fix: implement cancel endpoint and pool cancel mechanism

POST /api/tasks/{id}/cancel now works. Pool tracks a cancel func per
running task ID; Cancel(taskID) calls it and returns false if the task
isn't running. The execute goroutine registers/deregisters the cancel
func around the runner call.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 25 insertions, 2 deletions

diff --git a/internal/executor/executor.go b/internal/executor/executor.go
index f6932f4..62fed2e 100644
--- a/internal/executor/executor.go
+++ b/internal/executor/executor.go
@@ -35,6 +35,7 @@ type Pool struct {
 
 	mu        sync.Mutex
 	active    int
+	cancels   map[string]context.CancelFunc // taskID → cancel
 	resultCh  chan *Result
 	Questions *QuestionRegistry
 }
@@ -55,6 +56,7 @@ func NewPool(maxConcurrent int, runner Runner, store *storage.DB, logger *slog.L
 		runner:        runner,
 		store:         store,
 		logger:        logger,
+		cancels:       make(map[string]context.CancelFunc),
 		resultCh:      make(chan *Result, maxConcurrent*2),
 		Questions:     NewQuestionRegistry(),
 	}
@@ -81,6 +83,19 @@ func (p *Pool) Results() <-chan *Result {
 	return p.resultCh
 }
 
+// Cancel requests cancellation of a running task. Returns false if the task
+// is not currently running in this pool.
+func (p *Pool) Cancel(taskID string) bool {
+	p.mu.Lock()
+	cancel, ok := p.cancels[taskID]
+	p.mu.Unlock()
+	if !ok {
+		return false
+	}
+	cancel()
+	return true
+}
+
 // SubmitResume re-queues a blocked task using the provided resume execution.
 // The execution must have ResumeSessionID and ResumeAnswer set.
 func (p *Pool) SubmitResume(ctx context.Context, t *task.Task, exec *storage.Execution) error {
@@ -230,14 +245,22 @@ func (p *Pool) execute(ctx context.Context, t *task.Task) {
 		p.logger.Error("failed to update task state", "error", err)
 	}
 
-	// Apply task timeout.
+	// Apply task timeout and register cancel so callers can stop this task.
 	var cancel context.CancelFunc
 	if t.Timeout.Duration > 0 {
 		ctx, cancel = context.WithTimeout(ctx, t.Timeout.Duration)
 	} else {
 		ctx, cancel = context.WithCancel(ctx)
 	}
-	defer cancel()
+	p.mu.Lock()
+	p.cancels[t.ID] = cancel
+	p.mu.Unlock()
+	defer func() {
+		cancel()
+		p.mu.Lock()
+		delete(p.cancels, t.ID)
+		p.mu.Unlock()
+	}()
 
 	// Run the task.
 	err := p.runner.Run(ctx, t, exec)