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

Hi Hayato Kuroda,

Thanks for the patch. I think there are some non-fdw extensions out there
which could benefit from this logic. That's why I want to first learn some
more about high-level design/goals of the patch a little more.

+/*
> + * Call callbacks for checking remote servers.
> + */
> +void
> +CallCheckingRemoteServersCallbacks(void)

Why do we run this periodically, but not when a specific connection is
going to be used? Wouldn't running it periodically prevent detecting some
already-closed sockets at the time of the connection used (e.g., we checked
10 seconds ago, the server died 5 seconds ago)?

In other words, what is the trade-off for calling
pgfdw_connection_check_internal() inside GetConnection() when we are about
to use a "cached" connection? I think that might simplify the patch as well.

+/*
> + * Helper function for pgfdw_connection_check
> + */
> +static bool
> +pgfdw_connection_check_internal(PGconn *conn)
> +{

Can we have this function/logic on Postgres core, so that other extensions
can also use?

+ AddWaitEventToSet(eventset, WL_SOCKET_CLOSED, PQsocket(conn), NULL,
> NULL);

What if PQsocket(conn) returns -1? Maybe we move certain connection state
checks into pgfdw_connection_check_internal() such that it is more generic?
I can think of checks like: conn!=NULL, PQsocket(conn) != PGINVALID_SOCKET,
PQstatus == CONNECTION_OK

+ eventset = CreateWaitEventSet(CurrentMemoryContext, 1);
> + AddWaitEventToSet(eventset, WL_SOCKET_CLOSED, PQsocket(conn), NULL,
> NULL);
> +
> + WaitEventSetWait(eventset, 0, &events, 1, 0);
> +
> + if (events.events & WL_SOCKET_CLOSED)
> + {
> + FreeWaitEventSet(eventset);
> + return false;
> + }
> + FreeWaitEventSet(eventset);

Do you see any performance implications of creating/freeing waitEventSets
per check? I wonder if we can somehow re-use the same waitEventSet by
modifyWaitEvent? I guess no, but still, if this check causes a performance
implication, can we somehow cache 1 waitEventSet per connection?

Thanks,
Onder KALACI
Developing the Citus extension @Microsoft