Re: [BUGS] Old row version in hot chain become visible after a freeze

First new reply

On 9/6/17, 3:41 AM, "Alvaro Herrera" <alvherre@alvh.no-ip.org> wrote:
   Michael Paquier wrote:      > I have also spent a couple more hours looking at the proposed patch   > and
eye-ballingthe surrounding code, and my suggestion about   > heap_tuple_needs_freeze() is proving to be wrong. So I am
arrivingat   > the conclusion that your patch is taking the right approach to skip   > freezing completely if the tuple
isnot to be removed yet if it is for   > vacuum either DEAD or RECENTLY_DEAD.      I think in the "tupkeep" case we
mustnot mark the page as frozen in VM;   in other words I think that block needs to look like this:                  //
tupgone= false               {                   bool        tuple_totally_frozen;                      num_tuples +=
1;                  hastup = true;                      /*                    * Each non-removable tuple that we do not
keepmust be checked                    * to see if it needs freezing.  Note we already have exclusive
* buffer lock.                    */                   if (!tupkeep &&
heap_prepare_freeze_tuple(tuple.t_data,FreezeLimit,                                                 MultiXactCutoff,
                                            &frozen[nfrozen],
&tuple_totally_frozen))                          frozen[nfrozen++].offset = offnum;                      if (tupkeep ||
!tuple_totally_frozen)                      all_frozen = false;               }      Otherwise, we risk marking the
pageas all-frozen, and it would be   skipped by vacuum.  If we never come around to HOT-pruning the page, a
non-permanentxid (or a multixact? not sure that that can happen;   probably not) would linger unnoticed and cause a DoS
conditionlater   ("cannot open file pg_clog/1234") when the tuple header is read.      Now, it is possible that HOT
pruningwould fix the page promptly without   causing an actual DoS, but nonetheless it seems dangerous to leave
thingslike this.      --    Álvaro Herrera                https://www.2ndQuadrant.com/   PostgreSQL Development, 24x7
Support,Remote DBA, Training & Services   
 


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