Re: Patch proposal: New hooks in the connection path

Hi,

On 7/7/22 10:10 PM, Tom Lane wrote:
> Joe Conway <mail(at)joeconway(dot)com> writes:
>> It isn't clear to me if having a hook in the timeout handler is a
>> nonstarter -- perhaps a comment with suitable warning for prospective
>> extension authors is enough? Anyone else want to weigh in on this issue
>> specifically?
> It doesn't seem like a great place for a hook, because the list of stuff
> you could safely do there would be mighty short, possibly the empty set.
> Write to shared memory? Not too safe. Write to a file? Even less.
> Write to local memory? Pointless, because we're about to _exit(1).
> Pretty much anything I can think of that you'd want to do is something
> we've already decided the core code can't safely do, and putting it
> in a hook won't make it safer.
>
> If someone wants to argue for this hook, I'd like to see a credible
> example of a *safe* use-case, keeping in mind the points raised in
> the comments in BackendInitialize and process_startup_packet_die.

The use case would be to increment a counter in shared memory (or most
probably within an hash table) to reflect the number of times a startup
packet timeout occurred.

Reading the comments in/related to BackendInitialize() I understand
that's definitely not safe to write in shared memory for the
EXEC_BACKEND case, but wouldn't it be safe for the non EXEC_BACKEND case?

BTW, it makes me realize that the hook being fired in the bad startup
packet case:

        /*
         * Stop here if it was bad or a cancel packet. ProcessStartupPacket
         * already did any appropriate error reporting.
         */
        if (status != STATUS_OK)
+       {
+               if (FailedConnection_hook)
+                       (*FailedConnection_hook)
(FCET_BAD_STARTUP_PACKET, port);
                proc_exit(0);
+       }

is not safe for the EXEC_BACKEND case.

Regards,

--
Bertrand Drouvot
Amazon Web Services: https://aws.amazon.com