Implement in-memory signed URL cache with configurable TTL
epic-document-attachments-services-task-005 — Implement the in-memory LRU or Map-based cache inside AttachmentSignedUrlService. Cache key is the storage object path; value is the SignedUrlResult plus expiry timestamp. Expose a configurable TTL (default 55 minutes, slightly under Supabase's 60-minute signed URL lifetime). On cache hit within TTL, return cached URL without calling Supabase. On cache miss or expiry, generate a new signed URL and cache the result.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 1 - 540 tasks
Can start after Tier 0 completes
Implementation Notes
Use a `LinkedHashMap
The `getSignedUrl(String path)` method signature should return `Future
Testing Requirements
Unit tests (flutter_test): (1) cold cache generates URL and populates cache, (2) warm cache hit returns same URL without calling storage SDK — assert mock called once total across two requests, (3) expired cache entry triggers new URL generation — use injected clock or TTL=1s + await delay, (4) failed URL generation returns Left and does not pollute cache, (5) LRU eviction: fill cache to 200 entries, add one more, verify oldest entry is evicted, (6) TTL boundary: entry at exactly TTL-1ms is a hit, entry at exactly TTL+1ms is a miss. Use a fake/mock clock (pass `DateTime Function() clock` to constructor) to make TTL expiry tests deterministic without real delays.
The storage upload succeeds but the subsequent metadata insert fails. The rollback delete call to Supabase Storage could itself fail (network error, transient timeout), leaving an orphaned object in the bucket with no database record pointing to it — a cost and compliance risk that also breaks delete-on-cascade logic.
Mitigation & Contingency
Mitigation: Wrap the rollback delete in a retry loop (3 attempts, exponential back-off). Log orphaned-object incidents to a dedicated structured log stream for periodic audit. Consider a scheduled Supabase Edge Function that reconciles storage objects against database records and flags orphans.
Contingency: If orphaned objects accumulate, run the reconciliation edge function manually to identify and purge them. Add a monitoring alert for metadata insert failures after successful uploads so the issue is caught within minutes.
If the signed URL TTL is set too short, users browsing the attachment preview modal on slow connections will receive expired URLs before the content loads, causing a broken experience. If set too long, a URL shared outside the app (e.g., pasted into a chat) remains valid beyond the intended access window.
Mitigation & Contingency
Mitigation: Default TTL to 60 minutes, configurable via a named constant. The in-memory cache TTL should be set to TTL minus 5 minutes to ensure cached URLs are refreshed before they expire. Document the trade-off in code comments.
Contingency: If users report broken previews, shorten the cache TTL hotfix. If a URL leak is reported, rotate the Supabase storage signing secret to invalidate all outstanding signed URLs immediately.
The multi-attachment user story requires parallel uploads with individual progress indicators. Managing concurrent BLoC events for 3–5 simultaneous uploads risks state collisions, progress indicator mixups, or partial rollbacks that are difficult to reason about.
Mitigation & Contingency
Mitigation: Design the BLoC to maintain a per-attachment upload state map keyed by a client-generated UUID. Each upload runs as an isolated Future with its own result emitted as a typed event. Write integration tests for 3-concurrent-upload scenarios.
Contingency: If state collisions occur in production, fall back to sequential upload processing (one at a time) gated behind a feature flag until the concurrent model is stabilised.