Re: Reviewing freeze map code

Robert Haas wrote:

> 3. vacuumlazy.c includes this code:
>
> if (heap_prepare_freeze_tuple(tuple.t_data, FreezeLimit,
> MultiXactCutoff, &frozen[nfrozen]))
> frozen[nfrozen++].offset = offnum;
> else if (heap_tuple_needs_eventual_freeze(tuple.t_data))
> all_frozen = false;
>
> That's wrong, because a "true" return value from
> heap_prepare_freeze_tuple() means only that it has done *some*
> freezing work on the tuple, not that it's done all of the freezing
> work that will ever need to be done. So, if the tuple's xmin can be
> frozen and is aborted but not older than vacuum_freeze_min_age, then
> heap_prepare_freeze_tuple() won't free xmax, but the page will still
> be marked all-frozen, which is bad. I think it normally won't matter
> because the xmax will probably be hinted invalid anyway, since we just
> pruned the page which should have set hint bits everywhere, but if
> those hint bits were lost then we'd eventually end up with an
> accessible xmax pointing off into space.

Good catch. Also consider multixact freezing: if there is a
long-running transaction which is a lock-only member of tuple's Xmax,
and the multixact needs freezing because it's older than the multixact
cutoff, we set the xmax to a new multixact which includes that old
locker. See FreezeMultiXactId.

> My first thought was to just delete the "else" but that would be bad
> because we'd fail to set all-frozen immediately in a lot of cases
> where we should. This needs a bit more thought than I have time to
> give it right now.

How about changing the return tuple of heap_prepare_freeze_tuple to
a bitmap? Two flags: "Freeze [not] done" and "[No] more freezing
needed"

--
Álvaro Herrera http://www.2ndQuadrant.com/
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