session: ignore linearizability setting for auto-commit transactions (#…

…22746)

Signed-off-by: Yilin Chen <[email protected]>

session/session.go

@@ -502,7 +502,13 @@ func (s *session) doCommit(ctx context.Context) error {
 	s.txn.SetOption(kv.Enable1PC, s.GetSessionVars().Enable1PC)
 	// priority of the sysvar is lower than `start transaction with causal consistency only`
 	if s.txn.GetOption(kv.GuaranteeLinearizability) == nil {
-		s.txn.SetOption(kv.GuaranteeLinearizability, s.GetSessionVars().GuaranteeLinearizability)
+		// We needn't ask the TiKV client to guarantee linearizability for auto-commit transactions
+		// because the property is naturally holds:
+		// We guarantee the commitTS of any transaction must not exceed the next timestamp from the TSO.
+		// An auto-commit transaction fetches its startTS from the TSO so its commitTS > its startTS > the commitTS
+		// of any previously committed transactions.
+		s.txn.SetOption(kv.GuaranteeLinearizability,
+			!s.GetSessionVars().IsAutocommit() && s.GetSessionVars().GuaranteeLinearizability)
 	}
 
 	return s.txn.Commit(tikvutil.SetSessionID(ctx, s.GetSessionVars().ConnectionID))

session/session_fail_test.go

@@ -103,3 +103,20 @@ func (s *testSessionSerialSuite) TestClusterTableSendError(c *C) {
 	c.Assert(tk.Se.GetSessionVars().StmtCtx.WarningCount(), Equals, uint16(1))
 	c.Assert(tk.Se.GetSessionVars().StmtCtx.GetWarnings()[0].Err, ErrorMatches, ".*TiDB server timeout, address is.*")
 }
+
+func (s *testSessionSerialSuite) TestAutoCommitNeedNotLinearizability(c *C) {
+	tk := testkit.NewTestKitWithInit(c, s.store)
+	tk.MustExec("drop table if exists t1;")
+	defer tk.MustExec("drop table if exists t1")
+	tk.MustExec(`create table t1 (c int)`)
+
+	c.Assert(failpoint.Enable("github.com/pingcap/tidb/store/tikv/getMinCommitTSFromTSO", `panic`), IsNil)
+	defer failpoint.Disable("github.com/pingcap/tidb/store/tikv/getMinCommitTSFromTSO")
+
+	tk.Se.GetSessionVars().SetSystemVar("tidb_enable_async_commit", "1")
+	tk.Se.GetSessionVars().SetSystemVar("tidb_guarantee_linearizability", "1")
+	tk.MustExec("INSERT INTO t1 VALUES (1)")
+
+	tk.Se.GetSessionVars().SetSystemVar("tidb_enable_1pc", "1")
+	tk.MustExec("INSERT INTO t1 VALUES (2)")
+}

store/tikv/2pc.go

@@ -1102,6 +1102,7 @@ func (c *twoPhaseCommitter) execute(ctx context.Context) (err error) {
 	// than the snapshot TS of all existent readers. So we get a new timestamp
 	// from PD as our MinCommitTS.
 	if commitTSMayBeCalculated && c.needLinearizability() {
+		failpoint.Inject("getMinCommitTSFromTSO", nil)
 		minCommitTS, err := c.store.oracle.GetTimestamp(ctx, &oracle.Option{TxnScope: oracle.GlobalTxnScope})
 		// If we fail to get a timestamp from PD, we just propagate the failure
 		// instead of falling back to the normal 2PC because a normal 2PC will