Avoid using tuple from syscache for update of pg_database.datfrozenxid

pg_database.datfrozenxid gets updated using an in-place update at the
end of vacuum or autovacuum.  Since 96cdeae, as pg_database has a toast
relation, it is possible for a pg_database tuple to have toast values
if there is a large set of ACLs in place.  In such a case, the in-place
update would fail because of the flattening of the toast values done for
the catcache entry fetched.  Instead of using a copy from the catcache,
this changes the logic to fetch the copy of the tuple by directly
scanning pg_database.

Note that before 96cdeae, attempting to insert such a tuple to
pg_database would cause a "row is too big" error, so the end-of-vacuum
problem was not reachable.

This issue has been originally fixed in 947789f on v14~, and there have
been reports about this problem on v12 and v13, causing failures at the
end of VACUUM.  This completes the fix on all the stable branches where
pg_database can use a toast table, down to 12.

Author: Ashwin Agrawal, Junfeng Yang
Discussion: https://postgr.es/m/DM5PR0501MB38800D9E4605BCA72DD35557CCE10@DM5PR0501MB3880.namprd05.prod.outlook.com
Discussion: https://postgr.es/m/Y70XNVbUWQsR2Car@paquier.xyz
Backpatch-through: 12

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 922744e04db46db8926804be91e283448ec9953f..657af12a3990504b88fee00dfa2159cbf21a2a43 100644 (file)
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -5837,6 +5837,10 @@ heap_abort_speculative(Relation relation, ItemPointer tid)
  *
  * tuple is an in-memory tuple structure containing the data to be written
  * over the target tuple.  Also, tuple->t_self identifies the target tuple.
+ *
+ * Note that the tuple updated here had better not come directly from the
+ * syscache if the relation has a toast relation as this tuple could
+ * include toast values that have been expanded, causing a failure here.
  */
 void
 heap_inplace_update(Relation relation, HeapTuple tuple)

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index f5a0b600f9e58972fa7635d55ddb3e07838b904a..493a81c678126ef8cbc7338816a3ef4741f39ab0 100644 (file)
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -31,6 +31,7 @@
 #include "access/tableam.h"
 #include "access/transam.h"
 #include "access/xact.h"
+#include "catalog/indexing.h"
 #include "catalog/namespace.h"
 #include "catalog/pg_database.h"
 #include "catalog/pg_inherits.h"
@@ -1294,6 +1295,7 @@ vac_update_datfrozenxid(void)
    MultiXactId lastSaneMinMulti;
    bool        bogus = false;
    bool        dirty = false;
+   ScanKeyData key[1];
 
    /*
     * Restrict this task to one backend per database.  This avoids race
@@ -1411,10 +1413,25 @@ vac_update_datfrozenxid(void)
    /* Now fetch the pg_database tuple we need to update. */
    relation = table_open(DatabaseRelationId, RowExclusiveLock);
 
-   /* Fetch a copy of the tuple to scribble on */
-   tuple = SearchSysCacheCopy1(DATABASEOID, ObjectIdGetDatum(MyDatabaseId));
+   /*
+    * Get the pg_database tuple to scribble on.  Note that this does not
+    * directly rely on the syscache to avoid issues with flattened toast
+    * values for the in-place update.
+    */
+   ScanKeyInit(&key[0],
+               Anum_pg_database_oid,
+               BTEqualStrategyNumber, F_OIDEQ,
+               ObjectIdGetDatum(MyDatabaseId));
+
+   scan = systable_beginscan(relation, DatabaseOidIndexId, true,
+                             NULL, 1, key);
+   tuple = systable_getnext(scan);
+   tuple = heap_copytuple(tuple);
+   systable_endscan(scan);
+
    if (!HeapTupleIsValid(tuple))
        elog(ERROR, "could not find tuple for database %u", MyDatabaseId);
+
    dbform = (Form_pg_database) GETSTRUCT(tuple);
 
    /*