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Prevent "snapshot too old" from trying to return pruned TOAST tuples.

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Previously, we tested for MVCC snapshots to see whether they were too
old, but not TOAST snapshots, which can lead to complaints about missing
TOAST chunks if those chunks are subject to early pruning.  Ideally,
the threshold lsn and timestamp for a TOAST snapshot would be that of
the corresponding MVCC snapshot, but since we have no way of deciding
which MVCC snapshot was used to fetch the TOAST pointer, use the oldest
active or registered snapshot instead.

Reported by Andres Freund, who also sketched out what the fix should
look like.  Patch by me, reviewed by Amit Kapila.
This commit is contained in:
Robert Haas
2016-08-03 16:41:43 -04:00
parent a3c7a993d5
commit 3e2f3c2e42
6 changed files with 96 additions and 16 deletions
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37
src/backend/access/heap/tuptoaster.c
View File

@ -40,6 +40,7 @@
#include "utils/expandeddatum.h"
#include "utils/fmgroids.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
#include "utils/typcache.h"
#include "utils/tqual.h"
@ -81,6 +82,7 @@ static int toast_open_indexes(Relation toastrel,
int *num_indexes);
static void toast_close_indexes(Relation *toastidxs, int num_indexes,
LOCKMODE lock);
static void init_toast_snapshot(Snapshot toast_snapshot);
/* ----------
@ -1665,6 +1667,7 @@ toast_delete_datum(Relation rel, Datum value)
HeapTuple toasttup;
int num_indexes;
int validIndex;
SnapshotData SnapshotToast;
if (!VARATT_IS_EXTERNAL_ONDISK(attr))
return;
@ -1696,8 +1699,9 @@ toast_delete_datum(Relation rel, Datum value)
* sequence or not, but since we've already locked the index we might as
* well use systable_beginscan_ordered.)
*/
init_toast_snapshot(&SnapshotToast);
toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
SnapshotToast, 1, &toastkey);
&SnapshotToast, 1, &toastkey);
while ((toasttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
{
/*
@ -1730,6 +1734,7 @@ toastrel_valueid_exists(Relation toastrel, Oid valueid)
int num_indexes;
int validIndex;
Relation *toastidxs;
SnapshotData SnapshotToast;
/* Fetch a valid index relation */
validIndex = toast_open_indexes(toastrel,
@ -1748,9 +1753,10 @@ toastrel_valueid_exists(Relation toastrel, Oid valueid)
/*
* Is there any such chunk?
*/
init_toast_snapshot(&SnapshotToast);
toastscan = systable_beginscan(toastrel,
RelationGetRelid(toastidxs[validIndex]),
true, SnapshotToast, 1, &toastkey);
true, &SnapshotToast, 1, &toastkey);
if (systable_getnext(toastscan) != NULL)
result = true;
@ -1813,6 +1819,7 @@ toast_fetch_datum(struct varlena * attr)
int32 chunksize;
int num_indexes;
int validIndex;
SnapshotData SnapshotToast;
if (!VARATT_IS_EXTERNAL_ONDISK(attr))
elog(ERROR, "toast_fetch_datum shouldn't be called for non-ondisk datums");
@ -1859,8 +1866,9 @@ toast_fetch_datum(struct varlena * attr)
*/
nextidx = 0;
init_toast_snapshot(&SnapshotToast);
toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
SnapshotToast, 1, &toastkey);
&SnapshotToast, 1, &toastkey);
while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
{
/*
@ -1990,6 +1998,7 @@ toast_fetch_datum_slice(struct varlena * attr, int32 sliceoffset, int32 length)
int32 chcpyend;
int num_indexes;
int validIndex;
SnapshotData SnapshotToast;
if (!VARATT_IS_EXTERNAL_ONDISK(attr))
elog(ERROR, "toast_fetch_datum_slice shouldn't be called for non-ondisk datums");
@ -2082,9 +2091,10 @@ toast_fetch_datum_slice(struct varlena * attr, int32 sliceoffset, int32 length)
*
* The index is on (valueid, chunkidx) so they will come in order
*/
init_toast_snapshot(&SnapshotToast);
nextidx = startchunk;
toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
SnapshotToast, nscankeys, toastkey);
&SnapshotToast, nscankeys, toastkey);
while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
{
/*
@ -2289,3 +2299,22 @@ toast_close_indexes(Relation *toastidxs, int num_indexes, LOCKMODE lock)
index_close(toastidxs[i], lock);
pfree(toastidxs);
}
/* ----------
* init_toast_snapshot
*
* Initialize an appropriate TOAST snapshot. We must use an MVCC snapshot
* to initialize the TOAST snapshot; since we don't know which one to use,
* just use the oldest one. This is safe: at worst, we will get a "snapshot
* too old" error that might have been avoided otherwise.
*/
static void
init_toast_snapshot(Snapshot toast_snapshot)
{
Snapshot snapshot = GetOldestSnapshot();
if (snapshot == NULL)
elog(ERROR, "no known snapshots");
InitToastSnapshot(toast_snapshot, snapshot->lsn, snapshot->whenTaken);
}