br: fix flaky test TestGCServiceSafePoint (#58628)

close pingcap/tidb#58627

br/pkg/streamhelper/advancer.go

@@ -127,6 +127,9 @@ func newCheckpointWithSpan(s spans.Valued) *checkpoint {
 }
 
 func (c *checkpoint) safeTS() uint64 {
+	if c.TS == 0 {
+		return 0
+	}
 	return c.TS - 1
 }
 
@@ -429,7 +432,7 @@ func (c *CheckpointAdvancer) onTaskEvent(ctx context.Context, e TaskEvent) error
 		}
 		log.Info("get global checkpoint", zap.Uint64("checkpoint", globalCheckpointTs))
 		c.lastCheckpoint = newCheckpointWithTS(globalCheckpointTs)
-		p, err := c.env.BlockGCUntil(ctx, globalCheckpointTs-1)
+		p, err := c.env.BlockGCUntil(ctx, c.lastCheckpoint.safeTS())
 		if err != nil {
 			log.Warn("failed to upload service GC safepoint, skipping.", logutil.ShortError(err))
 		}

br/pkg/streamhelper/basic_lib_for_test.go

@@ -677,7 +677,7 @@ func newTestEnv(c *fakeCluster, t *testing.T) *testEnv {
 		Name: "whole",
 		Info: &backup.StreamBackupTaskInfo{
 			Name:    "whole",
-			StartTs: 5,
+			StartTs: 0,
 		},
 		Ranges: rngs,
 	}
@@ -782,7 +782,7 @@ func (t *testEnv) putTask() {
 		Name: "whole",
 		Info: &backup.StreamBackupTaskInfo{
 			Name:    "whole",
-			StartTs: 5,
+			StartTs: 0,
 		},
 		Ranges: rngs,
 	}