store: fix potential panic in GC worker

store/tikv/gcworker/gc_worker.go

@@ -55,8 +55,6 @@ type GCWorker struct {
 	lastFinish  time.Time
 	cancel      context.CancelFunc
 	done        chan error
-
-	session session.Session
 }
 
 // NewGCWorker creates a GCWorker instance.
@@ -155,8 +153,6 @@ 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()
 
@@ -274,18 +270,20 @@ func (w *GCWorker) prepare() (bool, uint64, error) {
 	// 4. GC update `tikv_gc_safe_point` value to t2, continue do GC in this round.
 	// Then the data record that has been dropped between time t1 and t2, will be cleaned by GC, but the user thinks the data after t1 won't be clean by GC.
 	ctx := context.Background()
-	_, err := w.session.Execute(ctx, "BEGIN")
+	se := createSession(w.store)
+	defer se.Close()
+	_, err := se.Execute(ctx, "BEGIN")
 	if err != nil {
 		return false, 0, errors.Trace(err)
 	}
 	doGC, safePoint, err := w.checkPrepare(ctx)
 	if doGC {
-		err = w.session.CommitTxn(ctx)
+		err = se.CommitTxn(ctx)
 		if err != nil {
 			return false, 0, errors.Trace(err)
 		}
 	} else {
-		w.session.RollbackTxn(ctx)
+		se.RollbackTxn(ctx)
 	}
 	return doGC, safePoint, errors.Trace(err)
 }
@@ -1075,7 +1073,6 @@ 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 {
 		se.RollbackTxn(ctx)
@@ -1103,6 +1100,7 @@ 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()) {
@@ -1202,8 +1200,10 @@ 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)
 	}
@@ -1232,10 +1232,9 @@ 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.BgLogger().Debug("[gc worker] save kv",
 		zap.String("key", key),
 		zap.String("value", value),
@@ -1326,13 +1325,6 @@ 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.BgLogger().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
 }
 

store/tikv/gcworker/gc_worker_test.go

@@ -445,7 +445,9 @@ const (
 
 func (s *testGCWorkerSuite) testDeleteRangesFailureImpl(c *C, failType int) {
 	// Put some delete range tasks.
-	_, err := s.gcWorker.session.Execute(context.Background(), `INSERT INTO mysql.gc_delete_range VALUES
+	se := createSession(s.gcWorker.store)
+	defer se.Close()
+	_, err := se.Execute(context.Background(), `INSERT INTO mysql.gc_delete_range VALUES
 		("1", "2", "31", "32", "10"),
 		("3", "4", "33", "34", "10"),
 		("5", "6", "35", "36", "10")`)
@@ -473,7 +475,6 @@ func (s *testGCWorkerSuite) testDeleteRangesFailureImpl(c *C, failType int) {
 	}
 
 	// Check the delete range tasks.
-	se := createSession(s.gcWorker.store)
 	preparedRanges, err := util.LoadDeleteRanges(se, 20)
 	se.Close()
 	c.Assert(err, IsNil)