RE: logical apply worker's lock waits in subscriber can stall checkpointer in publisher

Dear Fujii-san,

> Yeah, but I'd like to try the first option. Attached is a very WIP patch that
> attempts to implement it.
> 
> With this patch, when a walsender exits with >= FATAL,
> send_message_to_frontend() attempts to send the error message to the standby
> in non-blocking mode. If that fails, the walsender gives up on sending
> the message and exits immediately.

I'm still unclear it is OK to modify the fundamental code, but confirmed your
patch can solve the issue.

One concern for me is that the WALs might be more likely to be missed for
streaming replication case. What if the case walreceiver is bit busy thus send
buffer becomes full for a while?
Are there no issues because switchover after the walsender exits with FATAL is
not recommended?

Best regards,
Hayato Kuroda
FUJITSU LIMITED