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- A transitions through PREPARING, which truncates its log to the last mirrored offset. This removes any local data A may have accumulated after B originally stopped mirroring from it, realigning A's log with B's offsets.
- B's local producers (created after failover) have positive PIDs. A maps them to negative space using the mapping rule.
- When the barrier record is replayed on A, it does not trigger PSM expiration of any negative PIDs, because this only happens when transitioning to STOPPED state.
- The stop cycle can repeat safely in either direction because each failover produces a clean PSM state via the barrier, and each new mirroring session starts from a truncated, offset-aligned log.