RE: [Proposal] Add foreign-server health checks infrastructure

Dear Fujii-san,

Thanks for checking!

> These failed to be applied to the master branch cleanly. Could you update them?

PSA rebased patches. I reviewed my myself and they contain changes.
E.g., move GUC-related code to option.c.

> + this option relies on kernel events exposed by Linux, macOS,
>
> s/this/This

Fixed.

>
> + GUC_check_errdetail("pgfdw_health_check_interval must be set
> to 0 on this platform");
>
> The actual parameter name "postgres_fdw.health_check_interval"
> should be used for the message instead of internal variable name.

Fixed.

> This registered signal handler does lots of things. But that's not acceptable
> and they should be performed outside signal handler. No?

I modified like v09 or earlier versions, which has a mechanism for registering CheckingRemoteServersCallback.
It had been removed because we want to keep core simpler, but IIUC it is needed
if the signal handler just sets some flags.
The core-side does not consider the current status of transaction and running query for simpleness.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED