Re: BUG #4958: Stats collector hung on WaitForMultipleObjectsEx while attempting to recv a datagram

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> We just had a customer hit a very similar problem on 9.1.3, running on
> Windows Server 2008 SP2. ...
> The customer finds that they can reproduce this on a variety of
> systems under heavy load.

> Now, it looks to me like for this stack trace to happen,
> PgstatCollectorMain() has got to call pgwin32_waitforsinglesocket (at
> line 3002), and that function has to return true, so that got_data
> gets set to true. Then PgstatCollectorMain() will call recv(), which
> on Windows will really be pgwin32_recv, which will call
> pgwin32_waitforsinglesocket, which must now hang. The fact that the
> first pgwin32_waitforsinglesocket call returned true should mean that
> the stats collector socket is ready for read, while the fact that the
> second one did not return seems to imply that it's not ready for read,
> close, or accept. So it almost looks like Windows can change its mind
> about whether the socket is readable.

> Or maybe we're telling it to change its mind. This sounds an awful
> lot like something that could have been caused by the oversights fixed
> in commit b85427f2276d02756b558c0024949305ea65aca5. Was there a
> reason we didn't back-patch that?

Sure: it was unproven that that fixed anything at all, much less that it
was bug-free enough to be safe to backpatch. Neither of those things
has changed since May. If you want you can try making up a 9.1 with
those changes and giving it to this customer to see if it fixes their
problems --- but without some field testing of the sort, I'm pretty
hesitant to put it into back branches.

regards, tom lane