Implement offline biometric auth with deferred network checks
epic-biometric-session-authentication-core-services-task-008 — Implement offline session validation logic in BiometricAuthService that defers Supabase network calls when connectivity is absent. When offline, the service validates the locally cached session token expiry, permits biometric verification against the device hardware only, and queues a session refresh for when connectivity resumes. Prevents unexpected logouts for mentors in low-connectivity areas.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 7 - 84 tasks
Can start after Tier 6 completes
Implementation Notes
Inject a ConnectivityService abstraction (wrapping connectivity_plus) into BiometricAuthService for testability. Parse JWT expiry (exp claim) from the stored token in flutter_secure_storage using dart:convert + base64 decode of the payload segment — no external JWT library needed for just reading claims. For the deferred refresh queue, use WidgetsBindingObserver.didChangeAppLifecycleState to trigger the refresh when the app resumes to foreground with connectivity. Alternatively, use a simple Completer stored on the service that the app's ConnectivityService resolves when online.
Define a maxOfflineDurationHours constant (default 72) sourced from remote config or hardcoded — document the security trade-off. The offline success result should be clearly differentiated (BiometricAuthSuccess with an offlineMode bool) so callers can optionally display a 'working offline' banner.
Testing Requirements
Unit tests with mocked connectivity_plus stream covering: online path unchanged, offline + valid JWT → offline success, offline + expired JWT → session expired result. Integration test on a physical device with Airplane Mode enabled. Test the deferred refresh queue: mock connectivity restored event triggers refreshSession() call. Test the max offline window: mock a JWT with iat > 72 hours ago → offline auth denied even if not expired.
Test that queued refresh failure on reconnect triggers re-login flow.
Multiple concurrent callers (e.g., SessionResumeManager and a background sync service) could simultaneously detect a near-expired token and each invoke SupabaseSessionManager.refreshSession(), causing duplicate refresh API calls and potentially a token invalidation race condition on the Supabase Auth server. This can result in one caller receiving a valid refreshed token while another receives a 401, causing intermittent authentication failures.
Mitigation & Contingency
Mitigation: Implement a single-flight pattern inside SupabaseSessionManager so that concurrent refresh calls coalesce into one in-flight request. Use a Dart Completer or AsyncMemoizer to ensure all waiters receive the same refreshed token. Write a concurrent integration test to validate the single-flight behaviour.
Contingency: If the single-flight pattern introduces deadlocks or timeout complexity, fall back to a mutex-based lock with a 10-second timeout, logging a warning if the lock is held longer than expected, and triggering a full re-login if the refresh ultimately fails.
Supabase Row-Level Security policies evaluate the JWT claims (user_id, role, org_id) on every query. If the refreshed token contains stale or changed claims — for example if a coordinator's role was updated server-side — RLS may silently block data access even though the session appears valid from the client's perspective, causing confusing empty screens rather than an authentication error.
Mitigation & Contingency
Mitigation: After every token refresh, decode the new JWT and compare key claims (role, org_id) with the cached values. If claims have changed, emit a session-claims-changed event that triggers a role re-resolution and navigation reset. Document this behaviour in the SupabaseSessionManager API contract.
Contingency: If claims drift is detected in production and causes data visibility issues, provide a force-refresh mechanism in the UI (pull-to-refresh on home screen) that clears cached role state and re-fetches from Supabase, accompanied by a user-visible toast indicating the session was refreshed.
Allowing session resumption from cached local token when offline introduces a window where a revoked or invalidated session can still grant app access. For example, if a coordinator deactivates a peer mentor's account while the mentor is offline, the mentor continues to have access until connectivity is restored and the token is validated server-side.
Mitigation & Contingency
Mitigation: Set a maximum offline grace period (e.g., 24 hours) stored alongside the token in SecureSessionStorage. If the grace period is exceeded, force a full credential re-login regardless of connectivity status. Scope offline access to read-only operations only, requiring connectivity for any write that reaches Supabase.
Contingency: If the offline grace period logic is found to be insufficient for compliance, implement remote session invalidation via a lightweight push notification that clears SecureSessionStorage even when the app is backgrounded, using FCM with a data-only message.