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

Dear Katsuragi-san,

Thank you for reviewing! PSA new version.

> 0001:
> Extending pqSocketPoll seems to be a better way because we can
> avoid having multiple similar functions. I also would like to hear
> horiguchi-san's opinion whether this matches his expectation.
> Improvements of pqSocketPoll/pqSocketCheck is discussed in this
> thread[1]. I'm concerned with the discussion.

I checked the thread and seems correct. I can post +1 to the thread.
And the modification will be automatically reflected to the feature
if we use the same function, I thought.

> As for the function's name, what do you think about keeping
> current name (pqSocketCheck)? pqSocketIsReadable... describes
> the functionality very well though.

No objection, I can keep the shorter name.

> pqConnCheck seems to be a family of pqReadReady or pqWriteRedy,
> so how about placing pqConnCheck below them?

Moved.

> + * Moreover, when neither forRead nor forWrite is requested and timeout
> is
> + * disabled, try to check the health of socket.
> Isn't it better to put the comment on how the arguments are
> interpreted before the description of return value?
>
> +#if defined(POLLRDHUP)
> + input_fd.events = POLLRDHUP | POLLHUP |
> POLLNVAL;
> ...
> + input_fd.events |= POLLERR;
> To my understanding, POLLHUP, POLLNVAL and POLLERR are ignored
> in event. Are they necessary?

I read man poll(3) again, and I found that these event is ignored when
it sets to the events attribute. So removed.

> 0002:
> As for the return value of postgres_fdw_verify_connection_states,
> what do you think about returning NULL when connection-checking
> is not performed? I think there are two cases 1) ConnectionHash
> is not initialized or 2) connection is not found for specified
> server name, That is, no entry passes the first if statement below
> (case 2)).
>
> ```
> if (all || entry->serverid == serverid)
> {
> if (PQconnCheck(entry->conn))
> {
> ```

I think in that case we can follow postgres_fdw_disconnect().
About postgres_fdw_disconnect(), if the given server_name does not exist,
an error is reported. This is a current behavior so I want to keep it.
Besides, I added the description to document.

> 0004:
> I'm wondering if we should add kqueue support in this version,
> because adding kqueue support introduces additional things to
> be considered. What do you think about focusing on the main
> functionality using poll in this patch and going for kqueue
> support after this patch?

I think it is better because it can keep patches smaller. So I stopped attaching 0004.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED