executor: 'select * from information_schema.tables' fail after setting @@tidb_snapshot (#18676)

[ti-srebot]

cherry-pick #18676 to release-3.1

---

What problem does this PR solve?

Issue Number: close #18347

Problem Summary:

The reproduce steps:

	create database gzj_test_db
	create table gzj_test_db.ad_account_record_log (id int auto_increment primary key)
	select now()
	drop database gzj_test_db
        set @@tidb_snapshot = '2020-07-18 23:56:05'   // now() 
	select * from information_schema.tables

'Table gzj_test_db.ad_account_record_log' doesn't exist

What is changed and how it works?

What's Changed:

getAutoIncrementID should consider the @@tidb_snapshot variable, use infoschema.GetInfoSchema(ctx)

How it Works:

func GetInfoSchemaBySessionVars(sessVar *variable.SessionVars) InfoSchema {
	var is InfoSchema
	if snap := sessVar.SnapshotInfoschema; snap != nil {
		is = snap.(InfoSchema)
		logutil.BgLogger().Info("use snapshot schema", zap.Uint64("conn", sessVar.ConnectionID), zap.Int64("schemaVersion", is.SchemaMetaVersion()))
	} else {
		is = sessVar.TxnCtx.InfoSchema.(InfoSchema)
	}
	return is
}

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Manual test (add detailed scripts or steps below)

It's not written as an unit test, because the test case depends on TiKV.

func (s *testInfoschemaTableSuite) TestWrongSnapshotSchema(c *C) {
	// Test case for https://github.com/pingcap/tidb/issues/18347
	// getAutoIncrementID does not use the snapshot infoschema and cause this bug.
	tk := testkit.NewTestKit(c, s.store)
	tk.MustExec("create database gzj_test_db")
	tk.MustExec("create table gzj_test_db.ad_account_record_log (id int auto_increment primary key)")
	rs := tk.MustQuery("select now()").Rows()
	tk.MustExec("drop database gzj_test_db")

	tk.MustExec("set @@tidb_snapshot = ?", rs[0][0])
	// Before the bug fix, this query get "'Table gzj_test_db.ad_account_record_log' doesn't exist"
	tk.MustQuery("select * from information_schema.tables;")
}

Release note

• Fix a bug `getAutoIncrementID()` function does not consider the `tidb_snapshot` session variable, this bug may cause dumper tool fail with 'table not exist' error.

[ti-srebot]

/run-all-tests

[tiancaiamao]

The bug does not exist in 3.x, so no need to cherry pick here.