A transient, in-memory backfill chunk awaiting the advance gate.
table is the {schema, table} pair; seq the chunk sequence number; g the chunk's
EXACT GTID position string captured under the brief per-chunk LOCK TABLES … READ (a
provable lower bound on the chunk's read view); rows the materialized chunk rows (a
bounded list — at most one chunk is buffered at a time); max_pk the canonical PK of the
last row (the cursor advances to it when the chunk is emitted).
Rule 1
rows and max_pk are row VALUES (user data). The @derive {Inspect, only: [:table, :seq, :g]} renders only the structural identity and elides them, so an incidental inspect
of a buffered chunk can never surface a row value.