On 26.10.2011 18:42, Robert Haas wrote:
On Tue, Oct 25, 2011 at 12:37 PM, Jeff Davis<pg...@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.

  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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