---
phase: 09-scan-cancellation-keyboard-shortcuts
plan: 01
subsystem: library
tags: [context-cancellation, scan-control, wails-binding, goroutine-coordination]

# Dependency graph
requires:
  - phase: 08-infrastructure
    provides: Library struct, Scan() pipeline, events system
provides:
  - CancelScan, PauseScan, ResumeScan Wails-bound methods on Library
  - IsScanActive, IsScanPaused state query methods
  - waitIfPaused internal pause checkpoint helper
  - LibraryScanCancelled, LibraryScanPaused, LibraryScanResumed events
  - ScanMetrics.Cancelled field
affects: [09-scan-cancellation-keyboard-shortcuts]

# Tech tracking
tech-stack:
  added: []
  patterns:
    - "Per-scan cancellable context (scanCtx) threaded through pipeline, app context (l.ctx) for DB ops"
    - "Blocking channel pattern for pause/resume (scanPauseCh closed to unblock all workers)"
    - "Mutex-protected scan state fields with deferred cleanup"

key-files:
  created:
    - backend/library/scan_control.go
  modified:
    - backend/events/events.go
    - frontend/src/events.ts
    - backend/library/library.go
    - backend/library/metrics.go

key-decisions:
  - "scanCtx for worker cancellation, l.ctx for DB transactions — ensures in-flight commits complete"
  - "Blocking channel pattern for pause — workers check waitIfPaused before each extraction"
  - "Orphan cleanup and variant generation skipped on cancel — prevents incorrect file deletion"

patterns-established:
  - "Per-operation cancellable context pattern: create child context at operation start, defer cancel, clean up state in defer"
  - "Channel-based pause/resume: create channel on pause, close on resume, select with ctx.Done for cancel-during-pause"

requirements-completed: [SCAN-01, SCAN-02, SCAN-03]

# Metrics
duration: 16min
completed: 2026-03-07
---

# Phase 9 Plan 01: Scan Control Backend Summary

**Per-scan cancellable context with pause/resume channel coordination and 3 new scan lifecycle events**

## Performance

- **Duration:** 16 min
- **Started:** 2026-03-07T02:14:23Z
- **Completed:** 2026-03-07T02:31:08Z
- **Tasks:** 2
- **Files modified:** 5

## Accomplishments
- Created scan_control.go with CancelScan/PauseScan/ResumeScan/IsScanActive/IsScanPaused methods
- Threaded per-scan cancellable context through walk and worker pipeline (3 select statements)
- Added waitIfPaused checkpoint in worker pool so workers block when paused
- Orphan cleanup and variant generation safely skipped on cancelled scans
- Added LibraryScanCancelled/Paused/Resumed events with TypeScript sync via go generate

## Task Commits

Each task was committed atomically:

1. **Task 1: Add scan control events and metrics fields** - `c695024` (feat)
2. **Task 2: Add scan control fields to Library struct and create scan_control.go** - `cf22e52` (feat)

## Files Created/Modified
- `backend/library/scan_control.go` - CancelScan, PauseScan, ResumeScan, IsScanActive, IsScanPaused, waitIfPaused
- `backend/events/events.go` - LibraryScanCancelled, LibraryScanPaused, LibraryScanResumed constants
- `frontend/src/events.ts` - Auto-generated TypeScript event constants
- `backend/library/library.go` - Scan control fields on Library struct, per-scan context threading, cancellation-aware orphan/variant phases
- `backend/library/metrics.go` - Cancelled bool field on ScanMetrics

## Decisions Made
- Used scanCtx for worker cancellation and l.ctx for DB transactions — ensures in-flight batch commits always complete even when scan is cancelled
- Blocking channel pattern for pause — `make(chan struct{})` on pause, `close()` on resume, all workers select against it
- Orphan cleanup and variant generation skipped on cancel — existingPaths still contains unvisited files that would be incorrectly deleted

## Deviations from Plan

None - plan executed exactly as written.

## Issues Encountered
None

## User Setup Required
None - no external service configuration required.

## Next Phase Readiness
- Scan control backend complete, ready for Plan 02 (keyboard shortcuts config) and Plan 03 (frontend scan control UI)
- All 5 new methods are exported and Wails-bindable
- Events synced to TypeScript for frontend consumption

## Self-Check: PASSED

- All 5 key files verified on disk
- Both task commits found in git log (c695024, cf22e52)

---
*Phase: 09-scan-cancellation-keyboard-shortcuts*
*Completed: 2026-03-07*