Don't update relfrozenxid if any pages were skipped.

Vacuum recognizes that it can update relfrozenxid by checking whether it has
processed all pages of a relation. Unfortunately it performed that check
after truncating the dead pages at the end of the relation, and used the new
number of pages to decide whether all pages have been scanned. If the new
number of pages happened to be smaller or equal to the number of pages
scanned, it incorrectly decided that all pages were scanned.

This can lead to relfrozenxid being updated, even though some pages were
skipped that still contain old XIDs. That can lead to data loss due to xid
wraparounds with some rows suddenly missing. This likely has escaped notice
so far because it takes a large number (~2^31) of xids being used to see the
effect, while a full-table vacuum before that would fix the issue.

The incorrect logic was introduced by commit
b4b6923e03f4d29636a94f6f4cc2f5cf6298b8c8. Backpatch this fix down to 8.4,
like that commit.

Andres Freund, with some modifications by me.

diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index f4bffb7d907ec8f95238bfefa4a5ec7ea8ac09a2..1d2472e99186aa536ea9d4d1ec217b1031ed8e6e 100644 (file)
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -166,7 +166,8 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
        BlockNumber possibly_freeable;
        PGRUsage        ru0;
        TimestampTz starttime = 0;
-       bool            scan_all;
+       bool            scan_all;               /* should we scan all pages? */
+       bool            scanned_all;    /* did we actually scan all pages? */
        TransactionId freezeTableLimit;
        BlockNumber new_rel_pages;
        double          new_rel_tuples;
@@ -209,6 +210,21 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
        /* Done with indexes */
        vac_close_indexes(nindexes, Irel, NoLock);
 
+       /*
+        * Compute whether we actually scanned the whole relation. If we did, we
+        * can adjust relfrozenxid.
+        *
+        * NB: We need to check this before truncating the relation, because that
+        * will change ->rel_pages.
+        */
+       if (vacrelstats->scanned_pages < vacrelstats->rel_pages)
+       {
+               Assert(!scan_all);
+               scanned_all = false;
+       }
+       else
+               scanned_all = true;
+
        /*
         * Optionally truncate the relation.
         *
@@ -245,9 +261,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
                new_rel_tuples = vacrelstats->old_rel_tuples;
        }
 
-       new_frozen_xid = FreezeLimit;
-       if (vacrelstats->scanned_pages < vacrelstats->rel_pages)
-               new_frozen_xid = InvalidTransactionId;
+       new_frozen_xid = scanned_all ? FreezeLimit : InvalidTransactionId;
 
        vac_update_relstats(onerel,
                                                new_rel_pages, new_rel_tuples,