Bug #97885 Session state tracking does not take MDL locks into account
Submitted: 4 Dec 2019 22:30 Modified: 5 Dec 2019 11:00
Reporter: Manuel Ung Email Updates:
Status: Verified Impact on me:
None 
Category:MySQL Server: Locking Severity:S3 (Non-critical)
Version:8.0.18, 5.7.28 OS:Any
Assigned to: CPU Architecture:Any

[4 Dec 2019 22:30] Manuel Ung
Description:
As stated in the documentation (https://dev.mysql.com/doc/refman/8.0/en/session-state-tracking.html), session tracking is meant to track session state changes so that we know if a session can be transferred to a different replica.

However, if the user runs LOCK TABLES or GET_LOCK (both now acquire MDL locks), we are not indicating to the client that session state has changed. In this case, it would not be safe to resume the connection on a separate server.

How to repeat:
In mtr:

create table t (i int);
--enable_session_track_info

SET @@session.session_track_state_change=ON;
--echo # expect 1
SELECT GET_LOCK("session_state_change_user_lock", 3);

--reset_connection

SET @@session.session_track_state_change=ON;
--echo # expect 1
LOCK TABLES test.t READ;

--reset_connection

--disable_session_track_info
drop table t;

Suggested fix:
Acquiring an MDL lock should be considered a session state change.
[5 Dec 2019 11:00] Umesh Shastry
Hello Manuel Ung,

Thank you for the bug report and feedback.

regards,
Umesh