Mark Dilger <[email protected]> writes:
>> On Mar 9, 2021, at 1:35 PM, Tom Lane <[email protected]> wrote:
>> So, to accept a patch that shortens the line pointer array, what we need
>> to do is verify that every such code path checks for an out-of-range
>> offset before trying to fetch the target line pointer.
> Much as Pavan asked [1], I'm curious how we wouldn't already be in trouble if
> such code exists? In such a scenario, what stops a dead line pointer from
> being reused (rather than garbage collected by this patch) prior to such
> hypothetical code using an outdated TID?
The line pointer very well *could* be re-used before the in-flight
reference gets to it. That's okay though, because whatever tuple now
occupies the TID would have to have xmin too new to match the snapshot
that such a reference is scanning with.
(Back when we had non-MVCC snapshots to contend with, a bunch of
additional arm-waving was needed to argue that such situations were
safe. Possibly the proposed change wouldn't have flown back then.)
regards, tom lane
