Re: [PG19] A canceled notify read leaves the NOTIFY queue stuck

I wrote:
> Playing around with this example, it avoids the queue-filling problem,
> but there's still an undesirable aspect: the N process has sent the L
> process a NOTIFY, but the L process does not see that, at least not
> till another NOTIFY is sent (and then it reports both at once).
> That's because the L process already cleared notifyInterruptPending,
> so it won't re-enter ProcessIncomingNotify. This is measurably worse
> than pre-v19: before, L would get the notify message when it next did
> something, even without another NOTIFY. (I'm not immediately sure
> why that changed.)

Oh, I must have fat-fingered the test case somehow, because when
I try this again, v18 behaves like HEAD. So never mind that bit.

I'm still interested in whether we can avoid blocking on a safe
snapshot, but that's clearly material for future development.
So we can close this open item for v19.

regards, tom lane