allow setting session variable default value (#3203)

Author: jennifersp

server/context.go

@@ -235,6 +235,15 @@ func (s *SessionManager) getOrCreateSession(ctx context.Context, conn *mysql.Con
 	return sess, nil
 }
 
+// InitSessionDefaultVariable sets a default value to a parameter of a session at start.
+func (s *SessionManager) InitSessionDefaultVariable(ctx context.Context, conn *mysql.Conn, name, value string) error {
+	sess, err := s.getOrCreateSession(ctx, conn)
+	if err != nil {
+		return err
+	}
+	return sess.InitSessionVariableDefault(s.ctxFactory(ctx, sql.WithSession(sess)), name, value)
+}
+
 // NewContextWithQuery creates a new context for the session at the given conn.
 func (s *SessionManager) NewContextWithQuery(ctx context.Context, conn *mysql.Conn, query string) (*sql.Context, error) {
 	sess, err := s.getOrCreateSession(ctx, conn)

sql/base_session.go

@@ -159,16 +159,37 @@ func (s *BaseSession) InitSessionVariable(ctx *Context, sysVarName string, value
 	return s.setSessVar(ctx, sysVar, value, true)
 }
 
-func (s *BaseSession) setSessVar(ctx *Context, sysVar SystemVariable, value interface{}, init bool) error {
+// InitSessionVariableDefault implements the Session interface and is used to initialize variables (Including read-only variables)
+func (s *BaseSession) InitSessionVariableDefault(ctx *Context, sysVarName string, value interface{}) error {
+	sysVar, _, ok := SystemVariables.GetGlobal(sysVarName)
+	if !ok {
+		return ErrUnknownSystemVariable.New(sysVarName)
+	}
+
+	sysVar.SetDefault(value)
+	svv, err := sysVar.InitValue(ctx, value, false)
+	if err != nil {
+		return err
+	}
+
+	sysVarName = strings.ToLower(sysVarName)
+	s.systemVars[sysVarName] = svv
+	if sysVarName == characterSetResultsSysVarName {
+		s.charset = CharacterSet_Unspecified
+	}
+	return nil
+}
+
+func (s *BaseSession) setSessVar(ctx *Context, sysVar SystemVariable, val interface{}, init bool) error {
 	var svv SystemVarValue
 	var err error
 	if init {
-		svv, err = sysVar.InitValue(ctx, value, false)
+		svv, err = sysVar.InitValue(ctx, val, false)
 		if err != nil {
 			return err
 		}
 	} else {
-		svv, err = sysVar.SetValue(ctx, value, false)
+		svv, err = sysVar.SetValue(ctx, val, false)
 		if err != nil {
 			return err
 		}
@@ -202,6 +223,22 @@ func (s *BaseSession) GetSessionVariable(ctx *Context, sysVarName string) (inter
 	return sysVar.Val, nil
 }
 
+// GetSessionVariableDefault implements the Session interface.
+func (s *BaseSession) GetSessionVariableDefault(ctx *Context, sysVarName string) (interface{}, error) {
+	sysVarName = strings.ToLower(sysVarName)
+	sysVar, ok := s.systemVars[sysVarName]
+	if !ok {
+		return nil, ErrUnknownSystemVariable.New(sysVarName)
+	}
+	// TODO: this is duplicated from within variables.globalSystemVariables, suggesting the need for an interface
+	if sysType, ok := sysVar.Var.GetType().(SetType); ok {
+		if sv, ok := sysVar.Var.GetDefault().(uint64); ok {
+			return sysType.BitsToString(sv)
+		}
+	}
+	return sysVar.Var.GetDefault(), nil
+}
+
 // GetUserVariable implements the Session interface.
 func (s *BaseSession) GetUserVariable(ctx *Context, varName string) (Type, interface{}, error) {
 	return s.userVars.GetUserVariable(ctx, varName)

sql/planbuilder/set.go

@@ -341,8 +341,9 @@ func (b *Builder) simplifySetExpr(name *ast.ColName, varScope ast.SetScope, val
 
 		switch varScope {
 		case ast.SetScope_None, ast.SetScope_Session, ast.SetScope_Global:
-			_, value, ok := sql.SystemVariables.GetGlobal(varName)
-			if ok {
+			// cannot use sql.SystemVariables.GetGlobal as the default value can be defined at session start runtime.
+			value, err := b.ctx.GetSessionVariableDefault(b.ctx, varName)
+			if err == nil {
 				return expression.NewLiteral(value, types.ApproximateTypeFromValue(value)), true
 			}
 			err = sql.ErrUnknownSystemVariable.New(varName)

sql/session.go

@@ -66,6 +66,8 @@ type Session interface {
 	Client() Client
 	// SetClient returns a new session with the given client.
 	SetClient(Client)
+	// InitSessionVariableDefault sets this session's default value of the system variable with the given name.
+	InitSessionVariableDefault(ctx *Context, sysVarName string, value interface{}) error
 	// SetSessionVariable sets the given system variable to the value given for this session.
 	SetSessionVariable(ctx *Context, sysVarName string, value interface{}) error
 	// InitSessionVariable sets the given system variable to the value given for this session and will allow for
@@ -76,6 +78,9 @@ type Session interface {
 	// GetSessionVariable returns this session's value of the system variable with the given name.
 	// To access global scope, use sql.SystemVariables.GetGlobal instead.
 	GetSessionVariable(ctx *Context, sysVarName string) (interface{}, error)
+	// GetSessionVariableDefault returns this session's default value of the system variable with the given name.
+	// To access global scope, use sql.SystemVariables.GetGlobal instead.
+	GetSessionVariableDefault(ctx *Context, sysVarName string) (interface{}, error)
 	// GetUserVariable returns this session's value of the user variable with the given name, along with its most
 	// appropriate type.
 	GetUserVariable(ctx *Context, varName string) (Type, interface{}, error)