RE: Fix slot synchronization with two_phase decoding enabled

On Mon, Apr 28, 2025 at 7:33 PM Zhijie Hou (Fujitsu) wrote:
>
> Thanks for reviewing. Here is V3 patch that addressed it.
>
> BTW, I also did some tests to confirm the catalog_xmin could still be
> ahead in some case, and here is an example:
>
> 1. Create a failover replication slot named 'logicalslot' on primary
> and acquire it in the walsender.
>
> 2. Log two standby snapshots on primary. Before logging, call
> txid_current() To assign a xid, so that each standby snapshot will
> hold a new xid in its oldestrunningXid field:
> - txid_current();
> - `0/3000420` - `running_xacts` (no running transactions,
> oldestrunningXid = 755)
> - txid_current();
> - `0/3000488` - `running_xacts` (no running transactions,
> oldestrunningXid = 756)
>
> 3. The walsender sets `0/3000420` as the `candidate_restart_lsn`, 755 as
> `candidate_catalog_xmin`, skipping the second `running_xacts` because
> `candidate_restart_lsn`/`candidate_catalog_xmin` is already valid.
>
> 4. After receiving a reply from the apply worker, the walsender assigns
> `0/3000420` to `restart_lsn`, `755` to `catalog_xmin`. At this point, the
> replication slot 'logicalslot' has `restart_lsn` set to `0/3000420`,
> `catalog_xmin` set to `755`.
>
> 5. On the standby, execute `pg_sync_replication_slots()` to synchronize
> 'logicalslot'.
>
> 6. During synchronization, with the initial `restart_lsn` at `0/3000420`, the
> sync slot reaches a consistent point at this position. As a result, it does
> not update `candidate_restart_lsn` and `candidate_catalog_xmin` at
> consistent point (refer to `SnapBuildProcessRunningXacts()`).
>
> 7. The sync process identifies the second standby snapshot at
> `0/3000488` and
> uses its LSN as `candidate_restart_lsn`, and use the
> oldestrunningXid `756`
> as `candidate_catalog_xmin`, eventually updating it to `restart_lsn` and
> `catalog_xmin`.
>
> 8. Now, the synced slot holds `restart_lsn` at `0/3000488`, `catalog_xmin` at
> `756`, which are all ahead of the remote slot on the primary server.
>
> Attaching a script to reproduce the same.
>
> Note that, to reproduce this stably, we'd better modify the value of
> LOG_SNAPSHOT_INTERVAL_MS in bgwriter.c to a bigger number to avoid
> unexpected xl_running_xacts logging.

In addition to above steps, for those interested in reproducing the specific
scenario where two_phase_at advances past the synced confirmed_flush, I'm
attaching a new script. This script can reproduce the issue after applying the
injection points I provided.

Best Regards,
Hou zj