Конвенция для обработки ошибок + рефакторинг кода

internal/worker/errors.go

@@ -0,0 +1,8 @@
+package worker
+
+import "errors"
+
+// ErrConflict — операция недопустима в текущем состоянии загрузки (напр. apply
+// вне review/deferred, undo вне done). Это нормальный конфликт состояния, а не
+// сбой сервера: транспорт матчит его через errors.Is и отвечает 409, не 500.
+var ErrConflict = errors.New("conflict")

internal/worker/review.go

@@ -225,7 +225,7 @@ func (w *Worker) Apply(ctx context.Context, id int64) error {
 		return fmt.Errorf("apply: %w", err)
 	}
 	if d.State != store.StateReview && d.State != store.StateDeferred {
-		return fmt.Errorf("apply: download %d is in state %s (expected review/deferred)", id, d.State)
+		return fmt.Errorf("apply: download %d is in state %s (expected review/deferred): %w", id, d.State, ErrConflict)
 	}
 	ctx = w.scoped(ctx, capFileLayout, id, d.Infohash.String)
 
@@ -234,8 +234,11 @@ func (w *Worker) Apply(ctx context.Context, id int64) error {
 		return fmt.Errorf("apply: %w", err)
 	}
 	t, ok, err := w.torrentByInfohash(ctx, d.Infohash.String)
-	if err != nil || !ok {
-		return fmt.Errorf("apply: torrent not found: %v", err)
+	if err != nil {
+		return fmt.Errorf("apply: lookup torrent: %w", err)
+	}
+	if !ok {
+		return fmt.Errorf("apply: torrent not found")
 	}
 
 	w.transition(ctx, *d, store.StateLinking, "", "")
@@ -304,7 +307,7 @@ func (w *Worker) Relink(ctx context.Context, id int64) error {
 		return fmt.Errorf("relink: %w", err)
 	}
 	if d.State != store.StateReverted && d.State != store.StateCancelled {
-		return fmt.Errorf("relink: download %d is in state %s (expected reverted/cancelled)", id, d.State)
+		return fmt.Errorf("relink: download %d is in state %s (expected reverted/cancelled): %w", id, d.State, ErrConflict)
 	}
 	if !d.Infohash.Valid {
 		return fmt.Errorf("relink: download %d has no infohash", id)
@@ -462,7 +465,7 @@ func (w *Worker) Undo(ctx context.Context, id int64) error {
 		return fmt.Errorf("undo: %w", err)
 	}
 	if d.State != store.StateDone {
-		return fmt.Errorf("undo: download %d is in state %s (expected done)", id, d.State)
+		return fmt.Errorf("undo: download %d is in state %s (expected done): %w", id, d.State, ErrConflict)
 	}
 	ctx = w.scoped(ctx, capFileLayout, id, d.Infohash.String)
 	batch, err := w.store.LatestBatchID(ctx, id)
@@ -499,7 +502,7 @@ func (w *Worker) requireReviewable(ctx context.Context, id int64, op string) (*s
 		return nil, fmt.Errorf("%s: %w", op, err)
 	}
 	if d.State != store.StateReview && d.State != store.StateDeferred {
-		return nil, fmt.Errorf("%s: download %d is in state %s (expected review/deferred)", op, id, d.State)
+		return nil, fmt.Errorf("%s: download %d is in state %s (expected review/deferred): %w", op, id, d.State, ErrConflict)
 	}
 	return d, nil
 }