Re: pg_stat_replication when standby is unreachable

I looked a bit more into the code and it appears to me that the following
are true:

- A separate wal sender process is created on the primary side for each
connected standby.
- The wal sender process terminates (walsender.c / WalSndLoop) when there
is an error to write to the standby's socket.
- If the standby machine is reachable but postgres is not running there any
more, then the wal sender terminates almost immediately, probably because
the standby machine sends a TCP RST to the wal sender.
- If the standby machine is unreachable, then the wal sender will keep
trying to send wal data. However, since the wal sender uses a non-blocking
socket to talk to the standby, it will timeout and exit after
"replication_timeout" (configured in postgresql.conf).

So it seems like the wal sender should exit within replication_timeout or
sooner, and this will be reflected using an update to pg_stat_replication.
Therefore, I could just wait for up to replication_timeout before declaring
the standby as dead.

Thanks,
Abhishek