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