sysvar tidb_read_staleness does not have a type

[morgo]

Enhancement

From code inspection:

tidb/sessionctx/variable/sysvar.go

Lines 307 to 309 in 1268bf0

	{Scope: ScopeSession, Name: TiDBReadStaleness, Value: "", Hidden: false, SetSession: func(s *SessionVars, val string) error {
	return setReadStaleness(s, val)
	}},

It looks like the intention is that the value specified should be an integer, but the default for a system variable is TypeStr. If we change it to TypeInt, then we can set a minimum/maximum value and validation will automatically be performed to make sure it actually is an int (with MySQL-compatible error codes automatically returned)

This will improve this code:

tidb/sessionctx/variable/varsutil.go

Lines 474 to 484 in 1268bf0

	if sVal == "" || sVal == "0" {
	s.ReadStaleness = 0
	return nil
	}
	if s.SnapshotTS != 0 {
	return fmt.Errorf("tidb_snapshot should be clear before setting tidb_read_staleness")
	}
	sValue, err := strconv.ParseInt(sVal, 10, 32)
	if err != nil {
	return err
	}

Since the error returned by ParseInt will not be reached.

[morgo]

cc @Yisaer