MySQL Bugs: #86874: Group Replication variable updates are not safe against stop

Bug #86874	Group Replication variable updates are not safe against stop
Submitted:	29 Jun 2017 16:44	Modified:	9 Jan 2018 17:03
Reporter:	Pedro Gomes	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: Group Replication	Severity:	S3 (Non-critical)
Version:	5.7.19	OS:	Any
Assigned to:		CPU Architecture:	Any

Description:
On plugin.cc, lets take a look at 
update_recovery_completion_policy 

notice how it does 

  if (recovery_module != NULL)
  {
    recovery_module->
        set_recovery_completion_policy(

now look at the stack that leads to 
terminate_recovery_module()

It can be the case that if someone stops the plugin when someone changes a plugin variable 

  if (recovery_module != NULL)
  {

<< terminate_recovery_module() is invoked 

    recovery_module->
        set_recovery_completion_policy(

<< segmentation fault 

This may affect all variables that use this formula on update/check 

How to repeat:
No test was produced but in theory such problem is visible if you continuously stop the plugin while changing recovery variables 

Suggested fix:
Protect accesses somehow.

Maybe use Mutex_autolock. 

Question is, what if manage to execute a variable update when the locks was deleted?

Is there a safe a solution?

Posted by developer:
 
Thank you for your feedback, this has been fixed in upcoming versions and the following was added to the 8.0.4 changelog:
Changes to Group Replication variables while starting or stopping the plugin were not being correctly validated. Now, variables can only be changed if the plugin is not changing state.

Now published.