* Prevent split-brain: Persistent ClusterID and Join Validation
- Persist ClusterId in Raft store to survive restarts.
- Validate ClusterId on Raft command application (piggybacked on MaxVolumeId).
- Prevent masters with conflicting ClusterIds from joining/operating together.
- Update Telemetry to report the persistent ClusterId.
* Refine ClusterID validation based on feedback
- Improved error message in cluster_commands.go.
- Added ClusterId mismatch check in RaftServer.Recovery.
* Handle Raft errors and support Hashicorp Raft for ClusterId
- Check for errors when persisting ClusterId in legacy Raft.
- Implement ClusterId generation and persistence for Hashicorp Raft leader changes.
- Ensure consistent error logging.
* Refactor ClusterId validation
- Centralize ClusterId mismatch check in Topology.SetClusterId.
- Simplify MaxVolumeIdCommand.Apply and RaftServer.Recovery to rely on SetClusterId.
* Fix goroutine leak and add timeout
- Handle channel closure in Hashicorp Raft leader listener.
- Add timeout to Raft Apply call to prevent blocking.
* Fix deadlock in legacy Raft listener
- Wrap ClusterId generation/persistence in a goroutine to avoid blocking the Raft event loop (deadlock).
* Rename ClusterId to SystemId
- Renamed ClusterId to SystemId across the codebase (protobuf, topology, server, telemetry).
- Regenerated telemetry.pb.go with new field.
* Rename SystemId to TopologyId
- Rename to SystemId was intermediate step.
- Final name is TopologyId for the persistent cluster identifier.
- Updated protobuf, topology, raft server, master server, and telemetry.
* Optimize Hashicorp Raft listener
- Integrated TopologyId generation into existing monitorLeaderLoop.
- Removed extra goroutine in master_server.go.
* Fix optimistic TopologyId update
- Removed premature local state update of TopologyId in master_server.go and raft_hashicorp.go.
- State is now solely updated via the Raft state machine Apply/Restore methods after consensus.
* Add explicit log for recovered TopologyId
- Added glog.V(0) info log in RaftServer.Recovery to print the recovered TopologyId on startup.
* Add Raft barrier to prevent TopologyId race condition
- Implement ensureTopologyId helper method
- Send no-op MaxVolumeIdCommand to sync Raft log before checking TopologyId
- Ensures persisted TopologyId is recovered before generating new one
- Prevents race where generation happens during log replay
* Serialize TopologyId generation with mutex
- Add topologyIdGenLock mutex to MasterServer struct
- Wrap ensureTopologyId method with lock to prevent concurrent generation
- Fixes race where event listener and manual leadership check both generate IDs
- Second caller waits for first to complete and sees the generated ID
* Add TopologyId recovery logging to Apply method
- Change log level from V(1) to V(0) for visibility
- Log 'Recovered TopologyId' when applying from Raft log
- Ensures recovery is visible whether from snapshot or log replay
- Matches Recovery() method logging for consistency
* Fix Raft barrier timing issue
- Add 100ms delay after barrier command to ensure log application completes
- Add debug logging to track barrier execution and TopologyId state
- Return early if barrier command fails
- Prevents TopologyId generation before old logs are fully applied
* ensure leader
* address comments
* address comments
* redundant
* clean up
* double check
* refactoring
* comment
`topology.Leader()` was using a backoff that typically
resulted in at least a 5s delay when initially starting
a master and raft server. This changes the backoff
algorithm to use exponential backoff starting with 100ms
and waiting up to 20s for leader selection.
Related to #4307
1, there will be two leader when master server startup in a few seconds
2, raft server will get a leader even there is only one master, so there is no need to do hard code to set the server to be leader
