Merge pull request pingcap#544 from pingcap/shenli/refactor-globalvar

*: Use Bind/Get global sysvar accessor instead of type assertion

expression/variable.go

@@ -62,6 +62,7 @@ func (v *Variable) String() string {
 func (v *Variable) Eval(ctx context.Context, args map[interface{}]interface{}) (interface{}, error) {
 	name := strings.ToLower(v.Name)
 	sessionVars := variable.GetSessionVars(ctx)
+	globalVars := variable.GetGlobalSysVarAccessor(ctx)
 	if !v.IsSystem {
 		// user vars
 		if value, ok := sessionVars.Users[name]; ok {
@@ -82,7 +83,7 @@ func (v *Variable) Eval(ctx context.Context, args map[interface{}]interface{}) (
 			return value, nil
 		}
 	}
-	value, err := ctx.(variable.GlobalSysVarAccessor).GetGlobalSysVar(ctx, name)
+	value, err := globalVars.GetGlobalSysVar(ctx, name)
 	if err != nil {
 		return nil, errors.Trace(err)
 	}

expression/variable_test.go

@@ -27,8 +27,10 @@ type testVariableSuite struct {
 }
 
 func (s *testVariableSuite) SetUpSuite(c *C) {
-	s.ctx = mock.NewContext()
+	nc := mock.NewContext()
+	s.ctx = nc
 	variable.BindSessionVars(s.ctx)
+	variable.BindGlobalSysVarAccessor(s.ctx, nc)
 }
 
 func (s *testVariableSuite) TestVariable(c *C) {

plan/plans/show.go

@@ -353,6 +353,7 @@ func (s *ShowPlan) fetchShowTables(ctx context.Context) error {
 
 func (s *ShowPlan) fetchShowVariables(ctx context.Context) error {
 	sessionVars := variable.GetSessionVars(ctx)
+	globalVars := variable.GetGlobalSysVarAccessor(ctx)
 	m := map[interface{}]interface{}{}
 
 	for _, v := range variable.SysVars {
@@ -384,13 +385,13 @@ func (s *ShowPlan) fetchShowVariables(ctx context.Context) error {
 				value = sv
 			} else {
 				// If session scope variable is not set, get the global scope value.
-				value, err = ctx.(variable.GlobalSysVarAccessor).GetGlobalSysVar(ctx, v.Name)
+				value, err = globalVars.GetGlobalSysVar(ctx, v.Name)
 				if err != nil {
 					return errors.Trace(err)
 				}
 			}
 		} else {
-			value, err = ctx.(variable.GlobalSysVarAccessor).GetGlobalSysVar(ctx, v.Name)
+			value, err = globalVars.GetGlobalSysVar(ctx, v.Name)
 			if err != nil {
 				return errors.Trace(err)
 			}

plan/plans/show_test.go

@@ -53,8 +53,10 @@ type testShowSuit struct {
 var _ = Suite(&testShowSuit{})
 
 func (p *testShowSuit) SetUpSuite(c *C) {
-	p.ctx = mock.NewContext()
+	nc := mock.NewContext()
+	p.ctx = nc
 	variable.BindSessionVars(p.ctx)
+	variable.BindGlobalSysVarAccessor(p.ctx, nc)
 
 	p.dbName = "testshowplan"
 	p.store = newStore(c, p.dbName)

session.go

@@ -543,6 +543,9 @@ func CreateSession(store kv.Storage) (Session, error) {
 	variable.BindSessionVars(s)
 	variable.GetSessionVars(s).SetStatusFlag(mysql.ServerStatusAutocommit, true)
 
+	// session implements variable.GlobalSysVarAccessor. Bind it to ctx.
+	variable.BindGlobalSysVarAccessor(s, s)
+
 	// session implements autocommit.Checker. Bind it to ctx
 	autocommit.BindAutocommitChecker(s, s)
 	sessionMu.Lock()

sessionctx/variable/sysvar.go

@@ -581,3 +581,27 @@ type GlobalSysVarAccessor interface {
 	// SetGlobalSysVar sets the global system variable name to value.
 	SetGlobalSysVar(ctx context.Context, name string, value string) error
 }
+
+// globalSysVarAccessorKeyType is a dummy type to avoid naming collision in context.
+type globalSysVarAccessorKeyType int
+
+// String defines a Stringer function for debugging and pretty printing.
+func (k globalSysVarAccessorKeyType) String() string {
+	return "global_sysvar_accessor"
+}
+
+const accessorKey globalSysVarAccessorKeyType = 0
+
+// BindGlobalSysVarAccessor binds global sysvar accessor to context.
+func BindGlobalSysVarAccessor(ctx context.Context, accessor GlobalSysVarAccessor) {
+	ctx.SetValue(accessorKey, accessor)
+}
+
+// GetGlobalSysVarAccessor gets accessor from ctx.
+func GetGlobalSysVarAccessor(ctx context.Context) GlobalSysVarAccessor {
+	v, ok := ctx.Value(accessorKey).(GlobalSysVarAccessor)
+	if !ok {
+		panic("Miss global sysvar accessor")
+	}
+	return v
+}

stmt/stmts/set.go

@@ -100,7 +100,7 @@ func (s *SetStmt) Exec(ctx context.Context) (_ rset.Recordset, err error) {
 	log.Debug("Set sys/user variables")
 
 	sessionVars := variable.GetSessionVars(ctx)
-
+	globalVars := variable.GetGlobalSysVarAccessor(ctx)
 	for _, v := range s.Variables {
 		// Variable is case insensitive, we use lower case.
 		name := strings.ToLower(v.Name)
@@ -138,7 +138,7 @@ func (s *SetStmt) Exec(ctx context.Context) (_ rset.Recordset, err error) {
 				if err != nil {
 					return nil, errors.Trace(err)
 				}
-				err = ctx.(variable.GlobalSysVarAccessor).SetGlobalSysVar(ctx, name, svalue)
+				err = globalVars.SetGlobalSysVar(ctx, name, svalue)
 				return nil, errors.Trace(err)
 			}
 			return nil, errors.Errorf("Variable '%s' is a SESSION variable and can't be used with SET GLOBAL", name)

util/mock/context.go

@@ -79,7 +79,7 @@ func (c *Context) SetGlobalSysVar(ctx context.Context, name string, value string
 }
 
 // NewContext creates a new mocked context.Context.
-func NewContext() context.Context {
+func NewContext() *Context {
 	return &Context{
 		values: make(map[fmt.Stringer]interface{}),
 	}