Re: GiST VACUUM

On Thu, Jun 27, 2019 at 11:38 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> * I moved the logic to extend a 32-bit XID to 64-bits to a new helper
> function in varsup.c.

I'm a bit uneasy about making this into a generally available function
for reuse.  I think we should instead come up with a very small number
of sources of fxids that known to be free of races because of some
well explained interlocking.

For example, I believe it is safe to convert an xid obtained from a
WAL record during recovery, because (for now) recovery is a single
thread of execution and the next fxid can't advance underneath you.
So I think XLogRecGetFullXid(XLogReaderState *record)[1] as I'm about
to propose in another thread (though I've forgotten who wrote it,
maybe Andres, Amit or me, but if it wasn't me then it's exactly what I
would have written) is a safe blessed source of fxids.  The rationale
for writing this function instead of an earlier code that looked much
like your proposed helper function, during EDB-internal review of
zheap stuff, was this: if we provide a general purpose xid->fxid
facility, it's virtually guaranteed that someone will eventually use
it to shoot footwards, by acquiring an xid from somewhere, and then
some arbitrary time later extending it to a fxid when no interlocking
guarantees that the later conversion has the correct epoch.

I'd like to figure out how to maintain full versions of the
procarray.c-managed xid horizons, without widening the shared memory
representation.  I was planning to think harder about for 13.
Obviously that doesn't help you now.  So I'm wondering if you should
just open-code this for now, and put in a comment about why it's safe
and a note that there'll hopefully be a fxid horizon available in a
later release?

[1] https://github.com/EnterpriseDB/zheap/commit/1203c2fa49f5f872f42ea4a02ccba2b191c45f32

-- 
Thomas Munro
https://enterprisedb.com