotifier — it has no reactive state of its own). Inject FirebaseMessaging and SupabaseClient via constructor for testability. Use StreamSubscription for onTokenRefresh and cancel it in a dispose() method called by the owning provider's onDispose. Use device_info_plus to generate a stable device_id: on Android use androidInfo.id, on iOS use iosInfo.identifierForVendor.
For retry, implement a simple exponential backoff helper rather than pulling in a retry package. The upsert SQL should use ON CONFLICT (device_id) DO UPDATE SET fcm_token = EXCLUDED.fcm_token, updated_at = now(). Ensure this service is initialized after FirebaseApp.initializeApp() and after Supabase.initialize() in main.dart.
Testing Requirements
Write unit tests using flutter_test and mocktail. Mock FirebaseMessaging and the Supabase client. Test cases: (1) getToken returns valid string and upserts to DB, (2) getToken returns null — no DB write, (3) onTokenRefresh fires — new token upserted, (4) Supabase upsert throws SocketException — retries 3 times then emits error, (5) logout — deleteToken called and DB row removed, (6) unauthenticated state — DB call deferred. Achieve 90%+ line coverage on FCMTokenManager.
No integration tests required at this stage.
Flutter's background message handler for FCM must run in a separate Dart isolate. Incorrect dependency initialization in the isolate (e.g., attempting to access Riverpod providers or Supabase before initialization) will cause silent crashes on Android when the app is terminated, resulting in missed notifications that are invisible in crash reporting.
Mitigation & Contingency
Mitigation: Use a minimal top-level background handler function annotated with @pragma('vm:entry-point') that only stores the raw RemoteMessage payload to a platform channel or shared preferences. Process the payload in the main isolate on next app launch. Write an explicit test for terminated-state message handling on Android.
Contingency: If isolate crashes are observed, implement a native Android FirebaseMessagingService subclass that handles background messages without Flutter isolate complexity, falling back to a database-insert-only approach for terminated-state notifications.
Supabase Edge Functions can experience cold-start latency of 1–3 seconds after periods of inactivity. For high-frequency events like assignment creation, cumulative cold starts could cause dispatch delays exceeding the 30-second SLA, reducing the perceived reliability of the notification system.
Mitigation & Contingency
Mitigation: Configure the Edge Function with a keep-warm ping mechanism or use Supabase database webhooks that invoke the function directly on row insert to minimize cold-start frequency. Batch preference lookups within the function to reduce per-invocation Supabase round-trips.
Contingency: If latency SLA is consistently breached, move to a polling or Realtime-subscription architecture within the Edge Function, or pre-compute dispatch targets at preference-save time to eliminate per-dispatch preference queries.
If the deep link handler does not perform server-side role validation before rendering the target screen, a peer mentor who receives a mis-configured notification payload containing a coordinator-only route could access restricted data, violating the role-based access control invariants.
Mitigation & Contingency
Mitigation: The deep link handler must check the user's current role from the RoleStateManager before constructing the navigation route. Coordinator-only routes must be listed in a deny-list checked against the current role. The go_router route guard is a second line of defence.
Contingency: If a role bypass is discovered in testing, immediately add the affected route to the deep link handler deny-list and add a regression test. Audit all notification payload types for route targets that could expose cross-role data.
FCM v1 HTTP API enforces per-project send quotas. For large organisations with many active peer mentors receiving simultaneous assignment notifications, batch dispatch events (e.g., bulk coordinator assignments) could approach quota limits and result in dropped notifications with 429 errors logged silently.
Mitigation & Contingency
Mitigation: Implement exponential backoff retry logic in the Edge Function for 429 responses. Design bulk assignment flows to dispatch notifications in batches with a configurable delay between batches. Monitor FCM console quotas during load testing.
Contingency: If quota limits are hit, implement a notification queue table in Supabase and a separate Edge Function that processes the queue with rate limiting, ensuring eventual delivery without exceeding FCM quotas.