On 26.10.2011 18:42, Robert Haas wrote:
> On Tue, Oct 25, 2011 at 12:37 PM, Jeff Davis<pgsql@j-davis.com> wrote:
>> Aren't there a few other cases like this floating around the code? I
>> know the single-xid cache is potentially vulnerable to xid wraparound
>> for the same reason.
>
> I believe that we're in trouble with XIDs as soon as you have two
> active XIDs that are separated by a billion, ...
That's not what Jeff is referring to here, though (correct me if I'm
wrong). He's talking about the one-item cache in
TransactionIdLogFetch(). You don't need need long-running transactions
for that to get confused. Specifically, this could happen:
1. In session A: BEGIN; SELECT * FROM foo WHERE id = 1; COMMIT; The row has xmin = 123456, and it is cached as
committedin the
one-item cache by TransactionLogFetch.
2. A lot of time passes. Everything is frozen, and XID wrap-around
happens. (Session A is idle but not in a transaction, so it doesn't
inhibit freezing.)
3. In session B: BEGIN: INSERT INTO foo (id) VALUES (2); ROLLBACK; By coincidence, this transaction was assigned XID
123456.
4. In session A: SELECT * FROM foo WHERE id = 2; The one-item cache still says that 123456 committed, so we return
the tuple inserted by the aborted transaction. Oops.
-- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com