store: fix potential panic in GC worker (pingcap#14403)

store/tikv/gcworker/gc_worker.go

@@ -53,8 +53,6 @@ type GCWorker struct {
 	lastFinish  time.Time
 	cancel      context.CancelFunc
 	done        chan error
-
-	session session.Session
 }
 
 // NewGCWorker creates a GCWorker instance.
@@ -136,8 +134,6 @@ func (w *GCWorker) start(ctx context.Context, wg *sync.WaitGroup) {
 	logutil.Logger(ctx).Info("[gc worker] start",
 		zap.String("uuid", w.uuid))
 
-	w.session = createSession(w.store)
-
 	w.tick(ctx) // Immediately tick once to initialize configs.
 	wg.Done()
 
@@ -864,7 +860,6 @@ func (w *GCWorker) checkLeader() (bool, error) {
 	if err != nil {
 		return false, errors.Trace(err)
 	}
-	w.session = se
 	leader, err := w.loadValueFromSysTable(gcLeaderUUIDKey)
 	if err != nil {
 		_, err1 := se.Execute(ctx, "ROLLBACK")
@@ -895,6 +890,7 @@ func (w *GCWorker) checkLeader() (bool, error) {
 	}
 	lease, err := w.loadTime(gcLeaderLeaseKey)
 	if err != nil {
+		se.RollbackTxn(ctx)
 		return false, errors.Trace(err)
 	}
 	if lease == nil || lease.Before(time.Now()) {
@@ -998,8 +994,10 @@ func (w *GCWorker) loadDurationWithDefault(key string, def time.Duration) (*time
 
 func (w *GCWorker) loadValueFromSysTable(key string) (string, error) {
 	ctx := context.Background()
+	se := createSession(w.store)
+	defer se.Close()
 	stmt := fmt.Sprintf(`SELECT HIGH_PRIORITY (variable_value) FROM mysql.tidb WHERE variable_name='%s' FOR UPDATE`, key)
-	rs, err := w.session.Execute(ctx, stmt)
+	rs, err := se.Execute(ctx, stmt)
 	if len(rs) > 0 {
 		defer terror.Call(rs[0].Close)
 	}
@@ -1028,10 +1026,9 @@ func (w *GCWorker) saveValueToSysTable(key, value string) error {
 			       ON DUPLICATE KEY
 			       UPDATE variable_value = '%[2]s', comment = '%[3]s'`,
 		key, value, gcVariableComments[key])
-	if w.session == nil {
-		return errors.New("[saveValueToSysTable session is nil]")
-	}
-	_, err := w.session.Execute(context.Background(), stmt)
+	se := createSession(w.store)
+	defer se.Close()
+	_, err := se.Execute(context.Background(), stmt)
 	logutil.Logger(context.Background()).Debug("[gc worker] save kv",
 		zap.String("key", key),
 		zap.String("value", value),
@@ -1084,13 +1081,6 @@ func NewMockGCWorker(store tikv.Storage) (*MockGCWorker, error) {
 		lastFinish:  time.Now(),
 		done:        make(chan error),
 	}
-	worker.session, err = session.CreateSession(worker.store)
-	if err != nil {
-		logutil.Logger(context.Background()).Error("initialize MockGCWorker session fail", zap.Error(err))
-		return nil, errors.Trace(err)
-	}
-	privilege.BindPrivilegeManager(worker.session, nil)
-	worker.session.GetSessionVars().InRestrictedSQL = true
 	return &MockGCWorker{worker: worker}, nil
 }