tidb returns 'Unknown auth plugin' when some old clients didn't set auth plugin in login request

[time-and-fate]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Use a mysql 5.1 client to connect tidb 5.2

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

Connect success

3. What did you see instead (Required)

TiDB returns "1105 Unknown auth plugin" error.

4. What is your TiDB version? (Required)

v5.2.0

[time-and-fate]

/type compatibility

[time-and-fate]

After capturing network traffic, we found that some old clients won't set auth plugin in the login request.
In this case, TiDB will return the "1105 Unknown auth plugin" error but MySQL won't.
This is introduced in #24991.

[dveeden]

Reproducing this:
TiDB v5.2.0
MySQL Client v5.1.73

Setup:

CREATE USER 'u1'@'%' IDENTIFIED WITH mysql_native_password BY 'abc';
CREATE USER 'u2'@'%' IDENTIFIED WITH caching_sha2_password BY 'abc';

Test:

/home/dvaneeden/opt/mysql/5.1.73/bin/mysql -u u1 -h 127.0.0.1 -P 4000 -pabc -e QUIT
# OK

/home/dvaneeden/opt/mysql/5.1.73/bin/mysql -u u2 -h 127.0.0.1 -P 4000 -pabc -e QUIT
# OK

/home/dvaneeden/opt/mysql/5.1.73/bin/mysql -u u3 -h 127.0.0.1 -P 4000 -pabc -e QUIT
# ERROR 1105 (HY000): Unknown auth plugin

For the tests that didn't give an error this showed up in the logs, and using the connection didn't work:

[2021/09/09 08:26:25.384 +02:00] [WARN] [conn.go:234] ["authSwitchRequest response fail"] [conn=37] [error="[server:8052]invalid sequence 0 != 3"]
[2021/09/09 08:26:25.384 +02:00] [WARN] [conn.go:684] ["failed to check the user authplugin"] [conn=37] [error="[server:8052]invalid sequence 0 != 3"]
[2021/09/09 08:26:28.370 +02:00] [WARN] [conn.go:234] ["authSwitchRequest response fail"] [conn=39] [error="[server:8052]invalid sequence 0 != 3"]
[2021/09/09 08:26:28.370 +02:00] [WARN] [conn.go:684] ["failed to check the user authplugin"] [conn=39] [error="[server:8052]invalid sequence 0 != 3"]
[2021/09/09 08:26:31.248 +02:00] [WARN] [conn.go:234] ["authSwitchRequest response fail"] [conn=41] [error="[server:8052]invalid sequence 0 != 3"]
[2021/09/09 08:26:31.248 +02:00] [WARN] [conn.go:684] ["failed to check the user authplugin"] [conn=41] [error="[server:8052]invalid sequence 0 != 3"]

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.