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

shihao zhong <zhong950419(at)gmail(dot)com> writes:
>> Do we really need a serializable snapshot for reading the notify
>> queue? If we do, can we acquire it before making any important state
>> changes?

> I did some investigation today and I don't think we need a serializable
> snapshot.

I looked at this, and I agree: all we care about in async.c is which
transactions are still running, and there's no particular need to wait
for a safe snapshot before deciding that. (A different way to say
that, perhaps, is that NOTIFY messages are delivered on the same basis
as READ COMMITTED operations, even if your default transaction mode
is higher.)

> So we just need to call GetSnapshotData(), but that needs a snapshot.
> I created a static one in async.c because feels like we should not
> touch snapmgr for encapsulation. If you'd rather keep this in
> snapmgr.c, a small wrapper there would work too.

I don't like that choice at all. The only callers of
GetSnapshotData() are in snapmgr.c and predicate.c, and there are
enough weird optimization-driven assumptions around snapshot
management that I think any new caller had better be in one of those
modules so that people working in that area will notice it. I wrote
a wrapper that documents two of the assumptions I think are involved
here, and there could well be more. See what you think.

regards, tom lane