log-backup: the subscription conntion may be create too many times when TiKV node restarts too fast

[YuJuncen]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Change the code via the diff:

diff --git a/br/pkg/streamhelper/basic_lib_for_test.go b/br/pkg/streamhelper/basic_lib_for_test.go
index 882ac69ede..9598b5d376 100644
--- a/br/pkg/streamhelper/basic_lib_for_test.go
+++ b/br/pkg/streamhelper/basic_lib_for_test.go
@@ -133,7 +133,7 @@ func (t trivialFlushStream) Recv() (*logbackup.SubscribeFlushEventResponse, erro
                        return &item, nil
                default:
                }
-               return nil, t.cx.Err()
+               return nil, status.Error(codes.Canceled, t.cx.Err().Error())
        }
 }

This is a reasonable change because in real-world clusters, once the context for the server streaming canceled, gRPC yields this error instead of plain context.Canceled.

Then run the test TestHasFailureStores.

2. What did you expect to see? (Required)

The test passes.

3. What did you see instead (Required)

The test failed.

    /home/hillium/developer/tidb/br/pkg/streamhelper/subscription_test.go:107: 
        	Error Trace:	/home/hillium/developer/tidb/br/pkg/streamhelper/subscription_test.go:107
        	Error:      	Received unexpected error:
        	            	the following errors occurred:
        	            	 -  rpc error: code = Canceled desc = context canceled

4. What is your TiDB version? (Required)

Current master.