RE: speed up a logical replica setup

Dear Euler,

> 03.
> ```
> /*
> * Is the standby server ready for logical replication?
> */
> static bool
> check_subscriber(LogicalRepInfo *dbinfo)
> ```
>
> You said "target server must be a standby" in [1], but I cannot find checks for it.
> IIUC, there are two approaches:
> a) check the existence "standby.signal" in the data directory
> b) call an SQL function "pg_is_in_recovery"

I found that current code (HEAD+v14-0001) leads stuck or timeout because the recovery
would be never finished. In attached script emulates the case standby.signal file is missing.
This script always fails with below output:

```
...
pg_createsubscriber: waiting the postmaster to reach the consistent state
pg_createsubscriber: postmaster was stopped
pg_createsubscriber: error: recovery timed out
...
```

I also attached the server log during pg_createsubscriber, and we can see that
walreceiver exits before receiving all the required changes. I cannot find a path
yet, but the inconsistency lead the situation which walreceiver exists but startup
remains. This also said that recovery status must be checked in check_subscriber().

Best Regards,
Hayato Kuroda
FUJITSU LIMITED
https://www.fujitsu.com/