Function open_exclusive 

pub fn open_exclusive(
    workspace: &Path,
    session_id: &str,
    max_events: usize,
) -> Result<SessionEventLog, SessionStoreError>

Open (creating if necessary) the event log as the session’s single writer.

Fails with SessionStoreError::SessionBusy when another process already holds the session open instead of interleaving two writers on one event stream. Reopens in this process share state and succeed. Readers that must tolerate a live writer keep using open.