Persistent wait event sets and socket changes

Hi all

I've been looking into the wait event set interface added in 9.6 with an
eye to using it in an extension that maintains a set of non-blocking libpq
connections to other PostgreSQL instances.

In the process I've been surprised to find that there does not appear to be
any interface to remove a socket once added to the wait event set, or
replace it with a new socket.

ModifyWaitEvent(...) doesn't take a pgsocket. There doesn't seem to be a
way to remove an event from the set either.

Discussion in
https://www.postgresql.org/message-id/flat/20160114143931.GG10941%40awork2.anarazel.de
talks about a proposed SetSocketToWaitOn(...)/AddSocketToWaitSet and
RemoveSocketFromWaitSet etc. But it looks like it petered out and went
nowhere, apparently mainly due to not being needed by any current core
users.

See:

*
https://www.postgresql.org/message-id/CA%2BTgmoZ_2EgBCXinHsJn%3Dv8bfg3Y3uXpG1xHiQ8g7yRJzwt3Yg%40mail.gmail.com

*
https://www.postgresql.org/message-id/CAEepm%3D14htYKhJ67OEvX%3DXmtiB5677K6ehQACH1ySkebeYh8Nw%40mail.gmail.com

I'd like to add such interfaces at some point, but for now can work around
it by destroying and re-creating the wait event set when the fd-set
changes. So I'm posting mostly to confirm that it's not supposed to work,
and ask if anyone thinks I should submit a comment patch to latch.c
documenting it.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services