fix: access nullptr issue in LoadClusterNodes

[RiversJin]

The sequence LoadClusterNodes -> SetClusterNodes -> SetMasterSlaveRepl -> srv_->slot_migrator->SetStopMigrationFlag causes a null pointer access during initialization. The fix involves moving the initialization of slot_migrator earlier to resolve this issue.

PS: I'sorry that the logic where SetClusterNodes accesses slot_migrator was my modification, meaning I introduced this bug. :P

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
66.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[git-hulk]

@RiversJin, do you mean the null pointer may happen on this line? https://github.com/apache/kvrocks/blob/231b093c979ab80c11ac5433e52d43213dc94840/src/cluster/cluster.cc#L214C24-L214C37

[mapleFU]

By the way, anyway we should return error/emit log rather than coredump on access nullptr? Can we add some guard in visiting the pointer?

[RiversJin]

LoadClusterNodes -> SetClusterNodes -> SetMasterSlaveRepl -> srv_->slot_migrator->SetStopMigrationFlag

kvrocks/src/cluster/cluster.cc

Line 252 in 231b093

srv_->slot_migrator->SetStopMigrationFlag(false);

@git-hulk here

[git-hulk]

LoadClusterNodes -> SetClusterNodes -> SetMasterSlaveRepl -> srv_->slot_migrator->SetStopMigrationFlag

kvrocks/src/cluster/cluster.cc

Line 252 in 231b093

srv_->slot_migrator->SetStopMigrationFlag(false);

@git-hulk here

@RiversJin Seems it has been checked if the migrator is null before accessing? see: https://github.com/apache/kvrocks/blob/231b093c979ab80c11ac5433e52d43213dc94840/src/cluster/cluster.cc#L250C15-L250C28

[RiversJin]

LoadClusterNodes -> SetClusterNodes -> SetMasterSlaveRepl -> srv_->slot_migrator->SetStopMigrationFlag

kvrocks/src/cluster/cluster.cc

Line 252 in 231b093

srv_->slot_migrator->SetStopMigrationFlag(false);

@git-hulk here

Seems it has been checked if the migrator is null before accessing? see: https://github.com/apache/kvrocks/blob/231b093c979ab80c11ac5433e52d43213dc94840/src/cluster/cluster.cc#L250C15-L250C28

Oops, it seems I've mixed up the differences between the private code of my current company and the community code ..
However,according to the information above, there is another place that also relies on slot_migrator, so I'll first test locally to see if there is an issue with the current branch ...

[git-hulk]

@RiversJin Yes, but I think this PR could also be merged since it would be better to move it before loading the cluster node.