fix(backup): allow for empty snapshot dirs on reindexing

Fixes #3733

Author: Michal-Leszczynski

pkg/service/backup/service.go

@@ -786,6 +786,7 @@ func (s *Service) Backup(ctx context.Context, clusterID, taskID, runID uuid.UUID
 			Config:      s.config,
 			Client:      client,
 		},
+		PrevStage:            run.Stage,
 		Metrics:              s.metrics,
 		Units:                run.Units,
 		OnRunProgress:        s.putRunProgressLogError,

pkg/service/backup/worker.go

@@ -64,6 +64,7 @@ type workerTools struct {
 type worker struct {
 	workerTools
 
+	PrevStage     Stage
 	Metrics       metrics.BackupMetrics
 	Units         []Unit
 	Schema        *bytes.Buffer

pkg/service/backup/worker_index.go

@@ -147,7 +147,9 @@ func (w *worker) indexSnapshotDirs(ctx context.Context, h hostInfo) ([]snapshotD
 		}
 	}
 
-	if len(dirs) == 0 {
+	// In case of reindexing, it's possible that all snapshot dirs
+	// were already uploaded and deleted in the previous run (#3733).
+	if w.PrevStage != StageUpload && len(dirs) == 0 {
 		return nil, errors.New("could not find any files")
 	}