---
phase: 09-scan-cancellation-keyboard-shortcuts
plan: 01
type: execute
wave: 1
depends_on: []
files_modified:
  - backend/events/events.go
  - frontend/src/events.ts
  - backend/library/library.go
  - backend/library/scan_control.go
  - backend/library/metrics.go
autonomous: true
requirements:
  - SCAN-01
  - SCAN-02
  - SCAN-03

must_haves:
  truths:
    - "CancelScan() cancels the scan context and workers stop at their next checkpoint"
    - "PauseScan() blocks workers via a channel; ResumeScan() unblocks them"
    - "Cancelled scans skip orphan cleanup to avoid deleting unvisited files"
    - "Batch commits use l.ctx (app context), not the cancellable scanCtx, so in-flight transactions complete"
    - "ScanMetrics.Cancelled is true when a scan was cancelled"
  artifacts:
    - path: "backend/library/scan_control.go"
      provides: "CancelScan, PauseScan, ResumeScan, IsScanActive, IsScanPaused methods"
      exports: ["CancelScan", "PauseScan", "ResumeScan", "IsScanActive", "IsScanPaused"]
    - path: "backend/events/events.go"
      provides: "New scan control events"
      contains: "LibraryScanCancelled"
    - path: "backend/library/metrics.go"
      provides: "Cancelled field on ScanMetrics"
      contains: "Cancelled"
  key_links:
    - from: "backend/library/scan_control.go"
      to: "backend/library/library.go"
      via: "scanCancel context.CancelFunc and scanPauseCh channel on Library struct"
      pattern: "l\\.scanCancel|l\\.scanPauseCh"
    - from: "backend/library/library.go"
      to: "backend/events/events.go"
      via: "EventsEmit for scan lifecycle events"
      pattern: "events\\.LibraryScan"
---

<objective>
Add scan cancellation and pause/resume to the Go backend. Thread a per-scan cancellable context through the existing scan pipeline, add pause/resume via a blocking channel, and expose Wails-bound methods for frontend control.

Purpose: Backend foundation for SCAN-01/02/03 — frontend buttons wire to these methods in Plan 03.
Output: scan_control.go with CancelScan/PauseScan/ResumeScan, modified Scan() method, new events, updated metrics.
</objective>

<execution_context>
@/home/caleb/.config/opencode/get-shit-done/workflows/execute-plan.md
@/home/caleb/.config/opencode/get-shit-done/templates/summary.md
</execution_context>

<context>
@.planning/PROJECT.md
@.planning/ROADMAP.md
@.planning/STATE.md
@.planning/phases/09-scan-cancellation-keyboard-shortcuts/09-RESEARCH.md

@backend/library/library.go
@backend/library/metrics.go
@backend/events/events.go

<interfaces>
<!-- Library struct (library.go:78-87) — add scan control fields here -->
type Library struct {
    mu          sync.Mutex
    ctx         context.Context
    logger      *slog.Logger
    conf        *Config
    db          *database.DB
    rescanHooks RescanHooks
}

<!-- ScanMetrics (metrics.go:11-55) — add Cancelled bool field -->
type ScanMetrics struct {
    mu sync.Mutex
    // ... existing timing and count fields ...
    Added   int64 `json:"added"`
    Updated int64 `json:"updated"`
    Skipped int64 `json:"skipped"`
    Removed int64 `json:"removed"`
    Warnings []ScanWarning `json:"warnings"`
}

<!-- Existing events (events.go:44-48) -->
const (
    LibraryScanStarted  = "LibraryScanStarted"
    LibraryScanProgress = "LibraryScanProgress"
    LibraryScanComplete = "LibraryScanComplete"
)

<!-- Scan() method signature (library.go:175) -->
func (l *Library) Scan() (*ScanMetrics, error)

<!-- Key scan pipeline locations that check l.ctx.Done() -->
<!-- library.go:297-298: case <-l.ctx.Done(): return l.ctx.Err() (walk, sending to workChan) -->
<!-- library.go:324-325: case <-l.ctx.Done(): return l.ctx.Err() (walk, new file) -->
<!-- library.go:496-497: case <-l.ctx.Done(): return l.ctx.Err() (worker, sending to resultChan) -->

<!-- commitBatch called at library.go:433 — uses l.ctx implicitly for DB ops -->
</interfaces>
</context>

<tasks>

<task type="auto">
  <name>Task 1: Add scan control events and metrics fields</name>
  <files>backend/events/events.go, frontend/src/events.ts, backend/library/metrics.go</files>
  <action>
1. In `backend/events/events.go`, add a new const block for scan control events:
   ```go
   // Scan control events.
   const (
       LibraryScanCancelled = "LibraryScanCancelled"
       LibraryScanPaused    = "LibraryScanPaused"
       LibraryScanResumed   = "LibraryScanResumed"
   )
   ```
   Place it after the existing Library events block (line 48).

2. Run `go generate ./backend/events/...` to regenerate `frontend/src/events.ts`.

3. In `backend/library/metrics.go`, add a `Cancelled` field to `ScanMetrics`:
   ```go
   Cancelled bool `json:"cancelled"`
   ```
   Place it after the `Removed int64` field (line 51), before the `Warnings` field.
  </action>
  <verify>
    <automated>cd backend && go build ./... && go generate ./events/... && grep -q "LibraryScanCancelled" events/events.go && grep -q "LibraryScanCancelled" ../frontend/src/events.ts && grep -q "Cancelled" library/metrics.go</automated>
  </verify>
  <done>Three new scan control events exist in events.go and are synced to frontend/src/events.ts. ScanMetrics has a Cancelled bool field.</done>
</task>

<task type="auto">
  <name>Task 2: Add scan control fields to Library struct and create scan_control.go</name>
  <files>backend/library/library.go, backend/library/scan_control.go</files>
  <action>
1. In `backend/library/library.go`, add scan control fields to the `Library` struct (after `rescanHooks` at line 86):
   ```go
   // Scan control fields — protected by mu.
   scanActive  bool
   scanCancel  context.CancelFunc
   scanPaused  bool
   scanPauseCh chan struct{}
   ```

2. Create `backend/library/scan_control.go` with these Wails-bound methods:

   ```go
   package library

   import (
       "github.com/wailsapp/wails/v2/pkg/runtime"
       "yellowjacket/backend/events"
   )

   // CancelScan cancels an in-progress scan. Returns immediately;
   // scan goroutines stop at their next checkpoint.
   func (l *Library) CancelScan() {
       l.mu.Lock()
       cancel := l.scanCancel
       l.mu.Unlock()

       if cancel != nil {
           cancel()
       }
   }

   // PauseScan pauses an in-progress scan. Workers block at their
   // next pause checkpoint until ResumeScan is called.
   func (l *Library) PauseScan() {
       l.mu.Lock()
       defer l.mu.Unlock()

       if !l.scanActive || l.scanPaused {
           return
       }

       l.scanPaused = true
       l.scanPauseCh = make(chan struct{})

       runtime.EventsEmit(l.ctx, events.LibraryScanPaused)
   }

   // ResumeScan unblocks a paused scan.
   func (l *Library) ResumeScan() {
       l.mu.Lock()
       defer l.mu.Unlock()

       if !l.scanPaused {
           return
       }

       l.scanPaused = false
       close(l.scanPauseCh) // unblocks all waiting workers

       runtime.EventsEmit(l.ctx, events.LibraryScanResumed)
   }

   // IsScanActive returns whether a scan is currently running.
   func (l *Library) IsScanActive() bool {
       l.mu.Lock()
       defer l.mu.Unlock()
       return l.scanActive
   }

   // IsScanPaused returns whether the scan is currently paused.
   func (l *Library) IsScanPaused() bool {
       l.mu.Lock()
       defer l.mu.Unlock()
       return l.scanPaused
   }

   // waitIfPaused blocks the calling goroutine if the scan is paused.
   // Returns ctx.Err() if the context is cancelled while waiting.
   func (l *Library) waitIfPaused(ctx context.Context) error {
       l.mu.Lock()
       ch := l.scanPauseCh
       paused := l.scanPaused
       l.mu.Unlock()

       if !paused || ch == nil {
           return nil
       }

       select {
       case <-ch:      // closed = unpaused
           return nil
       case <-ctx.Done():
           return ctx.Err()
       }
   }
   ```

   Note: `waitIfPaused` takes a `context.Context` parameter (the scan-specific context), not `l.ctx`. Add `"context"` to the import block.

3. Modify `Scan()` in `backend/library/library.go`:

   a. At the top of Scan() (after `metrics := newScanMetrics()`, line 176), create a cancellable scan context:
      ```go
      scanCtx, scanCancel := context.WithCancel(l.ctx)
      defer scanCancel()

      l.mu.Lock()
      l.scanCancel = scanCancel
      l.scanActive = true
      l.scanPaused = false
      l.scanPauseCh = nil
      l.mu.Unlock()

      defer func() {
          l.mu.Lock()
          l.scanCancel = nil
          l.scanActive = false
          // If still paused, unpause so no dangling channel
          if l.scanPaused {
              l.scanPaused = false
              if l.scanPauseCh != nil {
                  close(l.scanPauseCh)
              }
          }
          l.scanPauseCh = nil
          l.mu.Unlock()
      }()
      ```

   b. Replace ALL occurrences of `<-l.ctx.Done()` inside Scan() with `<-scanCtx.Done()`, and `l.ctx.Err()` with `scanCtx.Err()` (the walk goroutine send-to-workChan selects and the walk error return, and the worker pool send-to-resultChan select). There are 3 occurrences: line ~297, ~324, ~496.

   c. In the worker pool loop (Phase 3, around line 474), add a pause checkpoint before processing each file. Add at the start of the `g.Go(func() error {` closure body:
      ```go
      if err := l.waitIfPaused(scanCtx); err != nil {
          return err
      }
      ```

   d. **CRITICAL — Batch commits use l.ctx, NOT scanCtx:** The `commitBatch` method and all DB operations within it should continue to use `l.ctx` (the app context), NOT the scan-specific `scanCtx`. This is already the case since `commitBatch` accesses `l.ctx` internally. DO NOT change `commitBatch` to use `scanCtx`. This ensures in-flight transactions always complete even when the scan is cancelled.

   e. **CRITICAL — Skip orphan cleanup on cancelled scan:** Before the orphan cleanup phase (Phase 5, around line 549), add a check:
      ```go
      // Skip orphan cleanup if the scan was cancelled — existingPaths
      // still contains unvisited files that would be incorrectly deleted.
      cancelled := scanCtx.Err() != nil
      if cancelled {
          metrics.Cancelled = true
          l.logger.Info("scan cancelled, skipping orphan cleanup")
      } else {
          // ... existing orphan cleanup code ...
      }
      ```
      Wrap the existing orphan cleanup code (existingPaths.Range through metrics.OrphanCleanup = ...) inside the `else` block.

   f. Also skip the "Phase 6: post-scan variant generation" if cancelled (wrap in same `if !cancelled` check or separate check).

   g. When the scan was cancelled, emit `LibraryScanCancelled` instead of (or in addition to) `LibraryScanComplete`. Update the finalize section:
      ```go
      if cancelled {
          runtime.EventsEmit(l.ctx, events.LibraryScanCancelled, metrics)
      } else {
          runtime.EventsEmit(l.ctx, events.LibraryScanComplete, metrics)
      }
      ```
  </action>
  <verify>
    <automated>cd backend && go build ./... && go vet ./library/...</automated>
  </verify>
  <done>Library struct has scan control fields. scan_control.go provides CancelScan/PauseScan/ResumeScan/IsScanActive/IsScanPaused. Scan() uses per-scan context, workers check for pause, orphan cleanup is skipped on cancel, and appropriate events are emitted.</done>
</task>

</tasks>

<verification>
```bash
cd backend && go build ./... && go vet ./library/... && go vet ./events/...
```
All backend code compiles. No vet errors. New scan control methods are exported and Wails-bindable.
</verification>

<success_criteria>
- `go build ./...` passes with no errors
- `CancelScan`, `PauseScan`, `ResumeScan`, `IsScanActive`, `IsScanPaused` are exported methods on `*Library`
- `waitIfPaused` is an unexported helper that blocks on pause channel
- Scan() creates a per-scan context and uses it for worker cancellation
- Orphan cleanup and variant generation are skipped when scan is cancelled
- `LibraryScanCancelled`, `LibraryScanPaused`, `LibraryScanResumed` events exist and are synced to TypeScript
- `ScanMetrics.Cancelled` bool field exists
</success_criteria>

<output>
After completion, create `.planning/phases/09-scan-cancellation-keyboard-shortcuts/09-01-SUMMARY.md`
</output>