*: fix a data race in privilege handle Update() (pingcap#3389)

privilege handle bind a ctx to call Update(), but it may be
called by multiple goroutines, so data race in that ctx.

change Update() to take a context parameter, avoid using the
same ctx to fix the race.

domain/domain.go

@@ -429,8 +429,8 @@ func (do *Domain) SysSessionPool() *pools.ResourcePool {
 // LoadPrivilegeLoop create a goroutine loads privilege tables in a loop, it
 // should be called only once in BootstrapSession.
 func (do *Domain) LoadPrivilegeLoop(ctx context.Context) error {
-	do.privHandle = privileges.NewHandle(ctx)
-	err := do.privHandle.Update()
+	do.privHandle = privileges.NewHandle()
+	err := do.privHandle.Update(ctx)
 	if err != nil {
 		return errors.Trace(err)
 	}
@@ -450,7 +450,7 @@ func (do *Domain) LoadPrivilegeLoop(ctx context.Context) error {
 			case <-watchCh:
 			case <-time.After(duration):
 			}
-			err := do.privHandle.Update()
+			err := do.privHandle.Update(ctx)
 			if err != nil {
 				log.Error("load privilege fail:", errors.ErrorStack(err))
 			}

executor/simple.go

@@ -309,7 +309,13 @@ func (e *SimpleExec) executeFlush(s *ast.FlushStmt) error {
 		// TODO: A dummy implement
 	case ast.FlushPrivileges:
 		dom := sessionctx.GetDomain(e.ctx)
-		err := dom.PrivilegeHandle().Update()
+		sysSessionPool := dom.SysSessionPool()
+		ctx, err := sysSessionPool.Get()
+		if err != nil {
+			return errors.Trace(err)
+		}
+		defer sysSessionPool.Put(ctx)
+		err = dom.PrivilegeHandle().Update(ctx.(context.Context))
 		return errors.Trace(err)
 	}
 	return nil

privilege/privileges/cache.go

@@ -566,15 +566,12 @@ func appendUserPrivilegesTableRow(rows [][]types.Datum, user userRecord) [][]typ
 
 // Handle wraps MySQLPrivilege providing thread safe access.
 type Handle struct {
-	ctx  context.Context
 	priv atomic.Value
 }
 
 // NewHandle returns a Handle.
-func NewHandle(ctx context.Context) *Handle {
-	return &Handle{
-		ctx: ctx,
-	}
+func NewHandle() *Handle {
+	return &Handle{}
 }
 
 // Get the MySQLPrivilege for read.
@@ -583,9 +580,9 @@ func (h *Handle) Get() *MySQLPrivilege {
 }
 
 // Update loads all the privilege info from kv storage.
-func (h *Handle) Update() error {
+func (h *Handle) Update(ctx context.Context) error {
 	var priv MySQLPrivilege
-	err := priv.LoadAll(h.ctx)
+	err := priv.LoadAll(ctx)
 	if err != nil {
 		return errors.Trace(err)
 	}