fix(thumbnails): don't re-create thumbnails if they already exist

A previous cleanup introduced a bug which caused thumbnails to be
re-created even if we already had a matching thumbnail in the storage.

changelog/unreleased/fix-use-existing-thumbnails.md

@@ -0,0 +1,6 @@
+Bugfix: Avoid re-creating thumbnails
+
+We fixed a bug that caused the system to re-create thumbnails for images, even
+if a thumbnail already existed in the cache.
+
+https://github.com/owncloud/ocis/pull/1xxxx

services/thumbnails/pkg/service/grpc/v0/service.go

@@ -145,8 +145,12 @@ func (g Thumbnail) handleCS3Source(ctx context.Context, req *thumbnailssvc.GetTh
 	}
 
 	key, tr, err := g.checkThumbnail(req, sRes)
-	if err != nil {
+	switch {
+	case err != nil:
 		return "", err
+	case key != "":
+		// we have matching thumbnail already, use that
+		return key, nil
 	}
 
 	ctx = imgsource.ContextSetAuthorization(ctx, src.GetAuthorization())
@@ -220,9 +224,14 @@ func (g Thumbnail) handleWebdavSource(ctx context.Context, req *thumbnailssvc.Ge
 	}
 
 	key, tr, err := g.checkThumbnail(req, sRes)
-	if err != nil {
+	switch {
+	case err != nil:
 		return "", err
+	case key != "":
+		// we have matching thumbnail already, use that
+		return key, nil
 	}
+
 	if src.GetWebdavAuthorization() != "" {
 		ctx = imgsource.ContextSetAuthorization(ctx, src.GetWebdavAuthorization())
 	}