Let task agents create and manage subtasks

internal/store/discard.go

@@ -19,8 +19,12 @@ const (
 )
 
 var (
-	ErrDiscardPending = errors.New("the task is already being discarded")
-	ErrMerging        = errors.New("the task is merging")
+	ErrDiscardPending       = errors.New("the task is already being discarded")
+	ErrMerging              = errors.New("the task is merging")
+	ErrMergingIntoParent    = errors.New("the task is merging into its parent task")
+	ErrHasSubtasks          = errors.New("the task has active subtasks")
+	ErrParentNotOpen        = errors.New("the parent task is closed or being discarded")
+	ErrParentMergeRequested = errors.New("the subtask is already merging into its parent")
 )
 
 type Discard struct {
@@ -44,7 +48,12 @@ func (s *Store) RequestDiscard(ctx context.Context, taskID, actor string) (Disca
 		projectID := ""
 		status := ""
 		mergingBy := ""
-		err := tx.QueryRowContext(ctx, `SELECT project_id, status, merging_by, discard_state FROM tasks WHERE id = ?`, taskID).Scan(&projectID, &status, &mergingBy, &state)
+		parentMergeBy := ""
+		subtasks := 0
+		err := tx.QueryRowContext(ctx, `
+			SELECT project_id, status, merging_by, parent_merge_by, discard_state,
+				(SELECT count(*) FROM tasks AS sub WHERE sub.parent_task_id = tasks.id AND sub.status = ?)
+			FROM tasks WHERE id = ?`, string(api.TaskStatusActive), taskID).Scan(&projectID, &status, &mergingBy, &parentMergeBy, &state, &subtasks)
 		if errors.Is(err, sql.ErrNoRows) {
 			return ErrNotFound
 		}
@@ -60,6 +69,10 @@ func (s *Store) RequestDiscard(ctx context.Context, taskID, actor string) (Disca
 			return ErrDiscardPending
 		case mergingBy != "":
 			return ErrMerging
+		case parentMergeBy != "":
+			return ErrMergingIntoParent
+		case subtasks > 0:
+			return ErrHasSubtasks
 		}
 		state = DiscardHeld
 		_, err = tx.ExecContext(ctx, `UPDATE tasks SET discard_state = ?, discarding_by = ?, discard_session = '', merge_queued_by = '', merge_queued_at = '' WHERE id = ?`,