On Wed, Oct 26, 2011 at 11:52 AM, Heikki Linnakangas
<heikki.linnakangas@enterprisedb.com> wrote:
> 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 committed in 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.
Oh, hmm. That sucks.
--
Robert Haas
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