Log a warning in pg_createsubscriber for max_slot_wal_keep_size

Hi,

Currently, there is a risk that pg_createsubscriber may fail to
complete successfully when the max_slot_wal_keep_size value is set too
low. This can occur if the WAL is removed before the standby using the
replication slot is able to complete replication, as the required WAL
files are no longer available.

I was able to reproduce this issue using the following steps:
Set up a streaming replication environment.
Run pg_createsubscriber in a debugger.
Pause pg_createsubscriber at the setup_recovery stage.
Perform several operations on the primary node to generate a large
volume of WAL, causing older WAL segments to be removed due to the low
max_slot_wal_keep_size setting.
Once the necessary WAL segments are deleted, continue the execution of
pg_createsubscriber.
At this point, pg_createsubscriber fails with the following error:
2024-12-29 01:21:37.590 IST [427353] FATAL: could not receive data
from WAL stream: ERROR: requested WAL segment
000000010000000000000003 has already been removed
2024-12-29 01:21:37.592 IST [427345] LOG: waiting for WAL to become
available at 0/3000110
2024-12-29 01:21:42.593 IST [427358] LOG: started streaming WAL from
primary at 0/3000000 on timeline 1
2024-12-29 01:21:42.593 IST [427358] FATAL: could not receive data
from WAL stream: ERROR: requested WAL segment
000000010000000000000003 has already been removed

This issue was previously reported in [1], with a suggestion to raise
a warning in [2]. I’ve implemented a patch that logs a warning in
dry-run mode. This will give users the opportunity to adjust the
max_slot_wal_keep_size value before running the command.

Thoughts?

[1] - https://www.postgresql.org/message-id/TY3PR01MB9889FEDBBF74A9F79CA7CC87F57A2%40TY3PR01MB9889.jpnprd01.prod.outlook.com
[2] - https://www.postgresql.org/message-id/be92c57b-82e1-4920-ac31-a8a04206db7b%40app.fastmail.com

Thanks and regards,
Shubham Khanna.