server: session variables was not changed when execute the mysql command 'ComChangeUser'

[knull-cn]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

• set some session variable
• execute 'ComChangeUser'
• show the session variable , the values was the previous session setted(not the default values)

2. What did you expect to see? (Required)

show session variable should the default values.

3. What did you see instead (Required)

the values was the previous session setted

4. What is your TiDB version? (Required)

v5.3.0

[knull-cn]

problem : https://asktug.com/t/topic/662908
for java example code

import java.sql.*;

import com.mysql.cj.jdbc.JdbcConnection;

public class Client {
    public static void main(String args[]) {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            JdbcConnection con = (JdbcConnection) DriverManager.getConnection("jdbc:mysql://localhost:4000/test", "root", "tidb");

            ResultSet rs = con.createStatement().executeQuery("select @a;");
            if (rs.next()) {
                String i = rs.getString(1);
                System.out.println("before set : a= " + i);
            }
            con.createStatement().execute("set @a = 1;");
            rs.close();
            rs = con.createStatement().executeQuery("select @a;");
            if (rs.next()) {
                String i = rs.getString(1);
                System.out.println("after set : a= " + i);
            }
           // switch anther user.
            con.changeUser("tidb", "tidb");

            rs = con.createStatement().executeQuery("select @a;");
            if (rs.next()) {
                String i = rs.getString(1);
                System.out.println("after changeUser : a= " + i);
            }

            con.close();
        } catch (Exception e) {
            System.out.println(e);
        }
    }
}

[knull-cn]

After #24991, TiDB create new ctx object in clientConn::openSessionAndDoAuth https://github.com/pingcap/tidb/blob/v5.2.0/server/conn.go#L768-L776 .

but in handleChangeUser, ctx only call close but not set nil , so can't create new ctx, and use old session variables , see here : https://github.com/pingcap/tidb/blob/v5.2.0/server/conn.go#L2170-L2178

this was import with v5.2.0 and affected all version after v5.2.0

[bb7133]

@knull-cn Thanks! Since you've found the root cause, are you interested in closing it by a PR?

[knull-cn]

@knull-cn Thanks! Since you've found the root cause, are you interested in closing it by a PR?

yes , It is my pleasure.

[tiancaiamao]

!!- Note for cherry-picker -!!: the fix PR for this one would introduce other bug #33892
So they must be handled together!