improve the backward compatibility of “SHOW SLAVE HOSTS“ command

[duanhuichao]

What did you do?

With the version upgrade, on the new version of MySQL Cluster, the query results of many system commands will display some new columns. These new columns show new features or information of MySQL.

In general, it is recommended to provide backward compatibility of system commands (e.g., show xxx) rather than hard-coded return value format (i.e., limiting the number of returned columns).

For DM, there are some commands that are difficult to backward-compatible:
(1) SHOW MASTER STATUS
(2) SHOW SLAVE HOSTS
(3) SHOW BINARY LOGS

These commands limit the return format by limiting the number of columns returned:

rows, err := db.QueryContext(ctx, `SHOW SLAVE HOSTS`)
rowColumns, err := rows.Columns()
if len(rowColumns) == 5 {
	err = rows.Scan(&serverID, &host, &port, &masterID, &slaveUUID)
} else {
	err = rows.Scan(&serverID, &host, &port, &masterID)
}

What did you expect to see?

Through the reflection mechanism, we only take the value of the required corresponding column and do not limit the number of returned columns.

import (
"reflect"
)

• Rows.Columns to get column names and positions.
• Passing a slice of pointers []interface{} to Rows.Scan to get the values.
• reflect.Value and Value.Addr to get a pointer to the destination value.
• Value.FieldByName to get the Value of a struct field if you want to map to struct fields.

see:
https://stackoverflow.com/questions/21986780/is-it-possible-to-retrieve-a-column-value-by-name-using-golang-database-sql

What did you see instead?

For the above system commands, when the number of columns returned by MySQL changes, DM cannot work.

Versions of the cluster

any versions

current status of DM cluster (execute query-status <task-name> in dmctl)

No response

[lance6716]

Hi, which version of MySQL addes new columns? And please address comment in your PR

[ryangao0513]

Hello,I encountered the same problem in txsql 8.0.22 patch 20210330

[lance6716]

happens again https://asktug.com/t/topic/813202

[lance6716]

/assign @lyzx2001