8000 Fix traversal of half-frozen update chains · koderP/postgres@b052d52 · GitHub
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alvherrealvherre
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Fix traversal of half-frozen update chains
When some tuple versions in an update chain are frozen due to them being older than freeze_min_age, the xmax/xmin trail can become broken. This breaks HOT (and probably other things). A subsequent VACUUM can break things in more serious ways, such as leaving orphan heap-only tuples whose root HOT redirect items were removed. This can be seen because index creation (or REINDEX) complain like ERROR: XX000: failed to find parent tuple for heap-only tuple at (0,7) in table "t" Because of relfrozenxid contraints, we cannot avoid the freezing of the early tuples, so we must cope with the results: whenever we see an Xmin of FrozenTransactionId, consider it a match for whatever the previous Xmax value was. This problem seems to have appeared in 9.3 with multixact changes, though strictly speaking it seems unrelated. Since 9.4 we have commit 37484ad "Change the way we mark tuples as frozen", so the fix is simple: just compare the raw Xmin (still stored in the tuple header, since freezing merely set an infomask bit) to the Xmax. But in 9.3 we rewrite the Xmin value to FrozenTransactionId, so the original value is lost and we have nothing to compare the Xmax with. To cope with that case we need to compare the Xmin with FrozenXid, assume it's a match, and hope for the best. Sadly, since you can pg_upgrade a 9.3 instance containing half-frozen pages to newer releases, we need to keep the old check in newer versions too, which seems a bit brittle; I hope we can somehow get rid of that. I didn't optimize the new function for performance. The new coding is probably a bit slower than before, since there is a function call rather than a straight comparison, but I'd rather have it work correctly than be fast but wrong. This is a followup after 20b6552 fixed a few related problems. Apparently, in 9.6 and up there are more ways to get into trouble, but in 9.3 - 9.5 I cannot reproduce a problem anymore with this patch, so there must be a separate bug. Reported-by: Peter Geoghegan Diagnosed-by: Peter Geoghegan, Michael Paquier, Daniel Wood, Yi Wen Wong, Álvaro Discussion: https://postgr.es/m/CAH2-Wznm4rCrhFAiwKPWTpEw2bXDtgROZK7jWWGucXeH3D1fmA@mail.gmail.com
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4 files changed

+46
-11
lines changed

4 files changed

+46
-11
lines changed

‎src/backend/access/heap/heapam.c

Lines changed: 39 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1718,8 +1718,7 @@ heap_hot_search_buffer(ItemPointer tid, Relation relation, Buffer buffer,
17181718
* broken.
17191719
*/
17201720
if (TransactionIdIsValid(prev_xmax) &&
1721-
!TransactionIdEquals(prev_xmax,
1722-
HeapTupleHeaderGetXmin(heapTuple->t_data)))
1721+
!HeapTupleUpdateXmaxMatchesXmin(prev_xmax, heapTuple->t_data))
17231722
break;
17241723

17251724
/*
@@ -1888,7 +1887,7 @@ heap_get_latest_tid(Relation relation,
18881887
* tuple. Check for XMIN match.
18891888
*/
18901889
if (TransactionIdIsValid(priorXmax) &&
1891-
!TransactionIdEquals(priorXmax, HeapTupleHeaderGetXmin(tp.t_data)))
1890+
!HeapTupleUpdateXmaxMatchesXmin(priorXmax, tp.t_data))
18921891
{
18931892
UnlockReleaseBuffer(buffer);
18941893
break;
@@ -1920,6 +1919,39 @@ heap_get_latest_tid(Relation relation,
19201919
} /* end of loop */
19211920
}
19221921

1922+
/*
1923+
* HeapTupleUpdateXmaxMatchesXmin - verify update chain xmax/xmin lineage
1924+
*
1925+
* Given the new version of a tuple after some update, verify whether the
1926+
* given Xmax (corresponding to the previous version) matches the tuple's
1927+
* Xmin, taking into account that the Xmin might have been frozen after the
1928+
* update.
1929+
*/
1930+
bool
1931+
HeapTupleUpdateXmaxMatchesXmin(TransactionId xmax, HeapTupleHeader htup)
1932+
{
1933+
TransactionId xmin = HeapTupleHeaderGetXmin(htup);
1934+
1935+
/*
1936+
* If the xmax of the old tuple is identical to the xmin of the new one,
1937+
* it's a match.
1938+
*/
1939+
if (TransactionIdEquals(xmax, xmin))
1940+
return true;
1941+
1942+
/*
1943+
* When a tuple is frozen, the original Xmin is lost, but we know it's a
1944+
* committed transaction. So unless the Xmax is InvalidXid, we don't know
1945+
* for certain that there is a match, but there may be one; and we must
1946+
* return true so that a HOT chain that is half-frozen can be walked
1947+
* correctly.
1948+
*/
1949+
if (TransactionIdEquals(xmin, FrozenTransactionId) &&
1950+
TransactionIdIsValid(xmax))
1951+
return true;
1952+
1953+
return false;
1954+
}
19231955

19241956
/*
19251957
* UpdateXmaxHintBits - update tuple hint bits after xmax transaction ends
@@ -5045,8 +5077,7 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
50455077
* the end of the chain, we're done, so return success.
50465078
*/
50475079
if (TransactionIdIsValid(priorXmax) &&
5048-
!TransactionIdEquals(HeapTupleHeaderGetXmin(mytup.t_data),
5049-
priorXmax))
5080+
!HeapTupleUpdateXmaxMatchesXmin(priorXmax, mytup.t_data))
50505081
{
50515082
UnlockReleaseBuffer(buf);
50525083
return HeapTupleMayBeUpdated;
@@ -5500,7 +5531,10 @@ FreezeMultiXactId(MultiXactId multi, uint16 t_infomask,
55005531
if (TransactionIdPrecedes(xid, cutoff_xid))
55015532
{
55025533
if (TransactionIdDidCommit(xid))
5534+
{
5535+
xid = FrozenTransactionId;
55035536
*flags = FRM_MARK_COMMITTED | FRM_RETURN_IS_XID;
5537+
}
55045538
else
55055539
{
55065540
*flags |= FRM_INVALIDATE_XMAX;

‎src/backend/access/heap/pruneheap.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -435,7 +435,7 @@ heap_prune_chain(Relation relation, Buffer buffer, OffsetNumber rootoffnum,
435435
* Check the tuple XMIN against prior XMAX, if any
436436
*/
437437
if (TransactionIdIsValid(priorXmax) &&
438-
!TransactionIdEquals(HeapTupleHeaderGetXmin(htup), priorXmax))
438+
!HeapTupleUpdateXmaxMatchesXmin(priorXmax, htup))
439439
break;
440440

441441
/*
@@ -774,7 +774,7 @@ heap_get_root_tuples(Page page, OffsetNumber *root_offsets)
774774
htup = (HeapTupleHeader) PageGetItem(page, lp);
775775

776776
if (TransactionIdIsValid(priorXmax) &&
777-
!TransactionIdEquals(priorXmax, HeapTupleHeaderGetXmin(htup)))
777+
!HeapTupleUpdateXmaxMatchesXmin(priorXmax, htup))
778778
break;
779779

780780
/* Remember the root line pointer for this item */

‎src/backend/executor/execMain.c

Lines changed: 2 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -2019,8 +2019,7 @@ EvalPlanQualFetch(EState *estate, Relation relation, int lockmode,
20192019
* buffer's content lock, since xmin never changes in an existing
20202020
* tuple.)
20212021
*/
2022-
if (!TransactionIdEquals(HeapTupleHeaderGetXmin(tuple.t_data),
2023-
priorXmax))
2022+
if (!HeapTupleUpdateXmaxMatchesXmin(priorXmax, tuple.t_data))
20242023
{
20252024
ReleaseBuffer(buffer);
20262025
return NULL;
@@ -2138,8 +2137,7 @@ EvalPlanQualFetch(EState *estate, Relation relation, int lockmode,
21382137
/*
21392138
* As above, if xmin isn't what we're expecting, do nothing.
21402139
*/
2141-
if (!TransactionIdEquals(HeapTupleHeaderGetXmin(tuple.t_data),
2142-
priorXmax))
2140+
if (!HeapTupleUpdateXmaxMatchesXmin(priorXmax, tuple.t_data))
21432141
{
21442142
ReleaseBuffer(buffer);
21452143
return NULL;

‎src/include/access/heapam.h

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -127,6 +127,9 @@ extern void heap_get_latest_tid(Relation relation, Snapshot snapshot,
127127
ItemPointer tid);
128128
extern void setLastTid(const ItemPointer tid);
129129

130+
extern bool HeapTupleUpdateXmaxMatchesXmin(TransactionId xmax,
131+
HeapTupleHeader htup);
132+
130133
extern BulkInsertState GetBulkInsertState(void);
131134
extern void FreeBulkInsertState(BulkInsertState);
132135

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