Implement JWT automatic token refresh mechanism
epic-bankid-vipps-login-services-task-002 — Add proactive token refresh logic to the Auth Session Manager: detect tokens approaching expiry (within configurable window), trigger background refresh using the Supabase refresh token, update stored tokens atomically, and handle refresh failures with appropriate session invalidation.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 1 - 540 tasks
Can start after Tier 0 completes
Implementation Notes
Implement concurrent call deduplication using a Completer
For the background timer, use Timer(refreshDeadline.difference(DateTime.now()), _backgroundRefresh) where refreshDelay = expiresAt.subtract(refreshWindow).subtract(const Duration(seconds: 30)). Guard against negative durations (token already in refresh window at login time) by clamping to Duration.zero. The atomic write of new tokens should call SecureStorageAdapter methods in a specific order (refreshToken first, then accessToken, then expiresAt) so that if a partial write occurs and the app restarts, getSession() will see a missing or stale accessToken and trigger a fresh login rather than using a corrupted partial state. Document this ordering decision with a code comment.
Testing Requirements
Write unit tests using flutter_test and mocktail. Mock both the SecureStorageAdapter and the Supabase Auth client. Test cases: (1) token with 4 minutes remaining triggers refresh (within 5-min window), (2) token with 10 minutes remaining does not trigger refresh, (3) successful refresh updates in-memory cache and SecureStorage with new tokens, (4) network error on first attempt triggers exactly one retry after 2 seconds, (5) network error on retry propagates as NetworkRefreshException, (6) auth error (e.g., 400 from Supabase) calls clearSession() and throws AuthSessionExpiredException, (7) two concurrent calls to refreshSessionIfNeeded() result in exactly one Supabase API call (deduplication), (8) background timer fires at correct time relative to expiresAt, (9) clearSession() cancels the background timer, (10) calling refreshSessionIfNeeded() after clearSession() is a no-op. Use fake timers (FakeAsync) to test timer behavior deterministically without real time delays.
The PKCE OAuth flow requires the code verifier to survive an app backgrounding during the Vipps redirect, which can trigger OS memory pressure and clear in-memory state. If the verifier is lost between authorization request and callback, the token exchange fails and the user is stranded with a confusing error.
Mitigation & Contingency
Mitigation: Store the PKCE code verifier in AuthTokenStore (Flutter Secure Storage) immediately after generation, before launching the Vipps redirect. Clear it only after a successful or explicitly failed token exchange.
Contingency: If state loss occurs in production, implement a retry flow that generates a new PKCE pair and restarts the authorization URL request, with a user-visible 'Try again' prompt rather than a generic error.
Resuming a Supabase session after biometric verification requires the session token to still be valid. If the session has expired in the background (e.g., after a long device offline period), biometric success will not produce a valid session, and the user will see a confusing 'Face ID worked but still logged out' experience.
Mitigation & Contingency
Mitigation: Before presenting the biometric prompt, check session token expiry. If expired, skip biometrics and route directly to full BankID/Vipps re-authentication. Only offer biometric re-auth if the stored refresh token is still within its validity window.
Contingency: If session expiry during biometric flow occurs in production, implement a graceful transition message ('Your session has expired — please log in again') that preserves the user's last-used authentication method preference.
BankID and Vipps may return different user identifiers (personnummer, phone number, sub claim) that must be correctly linked to an existing Supabase auth user. If the linking logic has edge cases (e.g., user previously registered via email/password), duplicate Supabase accounts may be created.
Mitigation & Contingency
Mitigation: Design the identity linking logic with explicit disambiguation: check for existing users by personnummer before creating a new Supabase identity. Implement the linking via Supabase Edge Function to keep the logic server-side and auditable.
Contingency: Implement an admin-facing account merge tool in the admin portal to resolve duplicate accounts if they occur. Add a Supabase unique constraint on the personnummer field to make duplicates fail loudly rather than silently.
The Vipps nin (personnummer) scope requires explicit approval from Vipps as part of the merchant agreement. If this scope approval is not in place before the production release, the Vipps flow will succeed but return no personnummer, making the primary business value (membership data gap fill) non-functional without user-visible error.
Mitigation & Contingency
Mitigation: Apply for Vipps nin scope approval as part of the merchant onboarding process, well before Phase 2 launch. Implement the service to gracefully handle absent nin claims and show users a clear message if personnummer could not be retrieved.
Contingency: If nin scope is delayed, ship the Vipps login flow without personnummer write-back first (delivering login value immediately) and add personnummer sync as a post-approval update with no UI changes required.