Re: Hot Standby: subxid cache changes

On Thu, 2009-02-12 at 14:23 +0200, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > On Thu, 2009-02-12 at 09:50 +0200, Heikki Linnakangas wrote:
> >> So far so good, but what about all the other callers of
> >> SubTransGetParent()? For example, XactLockTableWait will fail an
> >> assertion if asked to wait on a subtransaction which is then released.
> >
> > I agree that it could fail the assertion, though it is clear that the
> > assertion should now be removed.
>
> No, then you just get an infinite loop instead, trying to get the parent
> of 0 over and over again.

There is no infinite loop. Try it, or read TransactionIdIsInProgress().

> > The logic is: if there is no lock table entry for that xid *and* it is
> > not in progress *and* it is not in pg_subtrans, then it must have been
> > an aborted subtransaction of a currently active xact or it has otherwise
> > completed.
>
> Right, we got it right that far. But after the subtransaction has
> completed, the question is: what's its parent? That's what the patch got
> wrong.

We can find that out from procarray, since a subcommitted xid will still
be present in the subxid cache of its parent (by definition, otherwise
it will be marked in pg_subtrans).

It will be quicker to fix that than to make other changes.

--
Simon Riggs www.2ndQuadrant.com
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