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Commit08e261c

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Fix race condition with toast table access from a stale syscache entry.
If a tuple in a syscache contains an out-of-line toasted field, and wetry to fetch that field shortly after some other transaction has committedan update or deletion of the tuple, there is a race condition: vacuumcould come along and remove the toast tuples before we can fetch them.This leads to transient failures like "missing chunk number 0 for toastvalue NNNNN in pg_toast_2619", as seen in recent reports from AndrewHammond and Tim Uckun.The design idea of syscache is that access to stale syscache entriesshould be prevented by relation-level locks, but that fails for at leasttwo cases where toasted fields are possible: ANALYZE updates pg_statisticrows without locking out sessions that might want to plan queries on thesame table, and CREATE OR REPLACE FUNCTION updates pg_proc rows withoutany meaningful lock at all.The least risky fix seems to be an idea that Heikki suggested when wewere dealing with a related problem back in August: forcibly detoast anyout-of-line fields before putting a tuple into syscache in the first place.This avoids the problem because at the time we fetch the parent tuple fromthe catalog, we should be holding an MVCC snapshot that will preventremoval of the toast tuples, even if the parent tuple is outdatedimmediately after we fetch it. (Note: I'm not convinced that thisstatement holds true at every instant where we could be fetching a syscacheentry at all, but it does appear to hold true at the times where we couldfetch an entry that could have a toasted field. We will need to be a bitwary of adding toast tables to low-level catalogs that don't have themalready.) An additional benefit is that subsequent uses of the syscacheentry should be faster, since they won't have to detoast the field.Back-patch to all supported versions. The problem is significantly harderto reproduce in pre-9.0 releases, because of their willingness to flushevery entry in a syscache whenever the underlying catalog is vacuumed(cf CatalogCacheFlushRelation); but there is still a window for trouble.
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3 files changed

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3 files changed

+108
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‎src/backend/access/heap/tuptoaster.c

Lines changed: 81 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -928,6 +928,87 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
928928
}
929929

930930

931+
/* ----------
932+
* toast_flatten_tuple -
933+
*
934+
*"Flatten" a tuple to contain no out-of-line toasted fields.
935+
*(This does not eliminate compressed or short-header datums.)
936+
* ----------
937+
*/
938+
HeapTuple
939+
toast_flatten_tuple(HeapTupletup,TupleDesctupleDesc)
940+
{
941+
HeapTuplenew_tuple;
942+
Form_pg_attribute*att=tupleDesc->attrs;
943+
intnumAttrs=tupleDesc->natts;
944+
inti;
945+
Datumtoast_values[MaxTupleAttributeNumber];
946+
booltoast_isnull[MaxTupleAttributeNumber];
947+
booltoast_free[MaxTupleAttributeNumber];
948+
949+
/*
950+
* Break down the tuple into fields.
951+
*/
952+
Assert(numAttrs <=MaxTupleAttributeNumber);
953+
heap_deform_tuple(tup,tupleDesc,toast_values,toast_isnull);
954+
955+
memset(toast_free,0,numAttrs*sizeof(bool));
956+
957+
for (i=0;i<numAttrs;i++)
958+
{
959+
/*
960+
* Look at non-null varlena attributes
961+
*/
962+
if (!toast_isnull[i]&&att[i]->attlen==-1)
963+
{
964+
structvarlena*new_value;
965+
966+
new_value= (structvarlena*)DatumGetPointer(toast_values[i]);
967+
if (VARATT_IS_EXTERNAL(new_value))
968+
{
969+
new_value=toast_fetch_datum(new_value);
970+
toast_values[i]=PointerGetDatum(new_value);
971+
toast_free[i]= true;
972+
}
973+
}
974+
}
975+
976+
/*
977+
* Form the reconfigured tuple.
978+
*/
979+
new_tuple=heap_form_tuple(tupleDesc,toast_values,toast_isnull);
980+
981+
/*
982+
* Be sure to copy the tuple's OID and identity fields. We also make a
983+
* point of copying visibility info, just in case anybody looks at those
984+
* fields in a syscache entry.
985+
*/
986+
if (tupleDesc->tdhasoid)
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HeapTupleSetOid(new_tuple,HeapTupleGetOid(tup));
988+
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new_tuple->t_self=tup->t_self;
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new_tuple->t_tableOid=tup->t_tableOid;
991+
992+
new_tuple->t_data->t_choice=tup->t_data->t_choice;
993+
new_tuple->t_data->t_ctid=tup->t_data->t_ctid;
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new_tuple->t_data->t_infomask &= ~HEAP_XACT_MASK;
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new_tuple->t_data->t_infomask |=
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tup->t_data->t_infomask&HEAP_XACT_MASK;
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new_tuple->t_data->t_infomask2 &= ~HEAP2_XACT_MASK;
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new_tuple->t_data->t_infomask2 |=
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tup->t_data->t_infomask2&HEAP2_XACT_MASK;
1000+
1001+
/*
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* Free allocated temp values
1003+
*/
1004+
for (i=0;i<numAttrs;i++)
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if (toast_free[i])
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pfree(DatumGetPointer(toast_values[i]));
1007+
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returnnew_tuple;
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}
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9311012
/* ----------
9321013
* toast_flatten_tuple_attribute -
9331014
*

‎src/backend/utils/cache/catcache.c

Lines changed: 18 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -19,6 +19,7 @@
1919
#include"access/heapam.h"
2020
#include"access/relscan.h"
2121
#include"access/sysattr.h"
22+
#include"access/tuptoaster.h"
2223
#include"access/valid.h"
2324
#include"catalog/pg_operator.h"
2425
#include"catalog/pg_type.h"
@@ -1591,16 +1592,32 @@ CatalogCacheCreateEntry(CatCache *cache, HeapTuple ntp,
15911592
uint32hashValue,IndexhashIndex,boolnegative)
15921593
{
15931594
CatCTup*ct;
1595+
HeapTupledtp;
15941596
MemoryContextoldcxt;
15951597

1598+
/*
1599+
* If there are any out-of-line toasted fields in the tuple, expand them
1600+
* in-line. This saves cycles during later use of the catcache entry,
1601+
* and also protects us against the possibility of the toast tuples being
1602+
* freed before we attempt to fetch them, in case of something using a
1603+
* slightly stale catcache entry.
1604+
*/
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if (HeapTupleHasExternal(ntp))
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dtp=toast_flatten_tuple(ntp,cache->cc_tupdesc);
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else
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dtp=ntp;
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15961610
/*
15971611
* Allocate CatCTup header in cache memory, and copy the tuple there too.
15981612
*/
15991613
oldcxt=MemoryContextSwitchTo(CacheMemoryContext);
16001614
ct= (CatCTup*)palloc(sizeof(CatCTup));
1601-
heap_copytuple_with_tuple(ntp,&ct->tuple);
1615+
heap_copytuple_with_tuple(dtp,&ct->tuple);
16021616
MemoryContextSwitchTo(oldcxt);
16031617

1618+
if (dtp!=ntp)
1619+
heap_freetuple(dtp);
1620+
16041621
/*
16051622
* Finish initializing the CatCTup header, and add it to the cache's
16061623
* linked list and counts.

‎src/include/access/tuptoaster.h

Lines changed: 9 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -143,6 +143,15 @@ extern struct varlena *heap_tuple_untoast_attr_slice(struct varlena * attr,
143143
int32sliceoffset,
144144
int32slicelength);
145145

146+
/* ----------
147+
* toast_flatten_tuple -
148+
*
149+
*"Flatten" a tuple to contain no out-of-line toasted fields.
150+
*(This does not eliminate compressed or short-header datums.)
151+
* ----------
152+
*/
153+
externHeapTupletoast_flatten_tuple(HeapTupletup,TupleDesctupleDesc);
154+
146155
/* ----------
147156
* toast_flatten_tuple_attribute -
148157
*

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