fix(goes/esgallery): fix auto-save concurrency error

goes/esgallery/processor.go

@@ -25,6 +25,8 @@ import (
 // ProcessedTag is added to [gallery.Stack]s that were processed by a [*PostProcessor].
 const ProcessedTag = "processed"
 
+var autoSaveMaxTries = 5
+
 // Processor post-processes [gallery.Stack]s and uploads the processed images
 // to (cloud) storage.
 type Processor[StackID, ImageID ID] struct {
@@ -537,28 +539,49 @@ func (q *processorQueue[Gallery, StackID, ImageID]) shouldProcess(evt event.Even
 }
 
 func (q *processorQueue[Gallery, StackID, ImageID]) apply(result *ProcessorResult[StackID, ImageID], galleryID uuid.UUID) error {
-	q.cfg.debugLog("fetching gallery to apply result to ... [id=%s]", galleryID)
+	// The following is true only if the [WithAutoApply] option is provided with
+	// the `autoSave` parameter set to `true`:
+	//
+	// Between the time of fetching and saving the gallery, the gallery might
+	// have raised other aggregate events. To avoid optimistic concurrency
+	// issues, we re-try the whole process a few times.
+	var tries int
+	for {
+		tries++
+
+		q.cfg.debugLog("fetching gallery to apply result to ... [id=%s]", galleryID)
+		g, err := q.processor.fetchGallery(q.ctx, galleryID)
+		if err != nil {
+			return fmt.Errorf("fetch gallery: %w", err)
+		}
 
-	g, err := q.processor.fetchGallery(q.ctx, galleryID)
-	if err != nil {
-		return fmt.Errorf("fetch gallery: %w", err)
-	}
+		q.cfg.debugLog("applying processing result ... [galleryId=%s, stackId=%s]", galleryID, result.StackID)
+		if err := result.Apply(g); err != nil {
+			return err
+		}
+		result.Applied = true
 
-	q.cfg.debugLog("applying processing result ... [galleryId=%s, stackId=%s]", galleryID, result.StackID)
-	if err := result.Apply(g); err != nil {
-		return err
-	}
-	result.Applied = true
+		if q.processor.autoSave != nil {
+			q.cfg.debugLog("auto-saving gallery ... [id=%s]", galleryID)
+			if err := q.processor.autoSave(q.ctx, g); err != nil {
+				if aggregate.IsConsistencyError(err) {
+					q.cfg.debugLog("optimistic concurrency error while auto-saving: %v", err)
+
+					if tries >= autoSaveMaxTries {
+						return fmt.Errorf("optimistic concurrency error while auto-saving: %w", err)
+					}
 
-	if q.processor.autoSave != nil {
-		q.cfg.debugLog("auto-saving gallery ... [id=%s]", galleryID)
-		if err := q.processor.autoSave(q.ctx, g); err != nil {
-			return fmt.Errorf("auto-save gallery: %w", err)
+					q.cfg.debugLog("retrying auto-save because of optimistic concurrency error ...")
+					continue
+				}
+
+				return fmt.Errorf("auto-save gallery: %w", err)
+			}
+			result.Saved = true
 		}
-		result.Saved = true
-	}
 
-	return nil
+		return nil
+	}
 }
 
 func (q *processorQueue[Gallery, StackID, ImageID]) fail(err error) {