### Summary
A race condition between session heartbeat processing and session closure can cause the server to panic with `send on closed channel`. The `heartbeat()` method uses a blocking channel send while holding a mutex, and under specific timing with concurrent `close()` calls, this can lead to either a deadlock (channel buffer full) or a panic (send on closed channel after TOCTOU gap in `KeepAlive`).
### Impact
A remote client can trigger a server crash by sending rapid `KeepAlive` requests while a session is expiring or being closed. This is a denial-of-service vulnerability that crashes the entire data server process.
All versions are affected.
### Details
In `oxiad/dataserver/controller/lead/session.go`, the `heartbeat()` method performs a blocking `s.heartbeatCh <- true` send. If the channel buffer is full (size 1), this blocks while holding the session mutex, preventing `close()` from acquiring the lock to close the channel — a deadlock.
Additionally, in `session_manager.go`, `KeepAlive()` releases the session manager's read lock before calling `heartbeat()`, creating a TOCTOU window where the session can be removed and closed between the lookup and the heartbeat call.
### Patches
Fixed by changing `heartbeat()` to use a non-blocking `select` with a `default` case, and by holding the session manager read lock through the entire `KeepAlive()` operation.
### Workarounds
No workaround available.