v7 patches attached. No code changes from v6, just rebased on current master to remove minor offset, and the backpatch file is renamed with a "nocfbot-" prefix so the commitfest bot picks up only the master patch.
Xuneng correctly pointed out that RequestXLogStreaming rounds down, not up, so it isn't the cause of the gap. The actual mechanism is that archive recovery processes whole segment files: after both nodes replay the same archived segment N, the cascade's next read position lands at the start of segment N+1, while the upstream's GetStandbyFlushRecPtr returns replayPtr inside segment N.
Changes from v5:
- Updated the code comment and commit message to describe the correct root cause (archive recovery segment granularity, not RequestXLogStreaming truncation).
- Reset the catchup state when the upstream is no longer behind. Without this, if the walreceiver successfully streams, the connection breaks, and it loops back to find itself ahead again, the stale deadline from the previous wait would cause an immediate timeout.
Two patches attached: v6-0001 for master (extends the walrcv_identify_system API) and v6-backpatch-0001 for stable branches (global variable to preserve ABI).
