session: fix auto commit variable change effect on transaction commit (…

…#36631)

close #36581

executor/executor_txn_test.go

@@ -297,6 +297,7 @@ func TestTxnSavepoint0(t *testing.T) {
 		{"set autocommit=1", nil, ""},
 		{"savepoint s1", nil, ""},
 
+		{"set autocommit=0", nil, ""},
 		{"begin", nil, ""},
 		{"savepoint s1", []string{"s1"}, ""},
 		{"set autocommit=1", nil, ""},

session/session_test/session_test.go

@@ -3646,7 +3646,7 @@ func TestAutocommit(t *testing.T) {
 
 	// When autocommit is 0, transaction start ts should be the first *valid*
 	// statement, rather than *any* statement.
-	tk.MustExec("create table t (id int)")
+	tk.MustExec("create table t (id int key)")
 	tk.MustExec("set @@autocommit = 0")
 	tk.MustExec("rollback")
 	tk.MustExec("set @@autocommit = 0")
@@ -3656,15 +3656,49 @@ func TestAutocommit(t *testing.T) {
 	tk1.MustExec("insert into t select 1")
 
 	tk.MustQuery("select * from t").Check(testkit.Rows("1"))
+	tk.MustExec("delete from t")
+
+	// When the transaction is rolled back, the global set statement would succeed.
+	tk.MustExec("set @@global.autocommit = 0")
+	tk.MustExec("begin")
+	tk.MustExec("insert into t values (1)")
+	tk.MustExec("set @@global.autocommit = 1")
+	tk.MustExec("rollback")
+	tk.MustQuery("select count(*) from t where id = 1").Check(testkit.Rows("0"))
+	tk.MustQuery("select @@global.autocommit").Check(testkit.Rows("1"))
 
-	// TODO: MySQL compatibility for setting global variable.
-	// tk.MustExec("begin")
-	// tk.MustExec("insert into t values (42)")
-	// tk.MustExec("set @@global.autocommit = 1")
-	// tk.MustExec("rollback")
-	// tk.MustQuery("select count(*) from t where id = 42").Check(testkit.Rows("0"))
-	// Even the transaction is rollbacked, the set statement succeed.
-	// tk.MustQuery("select @@global.autocommit").Rows("1")
+	// When the transaction is committed because of switching mode, the session set statement shold succeed.
+	tk.MustExec("set autocommit = 0")
+	tk.MustExec("begin")
+	tk.MustExec("insert into t values (1)")
+	tk.MustExec("set autocommit = 1")
+	tk.MustExec("rollback")
+	tk.MustQuery("select count(*) from t where id = 1").Check(testkit.Rows("1"))
+	tk.MustQuery("select @@autocommit").Check(testkit.Rows("1"))
+
+	tk.MustExec("set autocommit = 0")
+	tk.MustExec("insert into t values (2)")
+	tk.MustExec("set autocommit = 1")
+	tk.MustExec("rollback")
+	tk.MustQuery("select count(*) from t where id = 2").Check(testkit.Rows("1"))
+	tk.MustQuery("select @@autocommit").Check(testkit.Rows("1"))
+
+	// Set should not take effect if the mode is not changed.
+	tk.MustExec("set autocommit = 0")
+	tk.MustExec("begin")
+	tk.MustExec("insert into t values (3)")
+	tk.MustExec("set autocommit = 0")
+	tk.MustExec("rollback")
+	tk.MustQuery("select count(*) from t where id = 3").Check(testkit.Rows("0"))
+	tk.MustQuery("select @@autocommit").Check(testkit.Rows("0"))
+
+	tk.MustExec("set autocommit = 1")
+	tk.MustExec("begin")
+	tk.MustExec("insert into t values (4)")
+	tk.MustExec("set autocommit = 1")
+	tk.MustExec("rollback")
+	tk.MustQuery("select count(*) from t where id = 4").Check(testkit.Rows("0"))
+	tk.MustQuery("select @@autocommit").Check(testkit.Rows("1"))
 }
 
 // TestTxnLazyInitialize tests that when autocommit = 0, not all statement starts

sessionctx/variable/sysvar.go

@@ -953,10 +953,11 @@ var defaultSysVars = []*SysVar{
 	}},
 	{Scope: ScopeGlobal | ScopeSession, Name: AutoCommit, Value: On, Type: TypeBool, SetSession: func(s *SessionVars, val string) error {
 		isAutocommit := TiDBOptOn(val)
-		s.SetStatusFlag(mysql.ServerStatusAutocommit, isAutocommit)
-		if isAutocommit {
+		// Implicitly commit the possible ongoing transaction if mode is changed from off to on.
+		if !s.IsAutocommit() && isAutocommit {
 			s.SetInTxn(false)
 		}
+		s.SetStatusFlag(mysql.ServerStatusAutocommit, isAutocommit)
 		return nil
 	}},
 	{Scope: ScopeGlobal | ScopeSession, Name: CharsetDatabase, Value: mysql.DefaultCharset, skipInit: true, Validation: func(vars *SessionVars, normalizedValue string, originalValue string, scope ScopeFlag) (string, error) {