Re: Bug in autovacuum.c?

Alvaro Herrera wrote:
> Excerpts from Bruce Momjian's message of vie abr 01 16:50:29 -0300 2011:
>
> > To do the right thing every computation that passes over the xid
> > wraparound bounary should subtract FirstNormalTransactionId, not just
> > those that fall in the boundry.  That would prevent the value from going
> > backward and still allow the mapping you liked;  it isn't worth it, but
> > that is the right answer.
>
> This code is only concerned calculating an immediate the wrap horizon
> for the autovacuuming run that's about to take place.  If it's wrong in
> one or three counts doesn't mean much.  Consider what would happen if
> load was high and it would have taken 100 extra milliseconds to get to
> that bit: ReadNewTransactionId would have returned a value 3
> transactions later.  Furthermore, before this value is even used at all
> for vacuuming, there has to be a whole lot of inter-process signalling,
> a fork, and a new backend startup.
>
> I think this should be left alone.  As you said, it isn't worth it.

Agreed it is not worth it but I think we should at least C comment
something.   I think at a minimum we should set it to
FirstNormalTransactionId.

I am not so concerned about this case but about other cases where we are
computing xid distances across the invalid range.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
new file mode 100644
index efc8e7c..244930f
*** a/src/backend/postmaster/autovacuum.c
--- b/src/backend/postmaster/autovacuum.c
*************** do_start_worker(void)
*** 1107,1114 ****
       */
      recentXid = ReadNewTransactionId();
      xidForceLimit = recentXid - autovacuum_freeze_max_age;
!     /* ensure it's a "normal" XID, else TransactionIdPrecedes misbehaves */
!     if (xidForceLimit < FirstNormalTransactionId)
          xidForceLimit -= FirstNormalTransactionId;

      /*
--- 1107,1120 ----
       */
      recentXid = ReadNewTransactionId();
      xidForceLimit = recentXid - autovacuum_freeze_max_age;
!     /*
!      * Adjust for xids in the invalid range, less than FirstNormalTransactionId.
!      * We map the xids as though the invalid range did not exist.
!      * We cannot simplify the test below because of underflow
!      * issues with unsigned values.
!      */
!     if (xidForceLimit < FirstNormalTransactionId || /* in invalid range? */
!         recentXid < autovacuum_freeze_max_age)         /* did we underflow? */
          xidForceLimit -= FirstNormalTransactionId;

      /*