Handle empty dashboard metrics safely

internal/service/safety_test.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"context"
+	"encoding/json"
 	"os"
 	"path/filepath"
 	"testing"
@@ -11,6 +12,17 @@ import (
 	"git.casaderoll.de/michael/urbm/internal/store"
 )
 
+func TestEmptyBackupMetricsEncodeAsJSONArray(t *testing.T) {
+	s := &Service{}
+	encoded, err := json.Marshal(s.BackupMetrics())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(encoded) != "[]" {
+		t.Fatalf("empty backup metrics JSON = %s", encoded)
+	}
+}
+
 func TestRepositoryLockHonorsContextCancellation(t *testing.T) {
 	s := &Service{repositoryLocks: map[string]chan struct{}{}}
 	repo := model.Repository{Type: model.RepositoryLocal, Location: "/repo"}

internal/service/service.go

@@ -134,7 +134,9 @@ func (s *Service) Runs() []model.Run    { return s.queue.Snapshot() }
 func (s *Service) BackupMetrics() []model.BackupMetric {
 	s.metricsMu.RLock()
 	defer s.metricsMu.RUnlock()
-	return append([]model.BackupMetric(nil), s.backupMetrics...)
+	metrics := make([]model.BackupMetric, len(s.backupMetrics))
+	copy(metrics, s.backupMetrics)
+	return metrics
 }
 func (s *Service) Logs() map[string]any {
 	return map[string]any{